UNIVERSITY OF CALIFORNIA
MEDICAL CENTER LIBRARY
SAN FRANCISCO
FROM THE LIBRARY OF
ALBION W. HEWLETT, M.D.
PHYSIOLOGY AND BIOCHEMISTRY
IN MODERN MEDICINE
BY
J. J. II. MACLEOD, M.B.
PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF TORONTO, TORONTO, CANADA; FORMERLY
PROFESSOR OF PHYSIOLOGY IN THE WESTERN RESERVE UNIVERSITY,
CLEVELAND, OHIO
ASSISTED BY ROY G. PEARCE, B.A., M.D.
Director of the Cardiorespiratory Laboratory of Lakeside Hospital,
Cleveland, Ohio
AND BY OTHERS
WITH 233 ILLUSTRATIONS, INCLUDING
11 PLATES IN COLORS
ST. LOUIS
C. V. MOSBY COMPANY
1918
COPYRIGHT, 1918, BY C. V. MOSBY COMPANY
Press of
C. V. Mosby Company
St. Louis
-Ml 6
TO
M. W. M.
PREFACE
The necessity of allotting the various subjects of the medical curric-
ulum to different periods, so that the more strictly scientific subjects
are completed in the earlier years, has the great disadvantage that the
student, being no longer in touch with laboratory work, fails to employ
the scientific knowledge with full advantage in the solution of his clin-
ical problems. He is apt to regard the first two or three years in the
laboratory departments as inconsequential in comparison with the sup-
posedly more practical instruction offered during the subsequent clinical
years. He is taught by his laboratory instructors to observe accurately,
and to correlate the observed facts, so that he may be enabled to draw
conclusions as to the manner of working of the various functions of the
animal body in health, and before proceeding to his clinical studies, he
is required to show a proficiency in scientific knowledge, because it is
recognized that this must serve as the basis upon which his knowledge
of disease is to be built. When the clinic is reached, however, the meth-
ods of the scientist are not infrequently cast aside and an understanding
of disease is sought for largely by the empirical method ; namely, by the
endeavor to see and examine innumerable patients, to diagnose the case
according to the grouping of the signs and symptoms, and to treat it by
the prescribed methods of experience. So much has to be learned and so
much has to be seen during the clinical years, that the student gives little
thought to the natui'e of the functional disturbance which is responsible
for the symptoms; he fails to realize that after all, there is no essen-
tial difference between the condition brought about in his patient by
some pathologic lesion, and that which may be produced in the labora-
tory by experimental procedures, by drugs or by toxins. It must of
course be recognized that just as the science of medicine originated by
the grouping of symptoms into more or less characteristic diseases for
which the most favorable method of treatment had to be discovered by
experience, so must a certain part of the medical training be more or
less empirical but it should at the same time be realized that such a
method is only a means to an end, and that the real understanding of
disease can be acquired only when every abnormal condition is inter-
preted as a primary or secondary consequence of some perverted bodily
function, and when the training in observation and the inductive method
is carried from the laboratory into the clinic.
VI PREFACE
It is a constant experience of clinical instructors who would employ
scientific methods of instruction, that they find the students not only
indifferent to an analysis of their cases from the functional standpoint,
but also that they are too inadequately, prepared in fundamental phys-
iologic knowledge, to make the analysis possible. The student may
have a superficial acquaintance with the main facts of physiologic science
but have failed to acquire the enquiring habit of mind which will en-
able him, through reflection, comparison, and personal research, to ap-
ply the knowledge in practical, medicine and surgery.
For this lack of correlation between the laboratory and clinical stud-
ies, the clinical instructors are not alone responsible. The laboratory
courses are frequently given without any attempt being made to show
the student the bearing of the subject in the interpretation of disease,
or to train him so that in his later years he may be able to adapt the
methods of investigation which he learned in the laboratory, to the study
of morbid conditions. It is self-evident that (without any knowledge
of disease) the extent to which the student in the earlier years of the
course could be expected to appreciate the clinical significance of what
he learns in the laboratory is limited, but this should not deter the in-
structor from indicating whenever he can, the general application of
scientific knowledge in the interpretation of diseased conditions. But
the chief remedy of the evil undoubtedly lies partly in the continuance
of certain of the laboratory courses into the clinical years, and partly
in the study of medical literature in which the application of physiology
and biochemistry in the practice of medicine is emphasized.
Notwithstanding the sufficient number of excellent textbooks in phys-
iology available to the medical student, there is none in which partic-
ular emphasis is laid upon the application of the subject in the routine
practice of medicine. In the present volume the attempt is made to
meet such a want, by reviewing those portions of physiology and bio-
chemistry which experience has shown to be of especial value to the
clinical investigator. The work is not intended to be a substitute,
either for the regular textbooks in physiology, or for those in functional
pathology. It is supplementary to such volumes. It does not start like
the modern test in functional pathology, with a consideration of the
diseased condition, and then proceed to analyze the possible causes and
consequences of the disturbances of function which this exhibits; but
it deals with the present-day knowledge of human physiology in so far
as this can be used in a general way to advance the understanding of
disease. In a sense it is therefore an advanced text in physiology for
those about to enter upon their clinical instruction, and at the same
PREFACE Vll
time, a review for those of a maturer clinical experience who may desire
to seek the physiological interpretation of diseased conditions.
In attempting to fulfil these requirements, it has been deemed essen-
tial to go back to the fundamentals of the subject, and to explain as
simply as possible the physical and physicochemical principles upon
which so large a part of physiological knowledge depends. Physiology
may be considered as an application of the known laws and facts of
physics and chemistry to explain the functions of living matter, and it is
only after the extent to which this application can be made has been
appreciated, that the knowledge may be used to serve as the foundation
upon which a superstructure of clinical knowledge can be built.
In order that the volume might be maintained of reasonable size, it
has been necessary to select certain parts of the subject for particular
emphasis, the basis of selection being the degree to which our knowledge
clearly shows the value of the application of physiological methods both
of observation and of thought in the study of diseased conditions. This
has not been done to the extent of omitting the apparently less essential
parts, for these have been treated in sufficient detail to link the others
together so as to preserve a logical continuity, and show the bearing of
one field of knowledge on another. There are however certain parts
of the science, particularly the physiology of nerve and muscle, of the
special senses, and of reproduction, for which application in the general
fields of medicine and surgery is limited, and these parts have been
omitted entirely. It has been judged that this perhaps somewhat arbi-
trary selection is justified on the ground that the ordinary text in
physiology covers these subjects sufficiently, except for the specialist,
for whom on the other hand, no adequate review would have been pos-
sible within the limits of such a volume as this. With reference to bio-
chemistry, no attempt is made to review the properties or describe the
characteristic tests of the various chemical ingredients of the body tis-
sues and fluids. This is already sufficiently done in the textbooks on
biochemistry, and in the numerous manuals on clinical methods. Bio-
chemical knowledge is treated rather from the physiologist's stand-
point, as an integral part of his subject, particular attention, neverthe-
less, being paid to the far-reaching applications, of this latest depart-
ment of medical science, in the elucidation of many obscure problems
of clinical medicine, such as those of diabetes, nephritis, acidosis, goiter
and myxedema. To make the volume of value to those who may not
have had time or opportunity to familiarize themselves with the techni-
cal methods of the physiologist and biochemist as used in the modern
clinic, a certain amount of space is devoted to a brief description of the
methods that appear at present to be receiving most attention, and to
1)0 of irreatest value.
Vlll PREFACE
Finally, it should be mentioned that the principles of serum diagnosis
and therapy are omitted, since these belong to a highly specialized science
requiring an intensive training of its own.
In the hope that the volume may be instrumental in arousing sufficient
interest to stimulate a more intensive study of the various subjects
which it introduces, a brief bi-bliography is given at the end of each
section. The references selected are to papers that are more partic-
ularly knoAvn to the author; they are not necessarily the most impor-
tant publications on the subject, but are often chosen because of the
useful reviews of previous Avork contained in them, rather than because
of their own originality. Some of the papers, however, are referred to
as authority for statements of fact which may arouse in the reader a
desire to ponder for himself the evidence upon which these are based.
The references are usually divided into two groups, "monographs" and
"original papers," and it is only occasionally that specific reference is
made to the former in the context. The original papers, on the other
hand, are referred to by numbers. With the general field of the subject
so well covered by such excellent textbooks as Bayliss' "Principles of
General Physiology," Stewart's, HowelPs, Starling's, and Halliburton 's
"Human Physiologies," and Leonard Hill's "Recent and Further Ad-
vances in Physiology," the author has felt free to pick and choose from
the monographs and original papers, topics that are ordinarily passed
over cursorily in the textbook, and when this has been done, the refer-
ences are somewhat more extensive. Such is the case for example in
the chapters relating to the chemistry of respiration, to the metabolism
of carbohydrates and fats, to the problems of dietetics and growth, to the
physicochemical basis of neutrality regulation in the animal body, and to
the action of enzymes.
Acknowledgment is gratefully made for the assistance and advice
in the preparation of the book, particularly to Doctor R. G. Pearce, for
the contribution of several chapters, to which his name is attached, and
for which he is entirely responsible ; and to Doctor E. P. Carter, whose
criticisms, after patient perusal of the unfinished manuscript, were of
inestimable value in its final revision. Acknowledgment is also made
to Doctor R. W. Scott and Professor F. E. Lloyd, for valuable criticism
and advice, and to the former for a chapter on the "Clinical Applica-
tion of Electrocardiographs." To Miss Achsa Parker, M.A., the author
owes a great debt of gratitude for the thorough and painstaking way in
which she prepared the manuscript for the press, and for her never-
tiring endeavors to have the spelling and punctuation in conformity
with Webster's Dictionarj'. For assistance in the preparation of the
index thanks are due to Miss Marian Armour and Mrs. MacFarlanc,
PREFACE IX
and for permission to use certain of the figures and illustrations, to the
various authors and publishers who granted it. For the excellent man-
agement and careful execution of the presswork, the author wishes to
thank the publishers, whose courteous and friendly dealings have always
made the work easier.
J. J. R. MACLEOD.
University of Toronto,
Toronto, Canada.
CONTENTS
PART I
THE PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL
PROCESSES
CHAPTER I PAGE
GENERAL CONSIDERATIONS 1
The Laws of Solution, 3 ; Gas Laws, 3 ; Osmotic Pressure, 4 ; Biological Methods
for Measuring Osmotic Pressure, 6; Hemolysis, 7; Plasmolysis, 8.
CHAPTER II
OSMOTIC PRESSURE (CONT'D) 10
Measurement by Depression of Freezing Point, 10; The Role of Osmosis Dif-
fusion, and Allied Processes in Physiologic Mechanisms, 11.
CHAPTER III
ELECTRIC CONDUCTIVITY, DISSOCIATION, AND IONIZATION 16
Biological Applications, 19.
CHAPTER IV
THE PRINCIPLES INVOLVEI> IN THE DETERMINATION OF HYDROGEN-ION CONCENTRATION 22
Titrable Acidity and Alkalinity, 22; Actual Degree of Acidity or Alkalinity,
23; Mass Action, 23; Application to the Measurement of H-ion Concentration,
26; Application in Determining the Real Strength of Acids or Alkalies, 28.
CHAPTER V
THE PRINCIPLES INVOLVED IN THE MEASUREMENT OP HYDROGEN-ION' CONCENTRATION
(CONT'D) 29
The Electric Method, 29 ; The Indicator Method, 32.
CHAPTER VI
REGULATION OF NEUTRALITY IN THE ANIMAL BODY AND ACIDOSIS 36
Buffer Substances, 36 ; Theory of Acidosis, 38 ; Measurement of the Reserve
Alkalinity, 41 ; Titration Methods, 41 ; CO2-combining Power, 42 ; Indirect
Methods, 46.
CHAPTER VII
COLLOIDS 50
Characteristic Properties, 50; Characteristics of True Colloidal Solutions,
51; Tyndall Phenomenon, 51; Relative Indiffusibility, 51; Electric Proper-
ties, 55; Brownian Movement, 57; Osmotic Pressure, 57.
Xll CONTENTS
CHAPTER VIII PAGE
COLLOIDS (COXT'D) GO
Suspensoids and Emulsoids, GO; Gelatinization, 61; Imbibition, 62; Action of
Electrolytes on Colloids, 63; Proteins as Colloids, 63; Surface Tension, 64;
Adsorption, 65 ; Everyday Reactions Depending on Adsorption, 66 ; Conditions
Influencing or Influenced by Adsorption, 67; Physiologic Processes Depending
on Adsorption, 69.
FERMENTS, OR ENZYMES ' 71
The Nature of Enzyme Action, 72 ; Properties of Enzymes, 73 ; Reversibility
of Enzyme Action; 77; Specificity of Enzyme Action, 79; Peculiarities of
Enzymes, 80; Types of Enzyme, 81; Enzyme Preparations, 82; Conditions for
Enzymic Activity, 82
PART II
THE CIRCULATING FLUIDS
CHAPTER X
BLOOD: ITS GENERAL PROPERTIES (Bv R. G. PEARCE) v . . 85
Quantity of Blood in the Body, 85; Water Content, 86; Proteins, 87; Fer-
ments and Antiferments, 89.
CHAPTER XI
THE BLOOD CELLS (BY R. G. PEARCE) 91
Red Blood Corpuscles, or Erythrocytes, 91 ; Origin, 92 ; Rates of Regeneration,
93; Hemolysis, 95; Leucocytes, 96; Blood Platelets, 97.
CHAPTER XII
BLOOD CLOTTING 98
Visible Changes in the Blood During Clotting, 98; Methods of Retarding
Clotting, 99; Nature of the Clotting Process, 101; Influence of Calcium Salts,
103; Influence of Tissues, 104.
BLOOD CLOTTING (COXT'D) 106
Theories of Blood Clotting, 106; Intravascular Clotting, 107; Measurement of
the Clotting Time, 108; Blood Clotting in Various Physiologic Conditions, 110;
Blood Clotting in Disease, 111 ; Hemorrhagic Diseases, 112 ; Thrombus Forma-
tion, 113.
CHAPTER XIV
LYMPH FORMATION AND CIRCULATION 115
General Considerations, 135; Experimental Investigations, 118; Edema, 120.
CONTENTS xiii
PART III
CIRCULATION OF THE BLOOD
CHAPTER XV PAGE
BLOOD PRESSURE 122
The Mean Arterial Blood Pressure, 123; Mercury Manometer Tracings, 123;
Spring Manometer Tracings, 12(5; Clinical Measurements, 128.
CHAPTER XVI
THE FACTORS CONCERNED IN MAINTAINING THE BLOOD PRESSURE 134
Pumping Action of the Heart, 134; Peripheral Resistance, 134; Amount of
Blood in the Body, 135; Effects of Hemorrhage and Transfusion, 139; Viscos-
ity of the Blood, 140; Elasticity of Vessel "Walls, 142.
CHAPTER XVII
THE ACTION OF THE HEART 144
The Pumping Action of the Heart, 144 ; Intracardiac Pressure Curves, 146 ;
Comparison of the Curves, 148.
CHAPTER XVIII
THE PUMPING ACTION OF THE HEART (CONT'D) 151
Contour of the Intracardial Pressure Curves, 151 ; Ventricular Curve, 151 ;
Auricular Curve, 153; The Mechanism of Opening and Closing of the Valves,
154; The Heart Sounds, 157: Causes of Sounds, 157; Records of Sounds
(Electrophonograms) , 1 58.
CHAPTER XIX
THE NUTRITION OF THE HEART 161
B-lood Supply, 161; Perfusion of the Heart Outside the Body, 161; Resuscita-
tion'of the Heart in Situ, 164; Relationship of the Chemical Composition of the
Perfusion Fluid in Cold-blooded and Warm-blooded Hearts, 165.
CHAPTER XX
PHYSIOLOGY OF THE HEARTBEAT 170
Origin and Propagation of the Beat, 170; Myogenic Hypothesis, 171; Neuro-
genic Hypothesis, 172; The Pacemaker of the Heart and Heart-block, 174;
Physiologic Characteristics of Cardiac Muscle, 176.
CHAPTER XXI
PHYSIOLOGY OF THE HEARTBEAT (CONT'D) 182
Origin and Propagation of the Beat in the Mammalian Heart, 182 ; Conduct-
ing Tissue in the Mammalian Heart, 182; Site of Origin of Beat, 187.
CHAPTER XXII
PHYSIOLOGY OF THE HEARTBEAT (CONT'D) 101
Alode of Propagation of the Beat in the Auricles ami from the Auricles to the
Ventricles, 191 ; Spread of Beat in the Ventricle, 193 ; Fibrillation of the Ven-
tricles ami Auricles, 195.
XIV CONTENTS
CHAPTER XXIII PAGE
THE BLOODFLOW IN THE ARTERIES 198
The Pulses, 198; General Characteristics, 198; Rate of Transmission of Pulse
Waves, 198; Contour of the Pulse Curve, 200; Velocity Pulse, 200; Palpable
Pulse, 202; Analysis of the Curve, 202; The Dicrotic Wave, 203; Causes of
Disappearance of the Pulse in the Veins, 205.
CHAPTER XXIV
RATE OF MOVEMENT OF THE BLOOD IN THE BLOOD VESSELS 206
Velocity of Flow, 206; Mass Movement of the Blood, 208; The Visceral Blood-
flow in Man, 212; Work of the Heart, 212; Circulation Time, 213; Movement
of Blood in the Veins, 214.
CHAPTER XXV
THE CONTROL OF THE CIRCULATION 216
Nerve Control, 217; Vagus Control in the Cold-blooded and the Mammalian
Heart, 217; Tonic Vagus Action, 221; Afferent Vagus Impulses, 222; Mechan-
ism of Vagus, 224 ; Termination of the Vagus Fibers in the Heart, 22o ; Sym-
pathetic Control, 227.
CHAPTER XXVI
THE CONTROL OF THE CIRCULATION (CONT'D) . 229
Nerve Control of Peripheral Resistance, 229; Detection of Vasomotor Fibers
in Nerves, 231; Origin of Vasomotor Nerve Fibers, 232; Vasomotor Nerve
Centers, 235 ; Independent Tonicity of Blood Vessels, 236.
CHAPTER XXVII
THE CONTROL OF THE CIRCULATION (CONT'D) 237
Control of the Vasomotor Center, 237; Hormone Control, 237; Nerve Control,
238; Pressor and Depressor Impulses, 239; Reciprocal Innervation of Vascular
Areas, 243; Influence of Gravity on the Circulation, 244.
CHAPTER XXVIII
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA . . . 247
Circulation in the Brain, 247; Anatomical Peculiarities, 247; Physical Condi-
tions of Circulation, 249; Vasomotor Nerves, 252; Intracranial Pressure, 253;
Circulation through the Lungs, 253; Circulation through the Liver, 255; The
Coronary Circulation, 257.
CHAPTER XXIX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC METHODS 259
Electrocardiograms, 259; The Ventricular Complex, 262. •
CHAPTER XXX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC METHODS (CONT'D) 266
Electrocardiograms of the More Usual Forms of Cardiac Irregularities, 266;
Sinus Arrhythmia, 266 ; Sinus Bradycardia, 266 ; The Extrasystole, 266 ; Parox-
ysmal Tachycardia, 269; Auricular Fibrillation, 269; Auricular Flutter, 269;
Heart-block, 270.
CONTENTS XV
CHAPTER XXXI PAGE
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC METHODS (CONT'D) 273
Polysphygmograms, 273; Venous Pulse Tracings, 273; Simultaneous Arterial
Pulse Tracings, 276; Abnormal Pulses, 276.
CHAPTER XXXII
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC METHODS (CONT'D) 281
Measurement of the Mass Movement of the Blood, 281; The Normal Flow,
282; Clinical Conditions Which Affect the Blood flow, 283.
CHAPTER XXXIII
'SHOCK 287
Gravity Shock, 287; Hemorrhage Shock, 288; Anesthetic Shock, 288; Spinal
Shock, 288; Nervous Shock, 289; Surgical Shock, 289; Experimental In-
vestigation of Shock, 289; Treatment, 295; Cause of Secondary Symptoms,
295.
CHAPTER XXXIV
RESPIRATION 299
The Mechanics of Respiration, 299; Pressure and Amount of Air in the Lungs,
299; Respiratory Tracings, 303; The Intrapleural Pressure, 304; Influence
on Blood Pressure, 306.
CHAPTER XXXV
THE MECHANICS OF RESPIRATION (CONT'D) (BY R. G. PEARCE) 310
Variations in Dead Space, Residual Air and the Mid- and Vital Capacities in
, Various Physiologic and Pathologic Conditions, 310.
CHAPTER XXXVI
THE MECHANICS OF RESPIRATION (CONT'D) (BY R. G. PEARCE) 315
The Mechanism of the Changes in Capacity of the Thorax and Lungs, 315;
The Movements of the Ribs, 315; The Action of the Musculature of the Ribs,
319; The Action of the Diaphragm, 320; The Effects of the Respiratory Move-
ments on the Lungs, 325.
CHAPTER XXXVII
THE CONTROL OF RESPIRATION 327
The Respiratory Centers, 327; Reflex Control of the Respiratory Center, 331.
CHAPTER XXXVIII
THE CONTROL OF RESPIRATION (CONT'D) 335
Hormone Control of the Respiratory Center, 335; Tension of CO., and O2 in
Arterial Blood, 337; Tension of CO., and O2 in Alveolar Air, 339; Tension of
CO, in Venous Blood, 342.
XVI CONTEXTS
CHAPTER XXXIX PAGE
THE CONTROL OF RESPIRATION (CONT'D) (By R. G. PEAJICE) 344
Estimation of the Alveolar Gases, 344; Method for Normal Subjects, 345;
Clinical Method, 347.
CHAPTER XL
THE CONTROL OF RESPIRATION' (CONT'D) 349
The Nature of the Respiratory Hormone, 349 ; Relationship between CO, of
Inspired Air and Pulmonary Ventilation, 350; Possibility that CO., Specifically
Stimulates the Center, 352; Relationship among Acidosis, Alveolar CO, and
Respiratory Activity, 354.
CHAPTER XLI
THE CONTROL OF RESPIRATION (CONT'D) 356
The Constancy of the Alveolar CO, Tension under Normal Conditions) 256;
Sensitivity of the Center to Changes in the CO, Tension of the Alveolar Air,
357; Alveolar CO2 Tension during Breathing in a Confined Space, 357, in
Rarefied Air, 360, and in Apnea, 362.
CHAPTER XLII
THE CONTROL OF RESPIRATION (CONT'D) 366
The Effect of Muscular Exercise on the Respiration, 356.
CHAPTER XLIII
THE CONTROL OF RESPIRATION (CONT'D) 371
Periodic Breathing, 371; Types of Periodic Breathing, 371; Causes of Periodic
Breathing, 372.
CHAPTER XLIV
RESPIRATION BEYOND THE LUNGS 378
Transportation of Gases by the Blood, 379; Transportation of Oxygen, 379;
Dissociation Curve, 383; Difference between Curves of Blood and Hemoglobin
Solution, 383; Rate of Dissociation, 386; Dissociation Constant, 388.
CHAPTER XLV
RESPIRATION BEYOND THE LUNGS (CONT'D) 390
Means by Which the Blood Carries the Gases, 390; Oxygen Requirement of
the Tissues, 393; Mechanism by Which the Demands of the Tissues for Oxy-
gen Are Met, 397.
CHAPTER XL VI
THE PHYSIOLOGY OF BREATHING IN COMPRESSED AIR AND IN RAREFIED AIR . . . 399
Mountain Sickness, 399; Compressed Air Sickness (Caisson Disease), 402;
Practical Application in Treatment, 406.
CHAPTER XLVII
THE CIRCULATORY AND RESPIRATORY CHANGES ACCOMPANYING MUSCULAR EXERCISE 410
Mechanical Factor, 410; Nervous Factor, 412; Hormone Factor, 413.
CONTENTS XVii
CHAPTER XLVIII PAGE
GENERAL PHYSIOLOGY OF THE DIGESTIVE GLANDS 418
Microscopic Changes during Activity* 418; Mechanism of Secretion, 420; Other
Changes during Activity, 421 ; Control of Glandular Activity, 422 ; Nervous
Control, 423.
PHYSIOLOGY OF THE DIGESTIVE GLANDS (CONT'D) 425
Hormone Control, 425; Nervous Control of the Pancreas, 427.
CHAPTER L
PHYSIOLOGY OF THE DIGESTIVE GLANDS (CONT'D) 430
Normal Conditions of Secretion, 430 ; Normal Secretion of Saliva, 431 ; Secre-
tion of Gastric Juice, 432; The Intestinal Secretions, 441.
CHAPTER LI
THE MECHANISMS OF DIGESTION 444
Mastication, 444; Deglutition, 445; The Cardiac Sphincter, 448; Vomiting,
449.
CHAPTER LII
THE MECHANISMS OF DIGESTION (CONT'D) 451
Movements of the Stomach, 451 ; Character of the Movements, 451 ; Effect
of the Stomach Movements on the Food, 454; Emptying of the Stomach,
456 ; Control of the Pyloric Sphincter, 456 ; Rate of Emptying of the Stomach,
458; Influence of Pathologic Conditions on the Emptying, 450; Gastroenter-
ostomy, 461.
CHAPTER LIII
THE MECHANISMS OF DIGESTION (CONT'D) . 463
Movements of the Intestines, 463 ; Movements of the Small Intestine, 463 ;
Movements of the Large Intestine, 468; Effect of Clinical Conditions on the
Movements, 470.
CHAPTER LIV
HUNGER AND APPETITE 471
Hunger Contractions of Stomach, 471; Remote Effects of .Hunger Contrac-
tions, 474 ; Hunger during Starvation, 475 ; Control of the Hunger Mechanism,
476.
CHAPTER LV
BIOCHEMICAL PROCESSES OF DIGESTION 481
Digestion in the Stomach, 481; Functions of the Hydrochloric Acid, 482;
Amount and Source of the Acid, 482; Action of Pepsin, 485; Clotting of
Milk in the Stomach, 488.
XV111 CONTENTS
CHAPTER LVI PAGE
BIOCHEMICAL PROCESSES OF DIGESTION (CONT'D) 489
Digestion in the Intestines, 489 ; Pancreatic Digestion, 489 ; The Bile, 492 ;
Chemistry of Bile, 494.
CHAPTER LVII
BACTERIAL DIGESTION IN THE INTESTINE 499
Bacterial Digestion of Protein, 501; Botulism, 503.
PART VI
THE EXCRETION OF URINE
CHAPTER LVIII
THE EXCRETION OF URINE (BY R. G. PEARCE) 507
Structure of Kidney, 507; Mechanism of the Excretion of Urine, 510; Theories
of Renal Function, 511; Diuretics, 518; Albuminuria, 519; Influence of the
Nervous System on the Secretion of Urine, 519.
• CHAPTER LIX
THE AMOUNT, COMPOSITION AND CHARACTER OF TJIIE URINE (BY R. G. PEARCE) . 521
Amount, 522; Specific Gravity, 522; Depression of Freezing Point, 523; Re-
action, 524; Solid Constituents, 525.
PART VII
METABOLISM
CHAPTER LX
METABOLISM 534
Energy Balance, 535 ; Methods for Measuring Energy Output, 536 ; Normal
Values, 538 ; Influence of Age and Sex, 541 ; Influence of Diseases, 542 ;
The Material Balance of the Body, 543; Methods for Measuring Output, 543;
Calculation of the Results, 544.
CHAPTER LXI
THE CARBON BALANCE 547
Respiratory Quotient, 547; Influence of Diet, 547; Influence of Metabolism,
549; Magnitude of the Respiratory Exchange, 550; Influence of Body Tem-
perature, 551.
CHAPTER LXII
A CLINICAL METHOD FOR DETERMINING THE RESPIRATORY EXCHANGE IN MAN (BY
R. G. PEARCE) 554
The Valves, 555; Tissot Spirometers, 556; Douglas Bag, 558; Haldane Gas-
analysis Apparatus, 559; Calculations, 562.
CONTENTS XIX
CHAPTER LXIII PAGE
STARVATION* 566
Excretion of Nitrogen, 566 ; Energy Output, 568 ; Nitrogenous Metabolites, 568 ;
Excretion of Purines, 569; Excretion of Sulphur, 569; Normal Metabolism,
570; Nitrogenous Equilibrium, 571; Protein Sparers, 571.
CHAPTER LXIV
NUTRITION* AND GROWTH 574
The Food Factor of Growth, 574; Relationship of Proteins to Growth and
Maintenance of Life, 574.
CHAPTER LXV
NUTRITION AND GROWTH (CONT'D) 583
Relationship of Carbohydrates and Fats to Growth, 583; Accessory Food
Factors, or Vitamines, 584; Relationship of Inorganic Salts, 586.
CHAPTER LXVI
DIETETICS 588
Calorie Requirements, 588; The Protein Requirement, 590; Accessory Food
Factors, 593; Digestibility and Palatability, 593.
CHAPTER LXVII
THE METABOLISM OF PROTEIN 595
Introductory, 595; Chemistry of Protein and of the Amino Acids, 597.
CHAPTER LXVIII
THE METABOLISM OF PROTEJN (CONT'D) 606
Amino Acids in the Blood and Tissues, 606; Fate of the Amino Acids, 610.
CHAPTER LXIX
THE METABOLISM OF PROTEIN (CONT'D) 613
End Products of Protein Metabolism, 613; Urea and Ammonia, 615; In-
fluence of Acidosis on Ammonia-urea Ratio, 616 ; Influence of Liver on Am-
monia-urea Ratio, 617; Perfusion of Organs, 618; Clinical Observations, 620.
CHAPTER LXX
THE METABOLISM OF PROTEIN (CONT'D) 622
Creatine and Creatinine, 622; Essential Chemical Facts, 622; Metabolism,
624; Influence of Food, Age, and Sex, 624; Origin of Creatine and Creatinine,
626.
CHAPTER LXXI
THE METABOLISM OF PROTEIN (CONT'D) 629
Undetermined Nitrogen and Detoxication Compounds, 629; Ethereal Sulphates
and Glycuronates, 632.
CHAPTER LXXII
URIC ACID AND THE PURINE BODIES 634
Chemical Nature of the Purines, 634 ; Chemical Nature of the Substances
Containing Purine and Pyrimidine Bases, 637; History of Nucleic Acid in the
Animal Body, 638; Balance between Intake and Output o,f Purine Substances
under Various Physiologic and Pathologic Conditions, 641.
XX CONTENTS
CHAPTER LXXIII PACE
URIC ACID AND THE PURINE BODIES (CONT'D) 643
Source of Endogenous Purines, 643 ; Influence of Various Physiologic Con-
ditions, of Drugs, and of Disease on the Endogenous Uric-acid Excretion,
647; Uric Acid of Blood, 648.
CHAPTER LXXIV
METABOLISM OF THE CARBOHYDRATES 652
Capacity of the Body to Assimilate Carbohydrates, 652 ; Assimilation Limits,
652; Saturation Limits, 654; Digestion and Absorption, 656; Sugar Level in
the Blood, 657; Value of Blood Examinations in Diagnosis of Diabetes, 659;
Relationship Between Blood Sugar and the Occurrence of Glycosuria, 660.
CHAPTER LXXV
METABOLISM OP THE CARBOHYDRATES (CONT'D) 662
Fate of Absorbed Glucose, Gluconeogenesis, 662 ; Storage of Sugar, 662 ;
Sources of Glycogen, 662; Gluconeogenesis in Normal Animals, 667.
CHAPTER LXXVI
METABOLISM OF THE CARBOHYDRATES (CONT'D) 669
Fate of Glycogen, 669 ; Regulation of the Blood Sugar Level, 671 ; Nerve
Control and Experimental Diabetes, 672; Nervous Diabetes in Man, 674;
Hormone Control and Permanent Diabetes, 676; Utilization of Glucose in
Tissues, 677; Relation of the Pancreas to Sugar Metabolism, 678; Diabetes
and the Ductless Glands, 678; Diabetic Acidosis or Ketosis, 683; Starvation
Treatment, 684.
CHAPTER LXXVII
FAT METABOLISM 686
Chemistry of Fatty Substances, 686; Digestion of Fats, 690; Absorption of
Fats, 691.
CHAPTER LXXVIII
FAT METABOLISM (CONT'D) 696
Fat of Blood, 696; Methods of Determination, 696; Variations in Blood Fat,
697; Depot Fat, 700; Fat in the Liver, 701.
CHAPTER LXXIX
FAT METABOLISM (CONT'D) 707
Production of Fatty Acid Out of Carbohydrate, 707; Method by Which the
Fatty Acid is Broken Down, 709.
CHAPTER LXXX
CONTROL OF BODY TEMPERATURE AND FEVER 714
Variations in Body Temperature, 714; Factors in Maintaining the Body Tem-
perature, 715 ; Control of Temperature, 719 ; Fever, 721 ; Causes, 721 ; Changes
in the Body during .Fever, 723 ; Heat-icgulating Center, 725 ; Significance of
FevOr, 726.
CONTENTS XXI
PART VIII
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
CHAPTER LXXXI PAGE
THE • ENDOCRINE ORGANS, OR DUCTLESS GLANDS 729
Methods of Investigation; 730; Adrenal Gland, 731; Cortex, 731; Medulla,
732; AdrenalectOmy, 733; Suprarenal Extracts, 734; Physiologic Action, 734.
CHAPTER LXXXII
ADRENAL GLAND (CONT'D) 738
Variations in Physiologic Activity, 738; Assaying the Epinephiine Content
of the Gland, 738; Epinephrine Content of the Blood, 739; Autoinjection
Method, 743 ; Adrenalemia, 745 ; Association of the Adrenal with Other En-
docrine Organs, 746.
CHAPTER LXXXIII
THYROID AND PARATHYROID GLANDS -. . 749
Structural Relationship, 749; Thyroid Gland, 750; Condition of Gland, 750;
Experimental Thyroidectomy, 752; "Disease of the Thyroid, 753; Relation
with Other Endocrine Organs, 757; Parathyroids, 758; Experimental Parathy-
roidectomy, 758 ; Relationship with Other Endocrine Organs, 761. •
CHAPTER LXXXIV
PITUITARY BODY 762
Structural Relationships, 762; Functions, 764; Clinical Characteristics, 771;
Relationship, with Other Endocrine Organs, 773.
CHAPTER LXXXV
THE PINEAL GLAND AND THE GONADS 776
Pineal Gland, 776; Gonads or the Generative Organs, 776; Generative Glands
of the Male, 776; Generative Organs of the Female, 778.
PART IX
THE CENTRAL NERVOUS SYSTEM
CHAPTER LXXXVI
THE EVOLUTION OF THE NERVOUS SYSTEM 781
CHAPTER LXXXVII
PROPERTIES OF EACH PART OF THE REFLEX ARC 788
Receptor, 788; Epicritic and Protopathic Receptors, 790; Peculiarities of the
Separate Sensations, 791 ; Temperature, 791 ; Touch, 793 ; Pain, 795.
CHAPTER LXX XVIII
THE PROPERTIES OF EACH PART OF THE REFLEX ARC (CONT'D) 796
The Nerve Network, 796; Network on Skin Nerves, 796; The Synapsis, 797;
The Nerve Cell, 799; The Intermediate or Internuncial Neuron, 802.
XX11 CONTENTS
CHAPTER LXXXIX
REFLEXES OF THE SPINAL ANIMAL AND SPINAL SHOCK 803
Spinal Shock in Laboratory Animals, 803; Spinal Shock in Man, 806; Cause
of Spinal Shock, 807.
CHAPTER XC
PHYSIOLOGIC PROPERTIES OF THE SIMPLE REFLEX ARC 809
Latent Period, 809; Grading- of Intensity, 809: After-effect, 810; Summation,
810; Irreversibility of the Direction of Conduction, 810; Refractory Period,
811; Successive Degeneration, 813.
CHAPTER XCI
RECIPROCAL INNERVATION •. 814
Reciprocal Inhibition, 814; Action of Strychnine and Tetanus Toxin, 819.
CHAPTER XCII
INTERACTION AMONG REFLEXES . , 821
Integration of Allied Reflexes, 822; Integration of Antagonistic Reflexes,
824; Other Factors Which Determine Occupancy of Final Common Path, 824;
Irradiation, 826.
CHAPTER XCIII
THE TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 828
The Tendon Jerks, 828 ; Afferent Spinal Pathways, 830.
CHAPTER XCIV
EFFECTS OF EXPERIMENTAL LESIONS OF VARIOUS PARTS OF THE NERVOUS SYSTEM . 835
Anterior Roots, 835 ; Posterior Roots, 836 ; Spinal Cord, and Braiii Stem, 839 ;
, Medulla, 839; Corpora Quadrigemina, 840; Removal of the Cerebral Hemi-
spheres, 840.
CHAPTER XCV
CEREBRAL LOCALIZATION 843
Ablation of the Motor Centers, 843; Stimulation of the Motor Centers, 844;
Clinical Observations, 849.'
CHAPTER XCVI
CEREBRAL LOCALIZATION (CONT'D) 850
Sensory Centers, 850; Sense Centers, 851; Association Areas, 852.
CHAPTER XCVII
CONDITIONAL AND UNCONDITIONAL REFLEXES 856
CHAPTER XCVIII
HIGHER FUNCTIONS OF TIIE CEREBRUM IN MAN; APHASIA 860
Psychopathological Applications, 862.
CHAPTER XCIX
FUNCTIONS OF THE CEREBELLUM 865
Localization of Function, 867; Circumscribed Extirpation, 869; Clinical Ob-
servations, 870.
CONTENTS XX111
CHAPTER C
THE CEREBELLUM AND THE SEMICIRCULAR CANALS; FUNCTIONAL TESTS .... 873
Association between the Eye Movements and the Semicircular Canals, 875.
CHAPTER CI
THE AUTONOMIC NERVOUS SYSTEM 877
General Plan of Construction, 877; Thoracicolumbar Outflow, or Sympathetic
System Proper, 880 ; Bulbosacral Outflow, or the Parasympathetic System, 882 ;
Axon Reflexes, 883; Functions of Autonomic Nerves, 884; Afferent Fibers of
the Autonomic System, 885.
ILLUSTRATIONS
1. Diagram of osmometer ... ................. 5
2. Hematocrite ........................ 7
3. Plasmolysis in cells from Tradescantia discolor ...:.. ..... 9
4. Apparatus for measurement of the depression of freezing point of solution . 11
5. Diagram of conductivity cells ................. 18
6. Wheatstone Bridge for the measurement of electric resistance ..... 18
7. Diagram to show type of electrodes used in studying electromotive force . . 30
9. Chart of tints as used in eolorimetric measurement of H-ion concentration.
(Color Plate.) ...................... 34
8. Diagram of apparatus for the measurement of the H-ion concentration . . 31
10. Diagram of apparatus for saturating, blood and plasma with expired air . 43
11. Van Slyke's apparatus for measuring the CCycombining power of blood in
blood plasma ...................... 44
32. Ultramicroscope (slit type) for the examination of colloidal solutions . . 52
13. To show diffusion into gelatin of a crystalloid stain, and the noridiffusion
of a colloid stain .................... 53
14. Diagram from W. Ostwald showing the relative size of various particles and
colloidal dispersoids compared with a red blood corpuscle and an
anthrax bacillus . . . '. ................ 54
15. Capillary analysis of colloids ................. 56
16. Diagram to show structure of gels ................ 61
17. Diagram to illustrate surface tension .............. 64
18. Traube's stalagmometer .................... 65
19. Diagram of the graphic coagulometer ............... 109
20. Coagulometer ........................ 110
21. Mercury manometer and signal magnet, arranged for recording the mean ar-
terial blood pressure in a laboratory experiment ......... 124
22. The arterial blood pressure recorded with a mercury manometer (lower trac-
ing) along with a tracing of the respiratory movement of the thorax . 325
23. Hurthle's spring manometer .................. 126
24. Arterial pressure recorded by a spring manometer .......... 126
25. Diagram based on experiments on dogs to show the systolic, diastolic and
mean blood pressures at different parts of the circulatory system . . 127
26. Apparatus for measuring the arterial blood pressure in man ...... 129
27. Effect of cutting the vagus nerve on the arterial blood pressure ..... 135
28. Effect of stimulating the peripheral end of the right vagus on the arterial
blood pressure . . ................... 136
29. Effect of stimulation of the left splanchnic nerve on the arterial blood pres-
sure ......................... 137
30. The effect of rapid and slow hemorrhage on the arterial blood pressure . . 138
31. Diagram of experiment to show that the diastolic pressure depends on the
elasticity of the vessel wall ................ 1^3
32. Diagram of Wiggers' optical manometer ............. 146
XXVI ILLUSTRATIONS
FIG. PAGE.
33. Optical records of intraventricular pressure 147
34. Superimposed pressure curves after being graduated 149
35. Von Frank's maximal and minimal valve, which is placed in -the course of
the tube between heart and mercury manometer 152
36. Diagram to show the positions of the cardiac valves 155
37. Diagram showing the position of the cardiac chambers and valves during
presystole and during the sphymic period 156
38. Elcctrophonograms along with intraventricular pressure curves from three
different experiments 159
39. One form of apparatus for recording tracings from an excised heart . . 163
40. Volume curve of ventricles of cat (lower curve) in a heart-lung perfusion
preparation 169
41. Heart and cardiac nerves of Limulus polyphemus 173
42. Heart-block produced by applying clamp 175
43. Tracing of contraction of ventricle, showing the effect of the local appli-
cation of heat to the auricle . •. 175
44. Frog heart showing the position of the first and second ligatures of Stannius 176
45. Effects of stimuli of increasing strength on skeletal and cardiac muscle to
illustrate the "all or nothing" principle in the latter 177
46. The effects of successive stimuli on skeletal and cardiac muscle to show the
prominence of the staircase phenomenon, or treppe, in the latter . . 178
47. The effects, of successive stimuli and of tetanizing stimuli on skeletal muscle
and cardiac muscle 179
48. Myograms of frog's ventricle, showing effect of excitation by break induc-
tion shocks at various moments of the cardiac cycle 180
49. Heart of tortoise as suspended 183
50. Dissection of heart to show auriculoventricular bundle 184
51. Photograph of model of the auriculoventricular bundle and its ramifications,
constructed from dissections of the heart 184
52. Diagram of an auricle showing the arrangemoit of the muscle bands; the
concentration point; and the outline of the node 186
53. Diagram to show the general ramifications of the conducting tissue in the
heart of the mammal 186
54. Diagram to illustrate the development and spread of the wave of negativity
in a strip of muscle (curarized sartorius) when stimulated at the end . 188
55. Simultaneous electrocardiograms to show the cause for extrinsic deflections 190
56. Diagram of experiment by Lewis showing the times at which the excitation
wave appeared on the front of the heart 194
57. Diagram of Chauveau's dromograph 200
58. Diagram to show principle of Pitot's tubes for measuring velocity pulse . . 201
60. Dudgeon's sphygmograph 201
61. Pulse tracing (sphygmogram) taken by sphygmograph 202
62. Forms of apparatus for measurement of blood velocities 207
63. Plethysmograph for recording volume changes in the hand and forearm . 210
64. Simultaneous tracings from auricle and ventricle of turtle's heart . . . 218
65. Effect of vagus stimulation on heart of turtle 218
66. Tracing to show that vagus stimulation may diminish transmission from
auricles to ventricles 219
ILLUSTRATIONS XXV11
FIG. PAGE
67. Tracing to show that vagus stimulation may facilitate transmission from
auricles to ventricles 220
68. Diagram to show the innervation of the heart in the frog or turtle. (Color
Plate.) 224
69. Frog heart tracing showing the action of nicotine 226
70. Schematic representation of the innervation of the heart of the mammal.
(Color Plate.) 226
71. Tracings showing the effects on the heartbeat of the frog resulting from
stimulation of the sympathetic nerves prior to their union with the
vagus nerve 228
72. Roy 's kidney oncometer 230
73. Fall of blood pressure from excitation of the depressor nerve 239
74. The effect of strong stimulation (heat) of the skin of the foot on the ar-
terial blood pressure and respiratory movements 241
75. Diagram showing the probable arrangements of the vasomotor reflexes . 242
76. Aortic blood pressure, showing the effect of posture 245
77. Tracing to show the effect of gravity on the arterial blood pressure . . 245
78. The effect of gravity on the aortic pressure after division of the spinal
cord in the upper dorsal region 246
79. Schema to show the relations of the Pacchionian bodies to the sinuses . . 248
80. Tracing showing simultaneous records of the arterial blood pressure, the
venous pressure, the intracranial pressure, the pressure in the venous
sinuses . . 251
81. Electrocardiographic apparatus as made by the Cambridge Scientific Ma-
terials Co 260
82. Normal electrocardiogram 261
83. Electrocardiogram (dog) taken simultaneously with curves from auricle and
ventricle 262
84. Eecords of electrocardiogram and movement of ventricle of frog showing
that when the apex is warmed a typical T-wave appears in place of a
wave in the opposite direction appearing when the apex is cooled . . 264
85. Sinus bradycardia 267
86. Auricular extrasystole 267
87. Ventricular extrasystoles arising in the right ventricle 267
88. Ventricular extrasystole arising in the left ventricle 267
89. Paroxysmal tachycardia 268
90. Auricular fibrillation 268
9X. Auricular flutter 2-70
92. Delayed conduction 270
93. Partial dissociation 271
94. Complete dissociation 271
95. Polysphygmograph . . . . • 274
96. Normal jugular tracing 274
97. Eeduced tracings from carotid, aorta, ventricle, auricle and jugular, to show
the general relationships of the various waves 275
98. Polysphygmograms including jugular, apex and radial tracings .... 275
99. Delayed conduction time 277
100. Dropped beats 277
101. Premature beats (extrasystoles) ventricular in origin 278
XXV111 ILLUSTRATIONS
FIG. PAGE
102. Paroxysmal tachycardia 278
103. Auricular flutter .... 279
104. Auricular flutter 279
105. Auricular fibrillation 280
106. Showing the appearance of the blood vessels in the ears of a rabbit in
a state of deep shock. (Color Plate.) . 290
107; Diagram showing amounts of air contained by the lungs in various phases
of ordinary and of forced respiration 301
108. Pneumograph 303
109. Body plethysmograph for recording respiration 304
110. Effect of abdominal and chest breathing on the pulse and blood pressure
of man 308
111. First dorsal vertebra, sixth dorsal vertebra and rib. Axis of rotation shown
in each case 316
112. Lower half of the thorax from the 6th dorsal to the 4th vertebra, seen
from the front • . . . . 318
113. Intercostal muscles of 5th and 6th spaces 319
114. Hamberger's schema to demonstrate the functional antagonism of internal
and external intercostals • 319
115. Schema to demonstrate that the function of the internal intercar-
tilaginous intercostals is identical with that of the external in-
terosseous intercostals 320
116. Diagram to show the effect of high and low positions of the diaphragm
on the costal angle 322
117. Diagram to show the effect of clinical displacements of the diaphragm
on the costal angle 323
118. Diagram to show cuts required for isolation of the phrenic center . . . 328
119. Diagram to show certain positions in the medulla and upper cervical
cord, where sections may be made without seriously disturbing the
respirations 329
120. Diagram to show where cuts are made to isolate the chief respiratory
center from afferent impulses 330
121. Diagram showing principle for measurement of the tension of CO2 in blood 338
122. The gas analysis pipette for the microtonometer shown in Fig. 123 . . . 339
123. Microtonometer, to be inserted into a blood vessel 339
124. Apparatus for collection of a sample of alveolar air by Haldane 's method 340
125. Fridericia's apparatus for measuring the CO2 in alveolar air 341
126. Curves to show the relationship between the O2 and CO., tensions in alveolar
air and arterial blood 341
127. Same as Fig. 126, except that in this case the tension of CO2 in the
alveolar air was experimentally altered 342
128. Arrangement of meters and connections of Pearce's method for measure-
ment of CO3 of alveolar air in normal subjects 346
129. Curve showing the respiratory response to CO, in the deccrebratc cat . . 351
130. Tensions of O2 and CO2 in alveolar air at different altitudes 361
131. Curves showing variations in alveolar gas tensions after forced breath-
ing for 'two minutes 364
132. Various types of periodic breathing ; 372
ILLUSTRATIONS Xxix
PIG. PAGE
133. Quantitative record of breathing air through a tube 260 cm. long and
2 cm. in diameter 374
134. Barcroft's tonometer for determining the curve of absorption of oxygen
by hemoglobin or blood 381
135. Barcroft's differential blood gas manometer 381
130. Barcroft blood gas manometer 382
137. Typical dissociation curve. (Color Plate.) 382
338. Average dissociation curves 384
139. Dissociation curves of hemoglobin 385
140. Dissociation curves of human blood 386
141. Curves showing relative rates of oxidation and reduction of blood as
influenced by temperature and by tension of CO2 387
142. Curve of CO, tension in blood 392
143. Cells of parotid gland showing zymogen granules . 419
144. Parotid gland of rabbit in varying states of activity examined in fresh state 419
145. Diagrammatic representation of the innervation of the salivary glands
in the dog. (Color Plate.) 422
146. Pancreatic acini stained with hematoxylin 427
147. Three preparations of pancreatic acini stained by eosinorange toluidin blue 428
148. Diagram showing miniature stomach separated from the main stomach by
a double layer of mucous membrane 434
149. Typical curve of secretion of gastric juice collected in 5-minute intervals
on mastication of palatable food for 20 minutes 437
350. Cubic centimeters of gastric juice secreted after diets of meat, bread,
and milk 440
151. Digestive power of the juice, as measured by the length of the protein
column digested in Mett's tubes, with diets of flesh, bread, and milk . 441
152. Loop of intestine after tying off the portions, cutting the nerves running to
the middle portion and returning the loop to the abdomen for some time 442
153. The changes which take place in the position of the root of the tongue,
the soft palate, the "epiglottis and the larynx during the second
stage of swallowing 446
154. Schematic outline of the stomach 452
155. Diagrams of outline and position of stomach as indicated by skiagrams
taken on man in the erect position at intervals after swallowing food
impregnated with bismuth subnitrate 452
156. Outlines of the shadows cast by the stomach at intervals of an hour each
after feeding a eat with food impregnated with bismuth subnitrate . . 453
157. Section of the frozen stomach (rat) some time after feeding with food
given in three differently colored portions 455
158. Outlines of shadows in abdomen obtained by exposure to x-rays 2 hours
after feeding with food containing bismuth subnitrate 458
359. Curves to show the average aggregate length of the food masses in the
small intestine at the designated intervals after feeding 459
160. Apparatus for recording contractions of the intestine 464
161. Diagrammatic representation of the process of segmentation in the intestine 465
162. Intestinal contractions after excision of the abdominal ganglia and
section of both vagi 466
XXX ILLUSTRATIONS
FIG. PAGE
163. The effect of excitation of both splanchnic nerves on the intestinal
contractions . 467
164. The effect of stimulation of right vagus nerve on the intestinal
contractions 468
165. Diagram of time it takes for a capsule containing bismuth to reach the
various parts of the large intestine 469
166. Diagram of method for recording stomach movements 472
167. Tracing of the tonus rhythm of the stomach three hours after a meal . . 473
168. Tracings from the stomach during the culmination of a period of vigorous
gastric hunger contractions 473
169. Showing augmentation of the knee-jerk during the marked hunger con-
tractions 475
170. Diagram of the uriniferous tubules, the .'arteries., and the veins of
the kidney 508
171. Cross section of convoluted tubules from kidney of rat 509
172. Diagram of blood supply of Malpighian corpuscle and of convoluted
tubules in amphibian kidney 515
173. Nerve supply of the kidney 520
174. Respiration calorimeter of the Russell Sage Institute of Pathology,
Bellevue Hospital, New York ! 536
175. Chart for determining surface area of man in square meters from Aveight
in kilograms and height in centimeters according to formula . . . 540
176. Diagram of At water-Benedict respiration calorimeter 543
177. Nose clip, face mask, and mouthpiece 555
178. Diagram of respiratory valves 556
179. The Tissot spirometer 557
180. The Douglas bag method for determining the respiratory exchange . . 558
181. Haldane gas apparatus and Pearce sampling tube 559
182. Curve constructed from data obtained from a man who fasted for thirty-
one days 567
183. Curves of growth of rats on basal rations plus the various proteins indicated 576
184. Curves of growth of rats on basal rations plus the proteins indicated . . 577
185. Photographs of rats of same brood on various diets 579
186. Vividiffusion apparatus of .T. J. Abel 607
187. Curves showing the amount of amino nitrogen taken up by different tis-
sues after the cutaneous injection of amino acids 608
188. Curves showing the concentration of amino-acid nitrogen in the blood dur-
ing fasting and protein digestion 609
189. Curves showing the percentage of glucose in blood after a constant injec-
tion of an 18 per cent solution into a mesenteric vein 658
190. Arrangement of apparatus for recording contractions of a uterine strip,
intestinal strip, or ring, etc 740
191. Tracing showing the effect of epinephrine on the intestinal contractions
and on the arterial blood pressure 741
192. Arrangement of apparatus for perfusion of the vessels of .a brainless frog 742
193. Microphotographs of thyroid gland of dog 751
194. Cretin, nineteen years old 754
195. Case of myxedema before and after treatment 755
196. Drawing from a photograph of a mesial sagittal section through the pitui-
tary gland of a human fetus 763
ILLUSTRATIONS XXXI
FIG. PACK
197. Tracing showing the action of pituitrin on the uterine contractions and
blood pressure in a dog 768
198. Tracing showing the constricting action of pituitrin on the bronchioles and
its effect on blood pressure in a spinal dog 769
199. Showing the appearance before and after the onset of acromegalic symptoms 771
200. Hand of a person affected with acromegaly 772
201. Diagram showing gradual evolution of nervous system in sponge, sea
anemone, and earthworm 783
202. Diagram of nervous system of segmented invertebrate, supraesophageal
ganglion, subesophageal ganglion, esophagus or gullet 784
203 Schema of simple reflex arc 785
204. Thermoesthesiometer 791
205. Cold spots and heat spots of an area of skin of the right hand .... 792
206. Diagram to show axon reflex of sensory nerve fiber of skin 797
207. Arborization of collaterals from the posterior root fibers around the cells
of the posterior horn 798
208. Normal cell from the anterior horn, stained to show Nissl's granules . . 799
209. Part of an anterior cornual cell from the calf's spinal cord, stained to
show neurofibrils 800
210. Living nerve cells examined by the ultramicroscope 801
211. Tracing from the hind limb of a spinal dog during the scratching move-
ments produced by applying stimuli at two skin points 812
212. Eecord from myograph connected with the extensor muscle of the knee . 815
213. Diagram showing the muscles and nerves concerned in reciprocal inner-
vation 816
214. Eeciprocal innervation 817
215. Sherrington 's diagram illustrating the mechanism of reciprocal innervation 818
216. Diagram showing the reflex arcs involved in the scratch reflex .... 822
217. Showing region of body of dog from which the scratch reflex can be elicited 823
218. Diagram showing the segmental arrangement of the sensory nerves . . 837
219. Outer aspect of the brain of the chimpanzee 847
220. Three sections through different parts of the cerebral cortex 852
221. The location of the chief motor and sensory areas on the outer and mesial
aspects of the human brain 853
222. Footprints after destruction of the cerebellum in a dog 866
223. Diagrams to represent respectively a ventral view of the left half and a
dorsal view of the right half of the human cerebellum illustrating the
scheme of subdivision according to Bolk 868
224. Schema of the parts of the mammalian cerebellum spread out in one plane 869
225 and 226. The inferolateral and the posterior aspect of the human cerebellum
indicating certain cerebellar localizations according to Barany . . . 871
227. The semicircular canals of the ear, showing their arrangement in the
three planes of space 874
228. Diagram illustrating the different arrangements of the internuncial neurons
of the voluntary and involuntary nervous systems 878
229. Diagram of the sympathetic nervous system to be used along with Fig. 232.
(Color Plate.) 878
230. Diagram showing the manner of connection of the fibers composing the
great splanchnic nerve. (Color Plate.) 878
XXX11 ILLUSTRATIONS
FIG. PAGE
231. Diagram showing the manner in which a preganglionic fiber, emanating
from the spinal nerve by the white ramus communicans, connects in
a ganglion of the sympathetic chain with a nerve cell, the axon of
which then proceeds as the postganglionic fiber by way of the gray
ramus communicans back to the spinal nerve, along which it travels
to the periphery. (Color Plate.) 880
232. Diagram showing the main parts of the autonomic nervous system to be
used along with Fig. 229. (Color Plate.) . . 882
233. Schematic representation of the involuntary nervous system. (Color Plate.) 884
PHYSIOLOGY AND BIOCHEMISTRY
IN MODERN MEDICINE
PART I
THE PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL
PROCESSES
CHAPTER I
GENERAL CONSIDERATIONS
The work of the physiologist consists, in large part, in ascertaining to
what extent the known laws of physics and chemistry find application
in explaining the phenomena of life. He gathers from the vast store-
house of physical and chemical knowledge whatever is of value in the
interpretation of the various mechanisms that work together to com-
pose the living machine, and having added to this knowledge he passes
it on for use by those who are concerned in the study and treatment of
disease.
Many of the most important steps in the advance of physiologic
knowledge in recent years have depended upon the discovery of some
hitherto unknown physical or chemical law, or upon the elaboration of
some accurate method for the measurement of the phenomena upon
which these or previously known laws depend. The discoveries of
van't Hoff, Arrheiiius, and Ostwald of the so-called laws of solution
were soon followed by important observations on their relationship to
the movement of fluids and dissolved substances through cell mem-
branes; the discoveries of Hardy, Willard Gibbs, etc., of the behavior of
colloids and of the phenomena of surface tension found application in
explaining many hitherto inexplicable peculiarities in the activities of
ferments; the discovery by Nernst, etc., of methods for the measurement
of the electro-motive force of dissolved substances was applied to de-
termine the actual reaction or hydrogen-ion concentration of animal
1
PHYSICOCH^MICAL BASIS OF PHYSIOLOGICAL PROCESSES
fluids, and to explain the generation of the electric currents which ac-
company muscular, nervous, and glandular activity.
It would be out of place here to devote much space to a detailed ac-
count of such matters. They belong more properly in the domain of
general than in that of human physiology. General physiology is con-
cerned with the study of the essential nature of the vital processes;
whereas human physiology is merely a branch of the subject in which
special attention is devoted to the application of the truths of general
physiology to the working of the human machine. For the physician
and surgeon a knowledge of human physiology is as essential as is a
knowledge of the construction of a piece of machinery for the engineer
who attempts its repair, but obviously to acquire this knowledge the
fundamental principles of general physiology must first of all be under-
stood. For these reasons the introductory chapters are devoted to a
brief review of the most important of the physicochemical principles
upon which .the working of the cell depends.
From the viewpoint of the physical chemist the cell consists of an
envelope of more or less permeable material inclosing a dilute solution
of crystalline substances in which colloid matter is suspended. It con-
tains, in other words, a solution of crystalloids and colloids, in which
these are in a state of equilibrium with each other. This equilibrium is
readily altered by various influences that may act on the cell, and the
resulting changes manifest themselves outwardly by alterations in the
shape and volume of the cell — growth and motion; by the extrusion of
some of its contents — secretion; or by the propagation to other parts of
the cell, or its processes, of the state of disturbed equilibrium — nervous
impulse. Besides the activities that are dependent upon physicochem-
ical changes, purely chemical processes go on in the cell. Many of
these consist in the breakdown and oxidation of complex unstable organic
molecules, a process identical with that occurring in combustion outside
the cell. Others involve the building up, stage by stage, of complex
substances out of the elements or out of simpler molecules. Chemical
transformations occur in the cell which, in the chemical laboratory, re-
quire the most powerful reagents and physicochemical forces, either the
•strongest of acids, alkalies, oxidizing agents, etc., or extreme degrees
of heat, electrical energy, etc. But this is not all, for in the cell these
chemical transformations are capable of being guided to a very remark-
able degree of nicety so as to produce intermediate products that are
used for some special purpose either by the cell that produced them or,
after transportation by the blood, etc., by cells in other parts of the
organism.
It is customary to speak of the' cell as a chemical laboratory, but it
LAWS OF SOLUTION 3
is more than this; it is a laboratory furnished not only with the equip-
ment of the chemist but directed in the harmonious operation of its
many activities by a guiding hand which far surpasses anything known
to man. Chemical transformations that require for their accomplishment
the greatest skill proceed without apparent difficulty in the cell. To
what are these changes due? What is the nature qf the chemical rea-
gents and forces, and what is the directive influence that guides them
in their varied activities? To these, which are among the great ques-
tions of general physiology, the reply may be given that the reagents
are the ferments or enzymes, and that the directive influence operates
through the susceptibility of enzymic activities to changes in the envi-
ronment in which the enzymes are acting. In many cases these changes
can be explained on a physicochemical basis as dependent upon the
known laws of mass action or surface tension; in other cases they de-
pend on purely chemical changes in the cell contents, such as changes
in reaction or the accumulation of chemical substances that act like
poisons on the enzyme. But there are still others that appear to depend
on influences which as yet are quite unknown to the physical chemist,
such as the changes in cell activity that can be brought about by the
nerve impulse.
These preliminary remarks will serve to indicate the problems with
which we must first occupy our attention. They concern the physico-
chemical nature of saline solutions and ef colloids, and the general na-
ture of enzyme action. The knowledge which we acquire will be found
to be of value, not only because it will help us to understand the nature
of the workings of the normal healthy cell, but because, here and there,
it will indicate possible causes for derangement in cellular function and
suggest rational means by which we may attempt to rectify the fault.
THE PHYSICOCHEMICAL LAWS OF SOLUTION
The Gas Laws
Three fundamental principles of general chemistry serve as the basis
for an understanding of the nature of solutions. The first is that if
we take a quantity of any gas equal to its molecular weight in grams
(called a gram-molecule or for sake of brevity a mol), it will occupy ex-
actly 22.4 liters at standard temperature and pressure ; the second is
that, as we compress a gas, its pressure will increase in exactly the
same proportion as the volume diminishes (the volume of a gas is inversely
proportional to its pressure) ; the third is that all gases expand by 1/273
4 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
part of their volume at 0° C. for every degree C. that their temperature
is raised.*
The pressure of a gas is measured by connecting a pressure gauge or
manometer with the vessel which contains the gas. Now, it is plain
that if the 22.4 liters, which is the volume occupied by a gram-molecular
quantity, were compressed so as to occupy a volume of 1 liter, its pressure
would be 22.4 times that of 1 atmosphere, or 22.4 x 760 mm. Hg — the
temperature remaining constant. Under these conditions we must im-
agine that the molecules of gas are crowded together by the compression,
and if we further conceive of these molecules as being in constant mo-
tion, then we can understand why the pressure should increase just in
proportion as we confine the space in which they can move.
One other property of gases must be borne in mind — namely, their
tendency to diffuse from places where the pressure is high to places
where it is low until the pressure is the same throughout.
OSMOTIC PRESSURE
These fundamental facts regarding the behavior of gases suggested
to van't Hoff the hypothesis that molecules of dissolved substances must
behave in a similar manner to those of gases. To put this hypothesis to
the test, it is necessary that we have some method for measuring the
pressure of dissolved molecules. We can not, as in the case of a gas,
use an ordinary manometer, for this would measure only the pressure
of the solvent on the walls of its container and would tell us nothing of
the pressure of the dissolved molecules. We must use some filter or
membrane that will allow the molecules of the solvent but not those of
the dissolved substance to pass through it. It is evident that if such a
filter is placed, for example, between a solution of sugar in water and
water alone, the molecules of the latter will diffuse into the solution
until this has become so diluted that the pressure of the dissolved mol-
ecules is equal on both sides of the membrane. Such a membrane is
called semipermeable ; the diffusion of molecules through it is called
osmosis, and the pressure which is generated, the osmotic pressure. If
we prevent the water molecules from actually diffusing by opposing
a pressure which is equal to that with which they tend to diffuse through
the membrane, we can tell the magnitude of the osmotic pressure (Fig. 1).
In applying these facts to test the hypothesis that molecules in solution
*This implies that at -273° C. the gas would occupy no volume. Before this temperature is
reached, however, the liquefaction of the gas sets in. The temperature -273° C. is known as absolute
zero. An observed temperature phis 273° is called the absolute temperature. Another way of stat-
ing the above law is therefore that the volume is directly proportional to the absolute temperature.
At 273° C. the volume of a gas at 0° C. would be doubled, or if expansion were prevented the
pressure would be doubled.
LAWS OF SOLUTION
obey the same laws as those in gaseous form, we must employ a semi-
permeable membrane which is rigid enough to withstand the pressure
and which forms part of the walls of a closed vessel connected with a
manometer. If we place in such an osmometer a solution containing the
molecular weight in grams of some substance dissolved in one liter of
solvent, a so-called gram-molecular solution, it is obvious that, if the
gas laws are to apply, the osmotic pressure should equal that of 22.4
liters of a gas compressed to the volume of one liter; in other words,
it should equal 22.4 x 760 mm. Hg. Although there are very consider-
able technical difficulties in making a semipermeable membrane that is
strong enough to withstand such a pressure, yet this has been accom-
! M
w
Fig. 1. — Diagram of osmometer. The cylindrical vessel (O), with a bottom of unglazed
clay, the pores of which are filled with a precipitate of copper fe,rrocyanide to form a semi-
permeable membrane, is suspended in an outer vessel, and is closed above by a tightly fitting
stopper pierced by a tube leading to a manometer (.M). O contains a strong solution of cane
sugar, and W contains water. The water molecules tend to pass through the semipermeable
membrane into the cane sugar solution, and since the cane sugar molecules can not pass in
the opposite direction, the pressure in O rises and is recorded in M. This equals the osmotic
pressure.
plished, and the fundamental principle has therefore been firmly estab-
lished that substances in solution obey the same laws as gases.
Further proof that the gas laws apply to solutions has been secured by
showing that the osmotic pressure (of a dilute solution) is directly pro-
portional to the concentration of the dissolved substance (the solute)
and to the absolute temperature. It also obeys the law of partial pres-
sures, which states that the total pressure exerted by a mixture (of gases
or dissolved molecules) is the sum of the pressures which each constit-
uent of the mixture would exert were it alone present in the space
occupied by the mixture.
6 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
Since the osmotic pressure is analogous to the pressure of a gas and
is therefore proportional to the molecular concentration (i. e., number
of molecules in unit space), it follows that a semipermeable membrane
can be used to determine the relative concentration of two solutions of
the same substance. When a watery solution of some substance is
placed in an osmometer that is surrounded by a similar but more dilute
solution, water molecules will diffuse into the osmometer until the pres-
sure is equal on the two sides of the semipermeable membrane; that is,
the water will pass from the solution having a lower osmotic pressure
into the solution having the higher pressure. When two solutions have
the same osmotic pressure, they are said to be isotonic; when that of one
is greater than that of the other, it is kypertonic; and when less, hypotonic.
Biological Methods for Measuring Osmotic Pressure
A practical biological application of these principles can very readily
be made if, instead of a rigid semipermeable membrane such as that
figured in the diagram, we employ one that is extensible and takes the
form of a closed sac ; then as diffusion of water occurs the sac will either
distend when it contains a stronger solution than that outside, or shrivel
or crenate when the reverse conditions obtain. Many animal and veg-
etable protoplasmic membranes are semipermeable, including the en-
velope of red blood corpuscles. Thus, if we examine blood corpuscles
under the microscope and add to them a saline solution of higher os-
motic pressure than blood serum, they will visibly diminish in size and
become irregular in shape; whereas if the solution is of lower osmotic
pressure, they will distend. If no change occurs, the osmotic pressure of
the cell contents must equal that of the saline solution in which the cells
are immersed, from which it is clear that we can readily determine the
magnitude of the osmotic pressure if we know the strength of the
saline solution.
Instead of measuring the individual cells under the microscope, we can
measure the space they occupy in the fluid in which they are suspended.
For this purpose a portion of the suspension is placed in a graduated
tube of narrow bore, which is rotated in a horizontal position by a cen-
trifuge after being closed at one end. The graduation at which the
upper edge of the column of cells stands after centrifuging is a measure
of the relative amount of cells and fluid in the suspension. Having
found this value for cells suspended in an isotonic solution, as for blood
corpuscles in blood serum, we may then proceed to ascertain it for the
same cells suspended in an unknown solution; if we find that the cells
occupy a greater volume, the saline solution must have an osmotic pres-
LAWS OF SOLUTION 7
sure that is lower than that of serum in approximate proportion to the
readings on the tube in the two cases, and vice versa.
The above apparatus, called a hematocrite (Fig. 2) has been very ex-
tensively used in the collection of data concerning the relative osmotic
pressures of different physiologic fluids.
Hemolysis
Another way for determining the relative osmotic pressure of dif-
ferent solutions consists in placing equal amounts (a few drops) of
blood in a series of test tubes containing solutions of different strengths,
and after allowing the tubes to stand for some time, noting in which of
them laking of the blood corpuscles occurs. In solutions which are
isotonic or hypertonic with the contents of the corpuscles, the latter
will settle to the bottom of the tube and the supernatant fluid will be
untinted with hemoglobin, but in solutions which are distinctly hypotonic,
the sediment will be less distinct and the supernatant fluid red.
Fig. 2. — Hematocrite. The graduated glass tubes are filled with the two specimens of
blood, or corpuscular suspension, and then rotated rapidly by a centrifuge. The relative heights
at which the corpuscular sediment stands in the two tubes is proportional to the osmotic
pressures of the fluid in which the corpuscles are suspended.
By noting (1) the lowest concentration (percentage composition) of
the solutions in which the corpuscles sink to the bottom and leave the
supernatant fluid colorless, and (2) the highest concentration in which
the corpuscles when they settle leave the supernatant fluid red, we can
determine the limiting concentrations for solutions of different sub-
stances. Thus, with bullock's blood the following results were obtained
(Hamburger) :
SUBSTANCE PERCENTAGE STRENGTH OF SOLUTION IN WHICH:
I II
SUPERNATANT FLUID SUPERNATANT FLUID
WAS COLORLESS WAS R^D
KN03
1.04
0.96
Nad
0.60
0.56
K2S04
1.16
1.06
C12H22O,
„ (Cane sugar)
6.29
5.63
CH,COOH (Pot. acetate)
1.07
1.00
MgS04.
7H2O
3.52
3.26
CaCL,
0.85
0.79
8 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
The mean of these limiting concentrations is the critical concentration
and indicates the strength of each solution that can be added to blood
without causing any damage to the corpuscles. This critical concen-
tration is not, as might at first sight be imagined, the same as that
which is isotonic with the contents of the corpuscles, but distinctly
below it. The reason for this becomes apparent if we observe the be-
havior of corpuscles suspended in an isotonic solution which is then
gradually diluted. As dilution proceeds, the corpuscles distend, until at
last their envelopes burst and the hemoglobin is discharged. The lim-
iting concentrations of a given salt vary for different corpuscles; thus,
the concentration of sodium chloride solution that just causes laking of
frog's blood corpuscles is 0.21 per cent, of human blood 0.47 per cent,
and of horse blood 0.68 per cent. It is the strength of the corpuscular
envelope rather than variations in the osmotic pressure of the contents
that is responsible for these differences.
The above described method of hemolysis, as it is called, can not be
used for comparisons of osmotic pressure in cases in which the solution
contains substances which alter the permeability of the corpuscular
envelop ; for example, it can not be used when urea, or ammonium
salts, or certain toxic bodies are present. This very fact is, however,
put to a useful purpose in ascertaining whether a given substance does
have a damaging influence on the corpuscular envelope by finding whether
hemolysis occurs when we suspend the corpuscles in a solution that is
isotonic with the corpuscular contents. We can further determine the
degree of this toxic influence by estimating by color comparisons
(colorimetry) the amount of hemoglobin that has diffused out of the
corpuscles.
Plasmolysis
An analogous method for determining osmotic pressure is that of
plasmolysis, in which the behavior of certain plant cells is observed
microscopically while they are in contact with solutions of different
strengths. When the surrounding solution is isotonic with the cell
contents, the latter fill the cell and extend up to the more or less rigid
cell wall (A in Fig. 3) ; but when the solution is hypotonic, the cell
contents become detached from the cell wall at one or more places —
plasmolysis (B and C). The semipermeable membrane in this case is
therefore not the cell wall but the layer of protoplasm on the surface
of the cell contents. The method can be used only for detecting solu-
tions that are hypertonic, for with those that are hypotonic the cells
merely become turgid and exert more pressure on the more or less
rigid cell wall. Many of the conclusions that have been drawn from
LAWS OF SOLUTION
results obtained by the plasmolytic method have recently been called in
question, because no regard has been taken of the power of the colloids
of the cell to adsorb (imbibe) water (see page 62).
The methods of hemolysis and plasmolysis have been used for the
investigation of many problems in medicine. In the case of certain
toxic fluids, such as snake venom, tetanus toxin, etc., determination of
the hemolytic power has proved of value in roughly assaying the dam-
aging influence on other cells than blood corpuscles. Studies in hemol-
ysis have also been especially valuable in working out the mechanism
by which cellular toxins in general develop their action, and the conditions
under Avhich this action may be counteracted, as by the development of
Fig. 3. — To show plasmolysis in cells from
plasmolysis in 0.22 M. cane 'sugar; C, pronounc
wall; p, the protoplasm. (After De Vries.)
cells from Tradescantia discolor. A. norma
discolor. A, normal cell; B,
:cd plasmolysis in 1.0 M. KNO3; h, the cell
antibodies. Furthermore, any solution that is to be injected into the
animal body, either intravenously or subcutaneously, should first of all
be tested by the above methods in order to find out whether it is isotonic
with the body fluids. If a hypertonic solution is injected, it will result
in the abstraction of water from the tissue cells, whereas a hypotonic
solution will cause the water content of these to increase. Advantage
has recently been taken of this water-abstracting effect of hypertonic
solutions in the treatment of wounds. By constantly bathing them with
strong saline solutions, an outflow of water is 'set up from the tissue
cells that border on the wound, and this tends to bring to the focus of
infection the defensive substances that are present in animal fluids.
CHAPTER II
OSMOTIC PRESSURE (Cont'd)
Measurement by Depression of Freezing Point
The limitations in the use of the plasmolytic and hemolytic methods
in the precise measurement of the osmotic pressure of the body fluids
have rendered it necessary to find some physical method that will be
generally applicable. Because of technical difficulties, it is impracticable
to measure the pressure directly by employing an osmometer, so that
some indirect method, depending on a readily measurable physical prop-
erty which varies in proportion to the osmotic pressure of the dissolved
substances, must be used. Fortunately, one such exists in the property
which dissolved substances have in lowering the temperature at which
the pure solvent solidifies; the freezing point of pure water, for example,
is lowered when substances are dissolved in it, and the extent of this
lowering, with certain reservations which will be explained later (page
16), is proportional to the molecular concentration of the solution and
independent of the chemical nature of the substance dissolved. This
lowering of temperature is designated by the Greek letter A, and to
measure it a thermometer is used which is not only extremely sensitive
but in which the level of the mercury column can be adjusted so that it
stands at a convenient level on the scale corresponding to the freezing
point of whatever solvent was used in making the solution under investi-
gation (Beckmann's thermometer) (Fig. 4). The exact position on
the scale of this thermometer at which the pure solvent freezes having
been ascertained, the observation is repeated with the solution whose
osmotic pressure is to be determined.
A gram-molecular solution in water (having therefore an osmotic pres-
sure of 170,240 mm. Hg) has a freezing point that is 1.86° C. lower than
that of pure water. This is known as the "freezing point constant,"
and it varies for different solvents, being 3.9 for acetic acid and 4.9
for benzene. If an unknown watery solution is found to have a freez-
ing point that is A° C. lower than that of water, its osmotic pressure
, Ax 17.024 _
will equal — — ^ mm, Hg,
l.OQ
10
OSMOTIC PRESSURE
11
The depression of the freezing points produced by the various body
fluids has been compared, the objects in view being to see whether
osmotic pressure is a property which changes under different physiologic
and pathologic conditions, and to find out by comparison of the osmotic
pressures of the fluids in contact with a membrane, whether physical
forces alone can be held responsible for the transference of substances
through it from one fluid to the other.
The Role of Osmosis, Diffusion, and Allied Processes in Physiologic
Mechanisms
An account of some of the investigations in which the foregoing
methods have been used will illustrate their value in revealing the
Fig. 4. — Apparatus for measurement of the depression of freezing point of solutions. The
solution is placed in the large test tube with the side arm, and in it is suspended the bulb
of a Beckmann thermometer with a platinum loop to serve for stirring. The upper end^of
the mercury column of the thermometer is shown magnified at the upper left corner. The
amount of mercury in the thermometer tube can be regulated by tapping the upper end with
the thermometer in various positions. The test tube is protected by an outer tube, which is
then placed in a vessel containing a freezing mixture.
mechanism involved in the transference of water and dissolved sub-
stances through cell membranes, as occurs in absorption of food in the
intestine, in the formation of lymph and urine, and so forth. In em-
ploying physical methods in the elucidation of such problems, it is
always most necessary to proceed with great care, since the physical
12 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
chemist works with pure solutions, while the physiologist has to use
fluids that are always complicated and frequently very variable in com-
position. We must simplify the problem as far as possible by having
clearly before us the exact nature of the biological problem which a com-
parison of physicochemical values, such as osmotic pressure, may ena-
ble us to elucidate, and we must consider the other physical forces
which may assist or modify the particular one we are investigating.
In the physical experiments described above, the s-emipermeable mem-
brane may be conceived of as composed of pores of such a size that
they permit only the smallest of molecules — those of water — to pass
through them. Semipermeable membranes with larger pores may, how-
ever, exist — that is, membranes which permit water molecules and mole-
cules of simple chemical substances to pass, but hold back those com-
posed of large complex molecules. Such a semipermeable membrane
would allow -the saline constituents but not the proteins of blood serum
to pass. It is, however, no longer semipermeable towards all of the dis-
solved substances, and the process of diffusion through it is more gener-
ally designated as one of dialysis than of osmosis.
Since the passage of dissolved molecules through membranes de-
pends upon the principle of diffusion, its rate will be proportional to
the osmotic pressures of the solutions on the two surfaces of the mem-
brane and to the size of the molecules, small molecules diffusing more
quickly than large . ones. Suppose a membrane permeable to sodium
chloride and water is placed between two fluids containing sodium
chloride in solution, but in greater concentration in one of them than
in the other: the sodium chloride will diffuse from the stronger to the
weaker solution, and water will diffuse still more quickly (because its
molecules are smaller) in the opposite direction, until the number of
sodium-chloride molecules in a given volume of solution is equal on
both sides of the membrane. For a time, therefore, the volume of the
stronger solution will increase. The differences which exist in the dif-
fusibility of dissolved molecules are analogous to those which have
long been known to exist in the diffusibility of gases, but the relation
between rate of diffusibility and molecular weight is not so simple as
the ratio between these two quantities in gases. These relationships,
however, indicate several further possibilities in the explanation of the
mechanism of exchange of substances through membranes, and must not
be overlooked, as they often are, in the interpretation of physiologic
phenomena. An excellent review of the possible conditions is given
by Starling in his " Human Physiology."4 For example, let us suppose
the substances on the two sides of a semipermeable membrane, such
as the peritoneal, to be different in diffusibility, as cane sugar,
OSMOTIC PRESSURE 13
which does not readily diffuse, and sodium chloride, which diffuses
quickly; the osmotic flow will take place from the sodium-chlorid solu-
tion to the cane sugar even when the sodium-chloride solution is stronger
than the sugar. In such a case, water molecules will pass from the fluid
having the higher osmotic pressure (NaCl) toward a fluid in which
this is lower (sugar).
Furthermore, the simple laws of osmosis may be upset by an attrac-
tive influence of the membrane toward certain substances [due to their
becoming dissolved or adsorbed in it (see page 65)] but not toward
others. Many membranes of this nature are known to the chemist
(e. g., rubber membranes in contact with gases, pyridine solutions, etc.),
and it is probable that such a property of selective solubility may play
a not unimportant role in the transference of substances across animal
membranes (Kahlenberg5).
These few conditions which may modify the direction of the osmotic
flow, are indicated here to show how involved such problems are, and
how careful we must be not to assume that, because a substance is trans-
ferred through a living membrane contrary to the simpler laws of os-
mosia and diffusion, it must involve the expenditure of forces different
from those operating in dead membranes.
Another force comes into operation under certain conditions — namely,
that of filtration. This is a purely mechanical process, in Avhich mole-
cules are forced through the pores of a filter (i. e., membrane) by dif-
ferences in pressure on its two sides.
We are now in a position to consider in how far the above physical
forces explain certain physiologic problems.
1. Is the absorption, into the blood and lymph circulating in the intes-
tinal walls, of substances in solution in the intestinal contents entirely
dependent upon the processes of filtration, diffusion and osmosis? The
absorption of weak solutions of highly diffusible substances is probably
very largely a matter of osmosis and diffusion, and water passes quickly
into the blood because of osmotic attraction, but that other forces ordi-
narily come into play is very clearly established by the following ob-
servations. If a piece of intestine is isolated from the rest by placing
two ligatures on it, and the isolated loop filled either with a solution con-
taining the same saline constituents in similar proportions as in blood
serum, or better still, with some of the same animal's blood serum, it
will be found after some time that all of the solution becomes absorbed
into the blood ; the contents of the loop are therefore absorbed into the
blood, even though the osmotic pressures of the dissolved substances are
the same on both sides of the membrane (Weymouth Reid6).
The intestinal membrane seems to possess towards readily diffusible
14 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
substances a permeability which varies, not at all with the physical
diffusibility of the substance, but with its value from a physiologic
standpoint. Thus, sodium sulphate and sodium chloride diffuse through
ordinary membranes with about equal facility, and yet if a solution con-
taining these two salts is placed in the intestine, the chloride will be
absorbed into the blood much more quickly than the sulphate. Sodium
sulphate in watery solution diffuses through a membrane fifteen times
more quickly than cane sugar, but from the intestinal lumen, cane
sugar is absorbed ten times more quickly than sodium sulphate. If.
however, the vitality of the epithelium is destroyed, as by first of all
bathing it with a solution of sodium fluoride, then the sulphate and
chloride will be absorbed at an equal rate.
Although diffusion and osmosis can not therefore play any significant
role in the normal process of absorption from the intestine, we must
not entirely discount them; under certain circumstances, these physical
forces may assert their influence as, for example, when concentrated
saline solutions are present. Such solutions will attract water from the
blood, and, other things being equal, more will be attracted the less
permeable the epithelium happens to be towards the saline employed.
Sulphates and phosphates will attract more water than chlorides or
acetates. This property of the saline solutions to attract water coun-
teracts the natural tendency for the water to be absorbed, and the
large volume of fluid stimulates peristalsis.
2. Do the physical processes of filtration, diffusion and osmosis suf-
fice to account for the production of urine ~by the kidneys? Under normal
conditions the molecular concentration of the urine, as determined by
the depression of freezing point, is considerably greater than that of
the blood. This indicates that excretion must have occurred contrary
to the laws of osmosis; in other words, that the renal cells must have
compelled dissolved molecules to be transferred from the blood to the
urine, although the difference in osmotic pressure would cause them to
pass in the opposite direction. This force, sometimes called for want
of a better name "vital activity," must depend on the operation of
processes that are quite distinct from those of diffusion, etc.; but that
they are necessarily of a nonphysical nature (e, g., vital) is less probable
than that they depend on some physical process the nature of which our
present knoAvledge does not permit us to understand.
By comparing the osmotic pressures of urine and blood, attempts
have been made to measure the work done by the kidney in the produc-
tion of urine. Thus, it has been found that A for normal urine (human)
is about 1.8, and for blood about 0.6, from which it may be calculated
that in the production of 1 kilogram of urine 150 kilogrammeters of
OSMOTIC PRESSURE 15
work are expended.* But that such comparisons of the osmotic pres-
sure of blood and urine are fallacious as an indication of the work of
the kidney is evidenced, not alone by the results of the above calcula-
tions, but also by the fact that under certain circumstances (as after
copious diuresis) the osmotic pressure of the urine may be considerably
lower than that of the blood. That opposite relationships should exist
indicates that differences in osmotic pressure between blood and urine
can signify little if anything regarding the work done by the kidney.
For some time after the application of osmotic pressure measurements
to the study of biological problems, it was thought that determination
of A in urine might be of clinical value as a criterion of renal efficiency,
especially in one kidney as compared with the other. For this purpose
A was determined in samples of urine removed from each ureter by
catheterization. The tests of renal efficiency based on the rate of excre-
tion of potassium iodide, phenolphthalein, etc., have however been found
of much greater value.
3. Is the formation of lymph purely a physical process? The osmotic
pressure of normal lymph is nearly always somewhat below that of
blood serum, although occasionally it has been found to be a trifle
higher. Physical processes, such as filtration, might therefore suffice
to account for its formation under most conditions. But when we con-
sider the excessive production of lymph that occurs 'as a result of cel-
lular activity or following the injection of certain substances, called
"lymphagogues," it is not so easy to explain the .production in such
terms, although some interesting attempts have been made to do so by
those that are wedded to the mechanistic view. For example, the very
marked increase in lymph flow which occurs as a result of muscular
exercise or glandular activity has been attributed to the fact that dur-
ing such processes large molecules become broken down into small ones
in the cell protoplasm, so that the osmotic pressure is raised and water
is attracted into the the cell until the latter becomes distended and a
process of filtration into the neighboring lymph spaces occurs (see
page 119).
There are several other physiologic processes of secretion and excre-
tion which might be considered in the present relationship, but the above
instances will suffice to illustrate the general principle upon which all of
them have to be considered.
"Osmotic pressure corresponding to A = -0.6° C. equals 5,662 mm. Hg (75 m. of H2O), and
that corresponding to A = -1-8° C. equals 16,986 mm. Hg (225 m. H2O). The difference is there-
fore equal to a column of water 150 m. high. According to these calculations it would appear that
the kidney in producing the average daily output of 1500 c.c. urine performs 225 kilogrammeters of
work in comparison with the 14,000 kilogrammeters which the heart is computed to perform in the
same time (page 212).
CHAPTER III
ELECTEIC CONDUCTIVITY, DISSOCIATION, AND IONIZATION
The osmotic pressure is not infrequently found to be considerably
greater than that expected from the strength of the solution. Although
A of a gram-molecular watery solution of cane sugar (342 gm. to the liter)
is 1.86 (see page 10), that of sodium chloride (58.5 gm. to the liter) is
considerably greater. If the hypothesis regarding the relationship of
molecular concentration to osmotic pressure is to hold good, it becomes
necessary to explain this apparent inconsistency; one must account for
a greater number of dissolved units than is represented by the actual
number of dissolved molecules (i.e., weight of dissolved substances).
It was observed that the power to conduct the electric current — electric
conductivity — in the case of solutions (e. g., of sugar) which have an
osmotic pressure that corresponds to the weight of dissolved substances
is practically nil, whereas the conductivity of those solutions which give
higher osmotic pressure is quite pronounced. Arrhenius made the hy-
pothesis that the conductivity depends on the splitting of molecules into
two or more portions or ions, each of which carries either a positive or a
negative electric charge, and that it is only when such dissociation occurs
that the electric current can be conducted through the solution, the ions
serving as it were as floats carrying the electric current. When sodium
chloride is dissolved in water, it splits into Na carrying a positive charge
and Cl carrying a negative charge, or Na H Cl -, as it is written ; on the
other hand, when sugar is dissolved, the molecules remain unbroken and
no electric charges are set free.
Substances which thus dissociate are called electrolytes, and those which
do not, nonelectrolytes. When the electric current is passed through a
solution of electrolytes, the ions which carry a positive charge move to
the electrode or pole by which the current leaves the solution — that is, in
the same directions as the current; and since this electrode is called the
cathode, these are called cations. Hydrogen and the metals belong to
this group. The ions carrying a negative charge go in the opposite direc-
tion, against the current — that is, towards the electrode by which the cur-
rent enters, or the anode; they are therefore called anions. They include
oxygen, the halogens and the acid groups, such as S04, C03, etc.
It must be understood that this dissociation into ions is already present
in the solution before any electric current passes through it, the ions
16
ELECTRIC CONDUCTIVITY, DISSOCIATION, IONIZATION 17
being however uniformly distributed throughout — that is, arranged so
that the negative charges of the anions precisely neutralize the positive
charges of the cations. The electric current causes the electrodes to be-
come charg-ed, the one positively, the other negatively, so that an attrac-
tive force is exerted on the ions of opposite sign. This causes the nega-
tively charged ions to migrate towards the positive electrode, and the
positively charged, towards the negative electrode. It is this migration
of the ions that endows the solution Avith conducting qualities.
In water, or in a solution of a nonelectrolyte, molecules of H20 or non-
electrolyte exist thus:
H,0 H20 H20
H20 H20 H20
H20 H20 H20
In a solution of an electrolyte, the molecules split into ions thus:
Na+ Cl- Na+ Cl- Na+ 01-
Na* Cl- Na+ Cl- Na* 01-
Na+ Cl- Na+ Cl- Na+ 01-
When an electric current passes through a solution of an electrolyte,
the ions tend to arrange themselves thus:
Cathode- Anode*
Na> Na+ Na* Cl- Cl- 01-
Na+ Na+ Na+ Cl- Cl- 01-
Na+ Na+ Na+ Cl- Cl- 01-
It follows from the above considerations that the conductivity of a siib-
stance in solution will depend on the degree to which it undergoes dissocia-
tion. Furthermore, if we assume that in so far as osmotic pressure
phenomena are concerned, each ion behaves in the same way as a mole-
cule, then it follows that the electrical conductivity must be proportional
to the extent to which the osmotic pressure is greater than we should ex-
pect it to be from the amount of substance actually dissolved.
In the Determination of the Conductivity it is obviously necessary to
use standard conditions of depth and width of the fluid through which the
current is passed, and to have some standard of comparison. The value
is then known as the specific conductivity, the standard for comparison
being the conductivity of a hypothetical liquid which, if enclosed in a
centimeter cube, would offer a resistance of 1 ohm between two opposite
sides of the cube acting as electrodes. The actual determination is usu-
18
PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
ally made in a cylindrical vessel of hard glass (from soft glass enough
alkali might be dissolved to affect the results), the electrodes being circu-
lar plates of platinum firmly cemented at a known distance from each
other (Fig. 5).* This conductivity cell, as it is called, is- connected
with a suitable electric apparatus for measuring the resistance offered
Fig. 5. — Diagram of conductivity cells. The platinum discs are represented by the thick
black lines. They are held in position by thick-walled glass tubes, through which they are
connected with the terminals by platinum wires. (From Spencer.)
by the solution to the passage of an electric current (Wheatstone Bridge)
(see Fig. 6). The resistance is of course inversely proportional to the
conductivity.
As a saline solution is progressively diluted, its specific conductivity
naturally decreases (since there are now fewer molecules between the
Fig. 6. — Wheatstone Bridge for the measurement of electric resistance: a-b, bridge wire; c,
the movable contact.
two opposite faces of the centimeter cube, and the space between ions or
molecules is increased). This result will not, however, tell us whether
the salt itself is undergoing any alteration in conducting power as a con-
sequence, for example, of greater dissociation. To ascertain this we must
*This distance is determined not by direct measurement but by calculation from results obtained
by testing the actual resistance of a solution whose specific resistance is accurately known.
ELECTRIC CONDUCTIVITY, DISSOCIATION, IONIZATION 19
obtain figures relating to the same quantity of salt at each dilution. If
we multiply the specific conductivity by the volume of solution in c.c.
which contains 1 gram-equivalent (see page 22), a value will be secured
which represents the conducting power of a gram-equivalent. This is
known as the equivalent or molecular conductivity* and is represented by
the sign X. When it is determined for progressively diluted solutions,
A gradually increases, indicating that the efficiency of the electrolyte itself
as a conductor increases with dilution, because it dissociates more. The
extent of this increase is found to become less and less as dilution
proceeds. By plotting the values of the molecular conductivity of suc-
cessive dilutions as a curve, the value at infinite dilution can be ascertained
by extrapolation. This value is represented by A oc .
Now, let us see how these facts bear out the theory of electrolytic dissocia-
tion. According to this hypothesis the conductivity depends on the num-
ber of ions (see page 17), and since it is at a maximum at infinite dilu-
tion, the value A°c must represent the total number of ions that can be pro-
duced by the dissociation of 1 gram-equivalent, and A that at some other
dilution. If, therefore, we divide A by A<x we obtain a value (called a)
which must represent the degree to which the electrolyte is ionized at the
various dilutions at which A is measured. From what has been said re-
garding the osmotic pressure of similar solutions, it is evident that the
value a could also be calculated by finding the extent to which the de-
pression of freezing point A is greater than would be expected from the
number of dissolved molecules. As a matter of fact, it has been found
that practically identical values are obtained for many substances, thus
furnishing almost incontrovertible proof in support of the dissociation
hypothesis. In the cases of weak acids and bases, it is possible to secure
a value, called the dissociation constant (K), which represents the rela-
tive values of a at all dilutions. Since the activity of acids and bases
is dependent upon the number of H- and OH-ions, respectively, set free
by dissociation, it follows that it must be proportional to K. It will be
necessary to postpone a consideration of the application of this constant
until we have studied mass action (page 23).
Biological Applications. — The practical value of such knowledge rests,
not so much on any direct simple application that can be made of it in
explaining physiologic processes, as on the essentially important bearing
which it has in enabling us to understand the nature and operation of
other physicochemical factors concerned in physiologic processes. With-
out a clear comprehension of the elemental laws of dissociation, it is
impossible to consider such problems as those which concern the activities
*In other words, the molecular conductivity is the specific conductivity divided by the number of
gram-equivalents contained in-1 c.c.
20 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
of enzymes (mass action, etc.), the occurrence of electric currents during
the physiologic activity of muscles, glands, and nerves, and the all-
important question of the reaction or H-ion concentration of the body
fluids.
Before proceeding to show how these facts concerning the nature of
solutions are applicable to the study of physiologic processes, it may be
well to indicate one or two instances in which measurements of electrical
conductivity and of dissociation have direct physiologic value. The circu-
lation time of the bloodflow through an organ can be determined by first
finding the electrical resistance of a short piece of the vein of the organ,
and then observing the change in resistance which is produced when the
conductivity of the blood in the vein is altered by the arrival in it of
saline injected into the artery. The interval elapsing between the injec-
tion into the artery and the changes in resistance in the vein obviously
equals the circulation time (G. N. Stewart).
The same investigator has used measurements by electrical conductiv-
ity to study the passage of electrolytes out of the red blood corpuscles into
the serum. Under normal conditions the blood serum has a certain elec-
trical conductivity equal to that of a 0.9 per cent sodium-chloride solution.
The conductivity of the defibrinated blood is only about one-half that of
serum, because it contains corpuscles which are nonconductors and there-
fore obstruct the free passage of the ions, just as a suspension of quartz
powder in a sodium-chloride solution lowers the conductivity of the lat-
ter. If anything occurs therefore to occasion a passage of the saline con-
tents of the corpuscles through their walls into the serum, an increase in
the electric conductivity will be produced. The value of this method in
the investigation of changes in permeability of the red corpuscles is de-
pendent on the fact that such migration of electrolytes out of the cor-
puscles may occur before any of the less diffusible hemoglobin itself has
escaped. The rise in conductivity precedes the hemolysis (see page 7).
Although determinations of the specific conductivity of blood and urine
under various pathologic conditions have also been made, the results
have not been found to possess any diagnostic value or clinical signifi-
cance. Measurements of the electric conductivity of blood have, -how-
ever, been used by Wilson7 and by Priestley and Haldane8 to detect the
degree of dilution when large quantities of water are ingested.
Another application of conductivity measurements in biochemistry has
been made in studying the digestive action of proteolytic enzymes (Bay-
liss). The general action of the enzymes is to break the large undisso-
ciated molecules of the higher proteins (albumin, casein, etc.), into
smaller molecules (amino acids, etc.), which are partly ionized. As diges-
ELECTRIC CONDUCTIVITY, DISSOCIATION, IONIZATION 21
tion proceeds, therefore, the conductivity of the digestion mixture pro-
gressively increases, and is a measure of the rate of digestion.
Applications of the dissociation hypothesis in physiology concern the
explanation of such phenomena as the production of electric currents
during muscular, glandular, and nervous activity. The exact details of
the application are not as yet sufficiently understood to warrant our at-
tempting to do more than indicate the general lines along which the
problems are being investigated. Let us, for example, consider how the
current of action of muscle may be explained in terms of the dissociation
hypothesis. To do so we must delve a little further into physicochem-
ical research, when we shall find that there are two further facts . con-
cerning ionized molecules that must be of importance in connection with
our problem. The first is that the contribution which each ion makes to
the equivalent (or molecular) conductivity of a solution is independent
of the other ion with which it is associated; and the second, that ions
differ considerably in their conducting power. Since the univalent ions,
K., Na., CL', N03', carry charges of the same magnitude,* and yet all do
not conduct to the same degree, they must move at different velocities
through the solution. We are driven, therefore, to the conclusion that,
exposed to the same electric force, different ions have different mobili-
ties ; that is to say, when an electric current passes through a solution of
an electrolyte, the positively charged ions move towards the cathode at a
different rate from that at which the negatively charged ions move
towards the anode. Confirmation of this conclusion is obtained by exam-
ination of the concentration changes around the two electrodes of an
electrolytic cell. The actual velocity of each ion can be determined by
experimental means.
*Thus Faraday showed that the amounts of the various ions liberated by electrolysis are in the
same ratio as their chemical equivalents.
CHAPTER IV
THE PRINCIPLES INVOLVED IN THE DETERMINATION OP THE
HYDROGEN-ION CONCENTRATION
TITRABLE ACIDITY AND ALKALINITY
All acids have one property in common — namely, that they contain
hydrogen — and when the acid becomes neutralized, it is this element
which becomes replaced by some other cation. Evidently, then, the
strength of an acid is proportional to the number of displaceable hydro-
gen atoms which it contains. It may contain other hydrogen atoms
which are so bound up in the molecule that they do not become displaced
when an alkali is mixed with the acid. For example, in organic acids
like acetic, CH3COOH, it is only the H atom attached to the COOH
group, but not those attached to the CH3 group, that is replaceable. It
must therefore be possible -to prepare for every acid a solution having
exactly the same neutralizing power as that of any other acid; that is,
the -same volume of solution must be required in each case to neutralize
a given quantity of alkali, the point of neutralization being judged by
the change in color of indicators. As a standard a gram-molecular solu-
tion of an acid with one displaceable H ion, such as hydrochloric, is
chosen. This we call a "normal acid" (N). To prepare a normal solu-
tion of acids having two displaceable H atoms, such as H2S04, we can not
however use a gram-molecular quantity, but must take one-half of it;
and similarly in the case of those with three H atoms, such as H3P04,
a one-third gram-molecular solution will be a normal acid solution. For
practical purposes, use is very generally made of solutions that are some
fraction of the normal, e. g., tenth or decinormal (written N/10), or hun-
dredth or centinormal (N/100).
In a similar way, alkaline solutions can be prepared, a normal alkali
being one which exactly corresponds in strength with a normal acid
(i.e., can exactly neutralize it). Now, the characteristic of alkalies is
that they produce in solution "OH" or hydroxyl ions; so that the process
of neutralization must consist in the union of the H ions of the acid with
the OH ions of the alkali to form water: KOH + HC1 = KC1 +H20. We
can, therefore, prepare normal solutions of alkalies by dissolving in 1
liter of water such quantities of alkali (in grams) as will yield the OH
required to react with the available hydrogen in normal acid solutions.
22
HYDROGEN-ION CONCENTRATION 23
Actual Degree of Acidity or Alkalinity. — According to the foregoing
method of titration a normal solution of a powerful mineral acid, such
as hydrochloric, is no stronger than a normal solution of a weak acid,
such as acetic or lactic. It requires no fewer c.c. of N alkali to neutralize
it. But the normal solution of the powerful acid tastes more acid, is
more toxic, dissolves metals more readily, and in all its other chemical
and physiologic properties acts much more quickly than the weak acid,
so that the titrable acidity or alkalinity can not express the real strength
of the acid or alkali, or the actual degree of acidity or alkalinity. It is
in this connection that the dissociation hypothesis aids us, for it suggests
that the degree to which the acid becomes dissociated into H' and the
remainder of the molecule will determine its real strength (see page 16).
The question is, how are we to measure the latter 1 One action of H ions
which we may measure is that known as catalytic — that is, the power to
accelerate reactions, such as the splitting of cane sugar (C12H220U) into
glucose and levulose, which otherwise would proceed very slowly (see
page 75). If then the real strength of an acid depends on the degree
of dissociation which it undergoes, figures representing the catalytic
power should correspond with those representing the relative conductivi-
ties of the acids in equivalent concentration (see page 19). That this is
actually the case is shown in the following table, in which the above values
of various acids are given compared with HC1, which is taken as 100.
ACID CATALYTIC POWER RELATIVE CONDUCTIVITY
HC1 100 100
Dichloracetic 27 25
Monochloracetic 4.8 4.9
Formic 1.5 1.7
Acetic 0.40 0.42
It will be evident that, if we could measure the concentration of free
H ions in a solution — that is, of H ions that are not matched by OH ions —
we should have a faithful index of its real acidity. This measurement
has been rendered possible by the application of two other physico-
chemical principles — namely, those of mass action and electromotive
force. Since the object of this volume is to present the scientific basis
for the various methods that are used in modern medicine, it will be nec-
essary for us to review the main principles of these two actions. "We shall
see that they apply, not only in the measurement of H-ion concentration,
but in many other physiologic processes.
Mass Action
When materials take part in a reaction, some molecules are decom-
posing while others are being formed. After some time, however, a
24 . PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES
condition is reached in which the changes in one direction are exactly
offset by those in the other. An equilibrium is said to have become estab-
lished between the reacting substances. Bearing in mind that the ions
and molecules entering into these reactions are constantly moving about
and coming in contact with one another, it is easy to see that if we were
to add an additional quantity of one kind of molecule or ion, there would
be a change all along the line until a new equilibrium was established.
If, on the Other hand, we were to remove one kind of molecule or ion
as fast as it is formed, the equilibrium could never be established, and
the reaction would proceed until all of this material had disappeared.
The natural rate at which any chemical reaction proceeds is dependent
upon a number of conditions, such as chemical affinity, temperature,
catalysis, and concentration. Of these conditions that of concentration
is most readily measured. If we maintain all of the conditions other
than that of concentration unchanged, and designate this combined in-
fluence as K (constant), we shall find that the speed of the reaction- is
proportional to the molecular concentration of the reacting substances
(i. e., the number of gram-molecular weights per liter). In other words,
the speed with which two substances, a and b, unite to form other sub-
stances, c and d, will be expressed by the equation,
k (a) x (b) <=± k' (c) x (d);*
which means that, when the reaction is complete, the composition of
the mixture will be dependent upon the ratio between k and k'. Since
however these are Both constants, their quotient is also constant (K), and
(a) x (b)
we have the equation, -W — }-=£- = K, indicating that no matter how
(c) x(d)
the concentrations a, b, c, and d are varied, reaction will take place in
one direction or the other until the concentrations have become adjusted
so that K remains unchanged.
As an example of the application of these laws, let us take the reaction
which occurs between alcohols and organic acids to form the substances
called esters — a reaction which is analogous to that between mineral
alkalies and acids to form neutral salts, and which is of special interest
to us because it is the reaction involved in the splitting of animal fats.
The equation for the reaction is:
C2H5OH + CILCOOH ?± aH.OOCCH., + H2O.
(ethyl (acetic (ethyl acetate,
alcohol) acid) an ester)
Or expressed according to the law of mass action:
[C2H5OH] x [CH3COOH]
[C2H6OOCCH3] x [H,O]
= K.
*The brackets indicate that gram molecular quantities are used.
HYDROGEN-ION CONCENTRATION 25
Now it is clear that if we increase, say, H20 in the above equation, then
in order that K may remain unchanged C2H5OOCCH3 must diminish or
the substances which form the numerator of the equation must increase,
or both these changes must occur. As a matter of fact, in such a case as
the above, both of these adjustments take place, for, as the ester breaks
down, it must thereby increase the concentration of acid and alcohol.
Since in aqueous solutions the reaction occurs in the presence of an excess
of water, it is evident that the tendency for an ester in the presence of
water is to break down into alcohol and acid, and this must occur in all
reactions in the body fluids in which water enters into the equation.
Physiologic Applications. — The application of the law of mass action
in the explanation of biochemical processes is very extensive. Most of
the reactions which enzymes or ferments are capable of influencing are
of the same general nature as that represented above, and the products
of their activities are usually the substances on the side of the equation
in which no water molecules appear — i. e., they are hydrolytic reactions.
Enzymes merely accelerate the reaction (page 72), so that if we start
with a mixture of the substances on either side of the equation, all they
do is to accelerate the production of a sufficient concentration of those
on the other side, until the equilibrium point is reached. For example,
an enzyme present in pancreatic juice, called lipase, accelerates the
breakdown of such esters as neutral fat, which consists of the triatomic
alcohol, glycerol, combined with the fatty acids palmitic (C15H31COOH),
stearic (C17H35COOH) and oleic (C7H33COOH):
C3H5 (O OC C17H35)3 + 3H,O^3CnH3sCOOH + C3H5 (OH)3.
(the neutral fat, (the fatty acid, (glycerol)
tristearin) stearic)
Under ordinary conditions the reaction proceeds until nearly all the
neutral fat has become decomposed because of the preponderance of
water, but if we start with a mixture of fatty acid and glycerol with
just enough water to permit the enzyme to act, the reaction will pro-
ceed in the opposite direction — i. e., so that some neutral fat will be
synthesized. This is called the reversible action of enzymes.
Because of the universal presence of water, it is plain that such re-
versible reactions could not alone be held responsible for the synthe-
sis of neutral fat or of similar substances in the animal body. The only
way by which synthesis could occur under these conditions would be
if the substance produced along with the water were removed from the
site of the reaction as soon as it was formed. This might occur by the
precipitation of the substance or by its becoming surrounded by an en-
velope of some inert material. In the synthesis of neutral fat which
26 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
occurs in the epithelium of the intestine out of the fatty acid and glycerol
absorbed from the intestinal contents, it is possible that the last men-
tioned -process occurs. In other cases the substance may be carried
away by the blood or lymph or urine as fast as it is formed.
The Law of Mass Action as Applied to the Measurement of H-ion
Concentration. — Let us now return to the reaction or H-ion concentration
of substances in solution. As the standard of neutrality, pure water is
chosen. Let us consider, then, how the laws of mass action can be
applied in order to enable us to determine the H-ion concentration of
pure water. It has been stated above that chemically pure water is in-
capable of conducting the electric current. This however is not strictly
the case, for it conducts to a very slight degree. According to the dis-
sociation hypothesis, it must therefore be represented as containing
molecules of H20 and ions of H- and OH', and according to that of mass
action there nmst be a balanced reaction between the molecules and ions
represented thus:
|H-]x [OH'l
H.,0 <=» H' + OH', or - = K.
Since the concentration of H- and OH' is extremely small, there must
always be such an overwhelming preponderance of H20 molecules that
no changes in the concentration of H- and OH' will be capable of appre-
ciably affecting the concentration of H20 ; in other words, one may omit
the denominator of the equation and write it [H-] x [OH'] =K. If
then we know the value of K, it will only be necessary to measure the
concentration of either H- or OH' in order to express in numerical terms
the reaction of the solution. It has been found that the value of K is
about 1 x 10'14,* and since the concentrations of H- and OH' are nec-
essarily equal in pure water, it follows that [H] = [OH] = V lx 10~14,
i. e., each ion has a concentration of 1 x 10'7. This means that water con-
tains approximately 1 gram mol. each of H- and OH' ions, or 1 gram
H- and 17 grams OH' ions, in 10 7 or 10,000,000 liters. A consequence
of the above law is that no matter how much the concentration of one
ion is increased by adding another substance, the solution must still
contain some of the other ion. Thus, in acid solutions con. H • must
increase and con. OH' must decrease in such proportion that the two
multiplied together equals about 1 x 10-14. The H-ion, concentration can
be used 'therefore to express the reaction of neutral, acid and alkaline
solutions.
In place of water, let us substitute decinormal hydrochloric acid
*The sign 1Q-14 is simply a convenient way of expressing the degree of dilution. It gives the
number of times the value standing in front of it must be multiplied by 10 in order to find the
degree of dilution.
HYDROGEN-ION CONCENTRATION 27
(0.1 N HC1) — that is, a hydrochloric acid solution containing one tenth
of the molecular weight of hydrochloric acid in grams dissolved in a
liter of water. At this dilution HC1 is 91 per cent dissociated; therefore
the H-ion concentration (or CH as it is written for short) is 0.091 N,
or, in mathematical notation, 9.1 x 10 2.
Method of Expressing CH. — To avoid the necessity of having to use
several figures to express CH, as has been done above, SSrenson has intro-
duced a scheme by which only one figure is required. This figure, des-
ignated by PH, is found by subtracting from the power of ten (i. e.,
the figure standing behind 10) the common logarithm of the figure ex-
pressing the normality of the acid.* In a decinormal HC1 solution,
therefore, we must subtract from the power 2, the common log. of 9.1,
which is .96 (ascertained from logarithm tables), leaving 1.04. . Take
another example: decinormal acetic acid is dissociated only to the ex-
tent of 1.3 per cent; CH is therefore 0.0013 normal, or 1.3xlQ-3. Since
the logarithm of 1.3 is .11, PH equals 3 -.11, or -2.89. t
The only objection to the use of the exponent PH as an expression of
the H-ion concentration is that it increases in magnitude as the acidity
becomes less; this is because the negative sign of the power is disre-
garded. As stated above, it is usual to express the strength of alkalies
as well as acids in terms of CH, or PH, because it is easier to measure the
concentration of H ions than of OH ions. A 0.1 NaOH solution is 84
per cent dissociated; therefore the "OH" ion is 0.084 N (i. e., 0.084 gram
equivalents OH per liter), and since the product of the H- and OH'
concentrations must always equal 10'14-14 (at 20° C.), it is clear that as
the H ion increases in concentration, the OH ion must reciprocally de-
crease. Expressed according to the above scheme, the 0.084 N NaOH
solution gives PH 13.06; thus, 0.084 = 8.4 x 10"2; the log. of 8.4 is .924,
and this subtracted from the power -2 = 1.08 as POH, or 14.14 - 1.08 =
13.06 as PH.**
Similarly, PH of 0.1 N NH4HO solution is 11.286. Its dissociation is
1.4 per cent; therefore the solution contains only 0.0014 gram equivalents
HO— i. e., 1.4 x 10-3 POH = 3 - 0.146 = 2.854 . • . PH 14.14 - 2.854 =
113864
*Strictly speaking, PH is the logarithm to the base 10 of the concentration of H ions in grams
per liter, the negative sign being understood.
flf we wish to express the value of PH in ordinary notation, we must find the antilogarithm
of the difference between the value of PH and the next higher whole number; e.g., if PH = 7.45,
the antilogarithm of 0.55 being 3.55, the CH is 3.55 x 1Q-8, or 0.000,000,0355 N, or 3.55 gm. mol.
H ion in 100,000,000 liters.
**It must be remembered that the power of a number indicates the number of times by which
that number must be multiplied by ten; thus, PH-* does not mean that the H ion is six times less
than PH°, but 1 x 10 x 10 x 10 x 10 x 10 x 10, or 1,000,000 times less. Similarly, Pn-3 is 1000 times
as great as PH-*, not twice as great.
A solution containing almost exactly 1 gram molecule of dissociated hydrogen in 10,000,000 fiters
constitutes a neutral solution (PH = 7).
JThe expressions PH and CH may be used indiscriminately, but when the numerical value is
given, it is most convenient to use the former.
28 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
Application of the Law of Mass Action in Determining the Real
Strength of Acids or Alkalies. — We have seen that it is the degree of
dissociation upon which the real strength of an acid depends and that
this varies with dilution (page 19). The equilibrium between the un-
dissociated and dissociated molecules may therefore be shifted in either
direction by changing the concentration; in other words, the process of
dissociation is a reversible reaction, and may be represented as
AB ±5 A' + B •. The law of mass action must apply in such a case, and
as a matter of fact it has been found that a constant can be calculated,
which is known as the dissociation constant.* It is an expression of the
inherent ability of the acid to dissociate into ions, and is therefore the
best measure of the strength of the acid. This is strictly the case for all
of the weaker acids, but strong mineral acids (and bases) do not give
a satisfactory constant, so that the comparison must not be made between
them and weaker ones. That the dissociation constant expresses the rela-
tive strength of organic acids can be shown by comparing its value with
that of the rate at which cane sugar is inverted (see page 23), this being
proportional to the concentration of the H ions present. K for some or-
ganic acids is : Acetic, 0.000018 ; Formic, 0.000214 ; Benzoic, 0.00006 ; Sal-
icylic, 0.00102.
a2
*The equation is -TT — r-rr- = K, where it is supposed that in volume V of the solution there is
1 gram-equivalent of electrolyte, and that the degree of dissociation is a; the quantity of undis-
sociated electrolyte stated in a fraction of a gram-equivalent will be 1-a, and the quantity of each
ion a. To illustrate, let us take acetic acid in various dilutions:
V a K x 10'
0.994 0.004 1.62
2.02 0.00614 1.88
15.9 0.0166 1.76
18.1 0.0178 0.78
CHAPTER V
THE PRINCIPLES INVOLVED IN THE MEASUREMENT OF THE
HYDROGEN-ION CONCENTRATION (Cont'd)
THE METHODS OF MEASUREMENT
The Electric Method
In order to understand the principle of the standard method used for
measuring the H-ion concentration, it is necessary that a few words be
said concerning the factors governing the development of electric cur-
rents in chemical batteries. There may be a further application of this
knowledge in connection with the generation of the electric currents
which occurs during physiologic activity, as in active glands and muscles.
When a metal is immersed in a solution of one of its salts, it has a
tendency to give off ions into the solution. Similar ions are, however,
already present in this solution, and these, by their osmotic pressure,
tend to oppose the passage of the ions coming from the metal. The
force with which the metal sends out its ions into the solution is called
the electrolytic solution pressure. If this is equal to the osmotic pres-
sure of the metallic ions in the solution, there will be no electric current
generated, but if it is greater or less than the osmotic pressure of the
metallic ion, an electric current will be set up. When the solution pres-
sure is the greater, the metal will become negatively charged, because its
ions carry positive charges into the solution (cations); on the contrary,
when the osmotic pressure is greater than the solution pressure, the metal
will have a positive charge, owing to the receipt of the positive cations
from the solution.
Because of a force called electrostatic attraction, the ions given off
from the metal can not travel any measurable distance from the oppositely
charged mass of metal, so that from one of the electrodes alone it is
impossible for us to lead off any electric current. For this purpose we
must form a circuit. This is done in the manner shown in Fig. 7 by
connecting side tubes coming from the electrode vessels with an inter-
mediate vessel containing a solution of high conductivity and by con-
necting the metals by wires. If the circuit is formed between the
same metals in solutions of the same concentration, no electric cur-
rent will be generated, because the two electrode potentials will be>
29
30
PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PKOCESSES
equal and in opposite directions to each other. On the other hand, should
the concentration of the metallic ion in the solutions be unequal, the
electromotive force will flow from the one electrode to the other, and
the pressure with which it flows will be equal to the difference in con-
centration of the two solutions. This is the principle of a concentration
cell, and if we know the concentration of one of the solutions composing
it, and then proceed to measure the electromotive force, we can obtain
the concentrations of the other solution by difference. To do this we
must employ a formula which takes into consideration the relation be-
tween the potential and the concentration of the solution.
The potential of an unknown electrode composed of a metal in con-
tact with a solution of one of its salts- may also be determined by making
it one pole of a battery of which the other pole is composed of a stand-
ard electrode of unchanging known potential. An electrode of the latter
Fig. 7. — Diagram to show type of electrodes used in studying electromotive force. The
metal in each electrode is connected (through a glass tube) with a platinum wire, to which
the apparatus for measurement of the voltage is connected. The metal dips into a solution
contained in the electrode vessel and filling the side tube. The latter dips into an inter-
mediate vessel containing saturated KC1 solution. The currents flow through the circuit under
the following conditions: (1) dissimilar metals dipping into the same fluid; (2) similar metals
dipping into different fluids; (3) dissimilar metals dipping into different fluids.
type can most readily be made by bringing pure mercury in contact
with a saturated solution of calomel (Hg2Cl2) in normal potassium chlo-
ride solution. Under suitable conditions (i. e., when the circuit is com-
pleted), a potential of +0.560 v. is developed in this so-called calomel
electrode* — that is, positive ions of mercury are deposited on the mercury
from the calomel solution, at this pressure. Suppose that we connect a
calomel electrode, through the intermediation of some solution which
*The calomel electrode consists of a suitably shaped glass vessel containing pure mercury, con-
nected by means of a platinum wire with a conductor, and filled with a saturated solution of pure
mercurous chloride in normal KC1 solution up to such a level that it also fills a side tube connected
with a vessel containing a saturated solution of potassium chloride. Into this vessel also runs a
similar side tube from the unknown electrode. By having an excess of utidissolved calomel in the
solution in the calomel electrode its saturated condition is maintained during the chemical changes
which accompany the production of the electric current.
HYDROGEN-ION CONCENTRATION
31
will serve as a good conductor, with another electrode, the two elec-
trodes being also connected by wires with electrical apparatus for
measuring the total potential of the battery; then by adding +0.560 v.
to or subtracting this value from the total potential (depending on the
sign of the unknown electrode) we can tell the potential of the unknown
electrode.
We have discussed these principles of electrochemistry because they
form the basis upon which depends the standard method for the deter-
mination of the H-ion concentration of fluids. Suppose, for example,
that in place of using a metal in the construction of one electrode, we
use an electrode consisting of a layer of pure hydrogen gas in contact
with a solution in which are free H ions; then the rate at which H ions
Fig. 8. — Diagram of apparatus for the measurement of the H-ion concentration. The cur-
rent generated in the battery (composed of calomel electrode, connecting vessel with KC1 solu-
tion, and the H electrode) or that from the normal element is transmitted through a reversing
key to the bridge wire, where the voltage is compared with a steady current flowing through the
bridge wire from an accumulator. The capillary electrometer is used to detect the flow of
current at various positions of the movable contact on the bridge wire. (Modified from
Sorensen.)
become added to the solution from the H layer, or taken from it, will de-
pend on the concentration of H ions in solution. In order to secure a
hydrogen electrode fulfilling the above requirements, it is necessary to
employ some means by which a layer of hydrogen may be furnished, and
fortunately this can be done by taking advantage of the property which
spongy platinum possesses of absorbing large quantities of this gas. It
is also necessary to keep an atmosphere of pure H in contact with the
fluid.
As is the case of the simpler cells described above, there are two
types which we might use for measuring the electromotive force gen-
erated in the unknown electrode: a concentration cell composed of two
32 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
hydrogen electrodes, of which one contains a solution of known H-ion
concentration, and the other the solution in which this is unknown;
and a cell of which one electrode is a standard calomel electrode and
the other, a hydrogen electrode containing the unknown solution.
The exact arrangement of the apparatus in which the calomel elec-
trode is used will be seen in the accompanying sketch. The hydrogen
electrode, it will be noticed, is a very small V-shaped tube, in which is
suspended a platinum wire coated with spongy platinum and dipping
into a solution which nearly fills the tube. The space above the solution
is filled with pure hydrogen. This and the calomel electrode are con-
nected with suitable electric measuring instruments, and the circuit is
completed by connecting the two electrodes by means of an intermediate
vessel containing a saturated solution of potassium chloride. This con-
necting solution is used because it has been found that the electric cur-
rents set up at the contact between different solutions are so small that
they can be disregarded.*
As outlined above, the hydrogen electrode is that which is used to
determine the H-ion concentration of blood, the particular point about
it, in comparison with the apparatus used for simpler solutions, being
that the hydrogen is not changed in the course of the experiment. This
precaution is to prevent the carbon dioxide of the blood from being
"washed out" of it by a frequently changing atmosphere of hydrogen.
Many inaccuracies in the earlier results obtained by this method were
due to the removal of carbon dioxide, which, as we shall see later, is
one of the chief acids contributing to the H-ion concentration of blood.
The Indicator Method
As pointed out in a previous chapter (page 22), the method of titra-
tion for acidity or alkalinity in which a standard solution of alkali or
acid is added until a certain change in the color of a suitable indicator
is detected, does not afford any information regarding the H-ion con-
centration actually present in the solution. It tells us the total con-
centration of available acid or base, both dissociated and undissociated.
By modification of the method of procedure, however, we may also use
indicators for determining the H-ion concentration. The principle of
this method depends on the fact that there are certain dyes which
change quite distinctly in tint with very slight changes in the H-ion
concentration, so that if we use dyes which possess this property at a
point near that of neutrality (i. e., between PH6.5 and Pn8), we can es-
*A description of the technic for measuring the electric potential developed by the cell would
be out of place here. Suffice to say that the strength of the current is compared with that of a
current of known strength furnished by a normal cell, the comparison being made by a bridge wire
F, a capillary electrometer II being employed to detect the direction and degree of current.
HYDROGEN-ION CONCENTRATION
33
timate the H-ion concentration of the body fluids with very remarkable
accuracy, provided certain precautions are taken to circumvent the
disturbing influence which the protein and salts in these fluids may have
on the color change.
To understand this use of indicators, it is important to bear in mind
that one solution reacting neutral to one indicator may have a H-ion
concentration which differs very markedly from that of another solu-
tion reacting neutral to another indicator. This is because indicators
react to different H-ion concentrations. A solution that is neutral to
phenolphthalein has a PH of about 9, whereas one neutral to methyl or-
ange has a PH of about 4. This can be very clearly shown by titrating
a solution of phosphoric acid with decinormal alkali. After a certain
amount of alkali has been added it will be noticed that methyl orange
changes from red to yellow, but after it has changed and is therefore
alkaline as judged by this indicator, it still remains distinctly acid to-
wards phenolphthalein (shows no red tint) even though considerably
more alkali is added. The methyl orange is, therefore, itself unrespon-
sive to weak acids such as remain after the greater part of the phos-
phoric acid has been neutralized by the alkali.
The series of indicators which has been employed for this purpose is
given in the accompanying table, along Avith the PH limits through which
they change in color.
LIST OF INDICATORS
CHEMICAL NAME
COMMON
NAME
CONCEN-
TRATION
COLOR
CHANGE
RANGE
PH
Thymol sulfon phthalein
per cent
(acid range)
Thymol blue
0.04
Red-yellow
1.2-2.8
Tetra bromo phenol sul-
fon phthalein
Brom phenol
blue
0.04
Yellow-blue
3.0-4.6
Ortho carboxy benzene
azo di methyl
aniline
Methyl red
0.02
Red-yellow
4.4-6.0
Ortho carboxy benzene
azo di propyl
aniline
Propyl red
0.02
Red-yellow
4.8-6.4
Di bromo ortho cresol
sulfon phthalein
Brom cresol
purple
0.04
Yellow-
purple
5.2-6.8
Di bromq thymol sulfon
Brom thymol
phthalein
blue
0.04
Yellow-blue
6.0-7.6
Phenol sulfon phthalein
Phenol red
0.02
Yellow-red
6.8-8.4
Ortho cresol sulfon
phthalein
Cresol red
0.02
Yellow-red
7.2-8.8
Thymol sulfon phthalein
Thymol blue
0.04
Yellow-red
8.0-9.6
(see above)
Ortho cresol phthalein
Cresol
phthalein
0.02
Colorless-red
8.2-9.8
These dyes may now be obtained in this country.
(W. M. dark and H. A. Lubs.)»
34 PHYSICOCHliMICAL BASIS OF PHYSIOLOGICAL PROCESSED
Briefly stated the method for measuring the H-ion concentration Con-
sists in preparing a series of solutions containing known concentrations
of H-ion — that is to say, of known PH — and adding to each solution an
equal amount of an indicator which exhibits easily distinguishable
changes in tint at H-ion concentrations approximating those believed
to be present in the unknown solution. The same indicator is added to
the unknown solution, which is then placed side by side with the stand-
ards to find with which of them it most closely matches. The series
of solutions of known H-ion concentration is prepared by mixing fif-
teenth normal solutions of Na2HP04 and KH2P04 in varying propor-
tions as given in the following table:
PREPARATION OF STANDARD SOLUTIONS
The solutions are mixed
in the proportions indicated below to obtain the desired
PH:*
PH 6.4 6.6
6.8
7.0
7 1
7.2
7.3
7.4
7.5
7.6 7.7
7.8
8.0
8.2 8.4
Primary Potas. 73 63
Phos., c.c.
Secondary Sodium 27 37
Phos., c.c.
51
49
37
63
32
68
27
73
23
77
19
81
15.8
84.2
13.2 11
86.8 89
8.8
91.2
5.6
94.4
3.2 2.0
96.8 98.0
(From Levy, Rowntree and Marriott.)
'Standard phosphate mixtures are prepared according to Sorensen's directions as follows:
1/15 mol. acid or primary potassium phosphate.- — 9.078 grams of the pure recrystallized salt
(KHoPCU) are dissolved in freshly distilled water and made up to 1 liter.
1/15 mol. alkaline or secondary sodium phosphate. — The pure recrystallized salt (Na2HPO4. 12H2O)
is exposed to the air for from ten days to two weeks, protected from dust. Ten molecules of water
of crystallization are given off and a salt of the formulnl Na«HPO4 .2112O is obtained; 11.876 grams
of this are dissolved in freshly distilled water and made up to 1 liter. The solution should give a
deep rose red color with phenolphthalein. If only a faint pink color is obtained, the salt is not
sufficiently pure.
The indicator method is extremely accurate when used with 'pure
solutions of acids, but, as mentioned above, it is apt to be inaccurate, at
least with most indicators, when protein or inorganic salts are pres-
ent in the solution, and of course it is quite unusable with colored
fluids such as blood. In order to overcome these difficulties, the
dialysis method has recently been evolved. It consists in placing the
fluid — blood, for example — in a dialyser sac composed of celloidin and
about as large as a small test tube. The sac is placed in a wider test
tube of hard glass containing an isotonic solution of sodium chloride
that has been carefully tested to ascertain that it is strictly neutral.
The amount of blood or serum required for this method is only 2 or
3 c.c., and the amount of salt solution placed outside the sac should be
about the same. It takes only from five to ten minutes for dialysis to
occur. The celloidin sac is then removed, a few drops of the indicator
are thoroughly mixed with the dialysate, and the tube compared with
the series of standards until the corresponding tint is matched. This
indicates the H-ion concentration in the dialysate. The tints produced
by using sulphonephenolphthalein are reproduced as nearly as possible
PH7-o PH7-/ PH7-2 PH7-3
10
PH7-5 PH7-6
Fig. 9. — Chart showing approximately the tints produced by adding sulphophenolphthalein to a series
of phosphate solutions of the H-ion concentrations indicated in each case by PH.
HYDROGEN-ION CONCENTRATION
35
in the accompanying chart. The H-ion concentration of the unknown
solution is that of the tint with which it matches in the series.
It might be thought that this method would be inaccurate because of
the loss of carbon dioxide from the blood. By actual experiment, how-
ever, it has been found that, if the blood is collected with certain pre-
cautions, the error is negligible. The method is, therefore, a most useful
one clinically.
The following table gives the hydrogen-ion concentration or true
reaction of the body fluids.
FLUID
PH
FLUID
PH
Blood
7.4
Muscle juice (fresh)
6.8
Urine
6.0
Muscle juice (autolyzed)
Variable
Saliva
6.9
Pancreas extract
5.6
Gastric juice (adult)
0.9-1.6
Peritoneal fluid
7.4
Gastric juice (infant)
5.0
Pericardial fluid
7.4
Pancreatic juice (dog)
8.3
Aqueous humor
7.1
Small intestinal contents
8.3
Vitreous humor
7.0
Small intestinal contents
(infant) 3.1
Cerebrospinal fluid
7.2
Bile from liver
7.8
Cerebrospinal fluid
8.3
Bile from gall bladder
5.3-7.4
Amniotic fluid
7.1
Perspiration
7.1
Amniotic fluid
8.1
Perspiration
4.5
Milk (human)
7.0-7.2
Tears
7.2
Milk (cow)
6.6-6.8
Milk (goat)
6.6
Milk (ass)
7.6
(W. M. Clark and H. A. Lubs.)
CHAPTER VI
THE REGULATION OF NEUTRALITY IN THE ANIMAL BOD.Y
AND ACIDOSIS
Nothing is more constant in the animal economy than the H-ion con-
centration (On) of the fluids which bathe the tissues. This regulation
is fundamentally of a physicochemical nature, depending on the inter-
action of alkalies with acids, of which carbonic and phosphoric acids
and the proteins are the most important.* When different amounts of
acids or alkalies are added to water, the range of variation in H ion is
very extensive, whereas in blood the range is very limited indeed, not
extending beyond PH7 and PH7.52 (i. e., CH never goes above that of a
0.000,000,1 N solution or below that of a 0.000,000,03 N solution). In
other words blood can withstand considerable additions of acid or al-
kali without much change.
Buffer Substances. — The chemical reactions upon which this remark-
able constancy in reaction depends have been explained by Lawrence
J. Henderson.10 The fundamental equations are as follows:
M,HPO4 + HA — MH2PO4 4 MA, and
MHC03 4- HA = H2CO, 4- MA,
when M = a basic radicle, and A, an acid radicle.
Now it has been discovered that weak acids, like carbonic and phos-
phoric, possess the remarkable property of maintaining the reaction
constant when they are present in a solution which also contains an
excess of their salts. Under these circumstances the concentration of
ionized hydrogen is almost exactly equal to the product of the dissocia-
tion constantt of the acid (see page 10) multiplied by the ratio be-
tween free acid and salt; in other words,
K V[HA]
7K X[BA]'
If carbonic acid is present in a solution of bicarbonates so that there
'According to circumstances, proteins may act either as acids or as alkalies. They are there-
fore called amphoteric.
tThe ionization constant has already been referred to as a figure which expresses the tendency
of a weak acid or base to dissociate in an aqueous solution. "It expresses the proportion in which
the nondissociated part is capable of existing in the presence of its ions," and therefore is a gauge
of the strength. The dissociation constant amounts to about 0.000,000,5 for carbonic acid ; that is,
the dissociation of HoCOs into H'-f-HCQg' at room temperature will be such that the concentra-
tion of H-ion equals a 0.000,000,5 N solution.
36
ACIDOSIS 37
are equivalent quantities of free H2C03 and bicarbonate — i. e., r^.-. =~r
1-bAj l
— the H-ion concentration will be exactly the same as the dissociation
constant of carbonic acid; therefore 0.000,000,5 N (PH = 6.31), or about
five times the value of neutrality, 0.000,000,1 N (PH = 7.31). If ten
times as much free carbonic acid as bicarbonate is present, then the H-ion
concentration will be fifty times that of neutrality, i. e., •,_ . ,- =^-
x 0.000,000,5 = 0.000,005 (PH = 5.31); if there is ten times less carbonic
acid than bicarbonate, the H-ion concentration will be one-half that of
neutrality, i. e., [!*fj = -^ * 0.000,000,5 = 0.000,000,05) (PH = 7.31) ; or
L-t£.£\.J .LU
if twenty times less, one fourth (PH = 7.6). Since a large amount of
bicarbonate is actually present in blood (enough to yield from 50 to 65 c.c.
C02 per 100 c.c. of blood) (see page 391), and the free carbonic acid
undergoes fluctuations which are only trivial when compared with those
which have been chosen in the above examples, it is clear that there must
be very little change in the H-ion concentration of the blood in comparison
with the variations which would occur were no bicarbonate present.
Another weak acid which acts like carbonic in maintaining neutral-
ity is acid phosphate (MH2P04), and for the same reason — namely, that
its dissociation constant is of similar magnitude to the H-ion concen-
tration. Although the blood plasma itself contains much less phosphate
than bicarbonate, the tissues contain a considerable amount, which en-
ables them to maintain their neutrality. This action of bicarbonates and
phosphates is styled the buffer action, meaning that it serves to damp
down the effect on the H-ion concentration which additions of acids or
alkalies would otherwise have. As pointed out by Bayliss, however, a
better word to use would be "tampon action," since the substances
actually soak up much of. the added H- or OH' ions. It is not confined
to the fluids of the higher animals, but is very widely distributed
throughout nature ; for example, in the ocean and in the fluids of marine
organisms and animalcules (see L. J. Henderson).11
Although the actual reaction by which neutrality is maintained is
purely of a physicochemical nature, some provision must obviously be
made so that the acid and basic substances that take part in it may be
supplied and those produced by the reactions removed as occasion re-
quires. The source of supply is partly exogenous and partly endogenous.
The exogenous source is the basic and acid substances present in the
food; and although we do not ordinarily attempt to control the amounts
of these substances ingested, we may do so, as, for example, by the
persistent administration of soda in cases of pathologic acidosis. The
endogenous source depends on the constant production in metabolism
38 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
of acids such as carbonic, phosphoric, lactic, and sulphuric, and of
alkalies such as ammonia. Amphoteric substances, like amino acids and
proteins, may functionate either as acids or as alkalies. Whatever may
be its source, a considerable reserve of alkali is undoubtedly available
in the animal organism. This required store of alkali appears to be
automatically liberated as a result of the physicochemical process.
The removal is affected by three pathways: (1) through the lungs
gaseous carbonic acid is eliminated; (2) through the kidneys, the fixed
acids; and (3) through the intestines, some of the phosphoric acid.
Carbonic acid is produced in large amounts in the normal process of
metabolism, and is excreted in a gaseous condition by the lungs. Varia-
tion in its excretion is the most important mechanism for controlling
temporary changes in CH. In order to make this clear, it may be well to
revert for a moment to the physicochemical equation by which carbonic
acid is enabled to maintain neutrality. This may be written: CH =
TT pQ
molecular ratio 2 rr(~ • . The ratio may be increased either by adding
NaMCL^
free carbonic acid to the blood (as by causing an animal to respire some
of the gas), or by the addition of some other acid (e. g., oxybutyric, as in
diabetes) which will decompose some of the NaHC03 and produce
H2C03. The increase which these changes would cause in CH of the
blood is prevented by the remarkable sensitivity of the respiratory cen-
ter to changes in CH. An increase which is much less than can be
measured by physicochemical means stimulates the center, causing in-
creased pulmonary ventilation, so that the carbonic acid is immediately
eliminated through the lungs. This elimination does not stop when the
old level of carbonic-acid concentration is reached, but proceeds until
TT r<r\
the original ratio TT/^A *s again attained in the blood, and CH is
JNaHLO.,
restored exactly to its original value. If it stopped at the old C02 con-
centration, the ratio would be too high because there is less NaHC03.
THE THEORY OF ACIDOSIS
Although these considerations indicate that variations may occur in
the bicarbonate content of the blood without any significant change in
CH, they also show that the bicarbonate content must be a criterion of
the acid-base balance of the blood, and probably of the body fluids in
general. As pointed out by Van Slyke,12 bicarbonate represents the ex-
cess of base which is left over after all the fixed acids have been neu-
tralized. It represents the base that is available for the neutralization of
any excess of such acids that may appear — a measure of the reserve of
"buffer substance" or, more specifically, the alkaline reserve of the body.
ACIDOSIS 39
Under normal conditions the amount of NaHCO3 in blood plasma is very
constant (amounting to 50-65 vols. per cent C02), and when it is reduced,
it indicates that an excess of fixed acid must be present. This is taken
by Van Slyke and others to constitute the real definition of acidosis —
namely, "a condition in which the concentration of bicarbonate in the
blood is reduced below the normal level." If the respiratory center
for any reason should not respond promptly enough to an increase in
TT rir\
the molecular ratio — Vr/^/k > an(^ ^H consequently become greater, the
NaHLOg
condition is called uncompensated acidosis, but if the center does respond
so that CH is held constant (although NaHC03 is decreased), the condition
is one of compensated acidosis.
For practical reasons, therefore, the study of pathologic acidosis de-
pends on an estimation of the bicarbonate content of the blood or, since
it is simpler to carry out and is of equal value, of the plasma. When
plasma is obtained by removing blood from a vein of the arm and cen-
trifuging immediately out of contact with air (so that C02 may not be
lost from it) it contains approximately 60 vols. per cent of C02. Since
we know that the partial pressure of C02 in blood is equal to 42 mm. Hg
(ascertained from determinations of the alveolar C02) (see page 344),
we can calculate how much of the 60 vols. per cent must be in simple
solution by application of the law of solution of gas in a liquid (page
336). It has been found that plasma at body temperature and at 760
mm. Hg (atmospheric pressure) dissolves 0.54 per cent C02, so that at
42
42 mm. it will dissolve _pr. x 100 x 0.54 = 3 vols. per cent. Transcribing
7bO
[H2C03] 3 1
the figures to our equation we get
[NaHC03] 60 20
This definition of acidosis leaves out of regard all conditions that may
TT /-1Q
raise the ratio 2 3 by the addition of H2C03 without decomposing
any of the NaHC03, such, for example, as occurs Avhen an excess of free
carbonic acid is present in the blood plasma. Since increases in free
C02 are not infrequent in both health and disease — e. g., asphyxial con-
ditions — the above definition is not sufficiently comprehensive. When
we come to study the control of the respiratory center, we shall see that
TT pQ
an increase in the ratio — 2^pof sufficient magnitude to cause an
IN clii-vy v/3
actual increase in CH can be produced by causing an animal to respire air
*This agrees sufficiently with the result as calculated from the known values of the equation
N HCO~ ~ ~TC~ ' Thus, if we take CH as 0.35 xlO-7, \ as 0.605 for blood conditions, and
L H2CO3 0.605 x 0.35 x 10-7 _ 1
fc as 4.4 x 10- (M.chaehs and Rona),, we get = 4.4 x IQ-T = J\
40 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
containing an excess of C02 — a true acidosis, but one for which no place
is found in the above definition.
Nevertheless, Van Slyke's definition has a real value, because it em-
phasizes the importance of a determination of the bicarbonate as a cri-
terion of the degree of the forms of acidosis usually met with in disease.
The bicarbonate under such conditions may become reduced either be-
cause of the appearance of improperly oxidized fatty acids, like /?-oxy-
butyric and acetoacetic, when carbohydrate metabolism is upset as in
diabetes or starvation, or because the acids produced by a normal
metabolism are inadequately eliminated by the kidneys, as in nephritis.
Accordingly, if the respiratory mechanism and increased mass move-
ment of the blood (for an increase in CH accelerates this also) should
TT C*(\
fail to eliminate C02 quickly enough so as to keep the -A-nrf ratio at
JNa-ri.L'U3
one twentieth, then CH will rise. This is not likely to happen until a
large part of the NaHC03 has been used up, so that an estimation of that
actually present must be a reliable index of the proximity to this
condition.
A sustained increase in CH is incompatible with life. The NaHC03 is
the buffer, the factor of safety which prevents its occurrence. Although
it is only in arterial blood (i. e., after elimination of excess of C02 by
TT r<(\
the lungs has been accomplished) that constancy in the ratio
can be expected, it is fortunate, for practical reasons, that venous blood
collected during muscular rest and without stasis should be only slightly
different.
When acids are added to the blood, they will first of all be neutralized
by the "buffers" of the plasma — namely, NaHC03 and protein, as we
have seen. But this is only the first line of defense against acidosis, for
buffer substances present in the corpuscles may also be used. This intra-
corpuscular reserve of alkali is mobilized partly by transference of K
and Na from corpuscle to plasma, but mainly by that of HC1 from the
plasma into the corpuscle, so releasing base in the former to combine
with the added acid (e. g., H2C03), according to the equation:
H2C03 + NaCl ?=> NaHC03 + HC1. The HC1 on entering the corpuscle
reacts with phosphates according to the equation: HC1 + Na2HP04 ?±
NaH2P04 + NaCl. This is a particularly important detail of the buffer
action of the blood, for it shows us how the phosphates of the corpuscles
are rendered available for neutralizing acids added to the plasma, where
there are practically no phosphates. Indeed the transference of acid
through the corpuscular envelope indicates that the same sort of thing
must go on with the other cells of the body, so that the plasma, itself
rather poor in buffer substances, has all those of the body at its disposal.
ACIDOSIS 41
THE MEASUREMENT OF THE RESERVE ALKALINITY
1. Titration Methods
There are several methods by which the reserve alkalinity of the blood
may be measured. The simplest in theory consists in seeing how much
standard acid must be added to a measured quantity of blood plasma in
order to reach the neutral point as judged by change in tint of some
indicator. The indicators employed (e. g., methyl orange) are such as
change their tints at H-ion concentrations that are well to the acid side
of neutrality (i. e., at a high CH or low PH). To bring the plasma to this
point of neutrality the added alkali will need to neutralize, not only the
bicarbonate of the plasma, but other acid-binding substances as well.
This will give us a false impression of the acid-binding powers of the
plasma, since, at the normal CH of the blood, proteins do not absorb acids
to anything like the extent they do at higher degrees of CH. Another
objection to the method is that the proteins interfere with the sensitive-
ness of the indicators.
The objections can be removed by determining the end point electro-
metrically or by indicators that change tint at about PH7. The most
practical way is to determine the change in CH produced by adding a
known volume of standard acid to blood plasma. The resulting change
in CH will then be greater the less the alkaline reserve. In the electro-
metric method irregularities that might be caused by variable amounts
of carbonic acid in the blood to start with are best controlled by removing
the C02 from the plasma after adding the standard acid. The procedure
therefore consists in mixing 1 c.c. plasma with 2 c.c. N/50 HC1 in a small
separating funnel, which is then evacuated so as to remove the C02,
after which the fluid is transferred to a hydrogen electrode and CH
measured (see page 29). In normal blood this should be 10 5-6 (PH5.6).
In acidosis, where there is a depleted alkaline reserve, the 2 c.c. of acid
will cause a much greater change in CH — in diabetic blood to below 5
or lower.
The technic involved in the above method is, however, too exacting for
routine clinical work. For such purposes the colorimetric method of Levy
and Rowntree may be employed.
THE METHOD OF LEVY AND ROWNTREE.IS — A test tube made of hard
("nonsol") glass of about 20 c.c. capacity, containing about a gram of
powdered neutral potassium oxalate, is filled with newly drawn blood,
immediately stoppered and placed on ice. Quantities of 2 c.c. each of
the blood are then placed in a series of seven small (nonsol) test tubes
and allowed to stand for five to six minutes in order to permit a narrow
42 PHYSICOCHSMICAL BASIS OF PHYSIOLOGICAL PROCESSES
layer of plasma to separate on the surface (this prevents laking of the
blood during the subsequent addition of acid or alkali). The blood in
the first tube is used for the determination of the normal H-ion. In
each of the next three tubes are added respectively 0.1, 0.2 and 0.3 c.c.
N/50 HC1, and to the last three, similar quantities of N/50 NaOH. After
inverting the tubes so as to mix the contents, the blood in each is trans-
ferred to celloidin sacs and the CH determined according to the method
described elsewhere (page 32).
The tubes are noted in which a change in tint from that of the normal
blood is evident, and the results are expressed as the c.c. of N/50 HC1
or NaOH which must be added to blood to change its CH. Thus, the
alkali buffer is the c.c. of N/50 NaOH which can be added to 2 c.c. of
blood without change of CH of the dialysate, and the acid buffer the c.c.
of N/50 HC1.
The method suffers from the following drawbacks:
1. Very small quantities of acid and alkali are employed.
2. It is often difficult to tell just exactly when a slight difference in
tint has been produced.
3. Even with the precautions described above, it is impossible to be
sure that the amount of C02 in the different samples of blood is the same,
which means, of course, that some bloods will, on this account alone, be
able to bind more alkali than others.
THE METHOD OF VAN SLYKE. — A method based on somewhat the same
principle, but which is more accurate because it meets the above objec-
tion, is that suggested by Van Slyke, Stillman and Cullen.14 Plasma is
freed of CO2 by placing it in a vacuum, and, is then mixed w7ith an equal
volume of N/50 HC1 (or NaOH) and the CH determined by the electric
method (see page 29). In the case of normal blood, after such an addi-
tion of acid, a practically normal CH will be found, whereas in the blood
of cases of acidosis it will be very distinctly increased (i. e., PH lower).
2. C02-combining Power
The above objections to the titration of blood plasma or dialysate
with standard solutions of acids are removed if we measure the com-
bining power of the blood alkali towards carbonic acid itself at normal
blood reaction. This may be done either in blood immediately after its
removal from the animal or in blood that has been first of all saturated
outside the body with carbonic acid at a partial pressure equal to that
existing in the body. Since for practical reasons venous blood must be
used — in the clinic at least— the former of these methods suffers from
the fault that varying amounts of carbonic acid will be added to the
blood during its passage through the tissues, and the error thereby
ACIDOSIS
43
incurred will become greatly aggravated if venous stasis has been pro-
duced in drawing the specimen for analysis. But the chief reason why
this method has not been extensively employed, as pointed out by Van
Slyke, is the technical difficulty of making the necessary analysis.
It is most satisfactory to collect venous blood after a period (one hour
at least) of muscular rest (so that there is no excess of C02) and without
venous stasis, and to centrifuge without permitting any considerable loss
of carbonic acid. The latter precaution is necessary because there is a
migration of acid radicles, e. g., HC1, from plasma into corpuscles when
the C02 of the former is increased, and in the reverse direction when the
C02 is decreased. If the C02 in the blood were not the same during cen-
trifuging as it is in the body, the separate plasma would not contain the
same amount of alkali — i. e., its reserve alkalinity would be altered.
Although theoretically, therefore, centrifuging should be performed in
Fig. 10. — Diagram of apparatus for saturating blood or plasma with expired air. The glass
beads in the bottle condense excess of moisture. The separating funnel, as soon as it has been
filled with expired air, should be closed by a stopper and the stopcock turned off. It is then
rotated so that the blood forms a film on its walls.
an atmosphere containing the same partial pressure of C02 as exists in
the body (i. e., the alveolar air) (see page 344), this has been found im-
practicable for general use, and is unnecessary if loss of C02 from the
specimen of blood is prevented by allowing it to flow into the syringe
very slowly (without any suction). It is mixed in the syringe with
powdered (neutral) potassium oxalate (enough to make a 1 per cent
solution with the blood), and immediately delivered into a centrifuge
tube under paraffin oil, which by floating on its surface serves to diminish
free diffusion of C02 to the outside air (even though such oils dissolve
more C02 than water). To mix the blood with the oxalate, the syringe
should be moved backward and forward several times, but it must not b*1
shaken.
After centrifuging, about 3 c.c. of plasma are removed and saturated
with C0? at the same tension as in alveolar air (i. e., 5.5 per cent).
44
PHYSICOCHKMICAL BASIS OF PHYSIOLOGICAL PROCESSES
is done by placing the plasma in a separating funnel of 300 c.c. capacity,
laying the funnel on its side and displacing the air in it by alveolar air
secured by quickly making as deep an inspiration as possible through
the tube and bottle containing glass beads (Fig. 10). The glass beads
remove excess of water vapor from the air. The funnel must be restop-
pered before the end of the expiration, so that no outside air enters. It
is then rotated, for about two minutes, in such a way that the plasma
forms a film on its walls. If it is necessary to postpone the saturating
of the plasma, this should be pipetted off from the corpuscles and pre-
served in hard glass test tubes coated with paraffin. From ordinary glass
Fig. 11. — Van Slyke's apparatus for measuring the COa-cotnbining power of blood in blood plasma.
For description, see context.
enough alkali is soon dissolved out to vitiate the results. After saturation
of the plasma with C02, the funnel is placed in the upright position and
the plasma allowed to collect in the narrow portion, after which 1 c.c.
is removed with an accurate pipette and analyzed for C02.
The analysis may be done by using either the Van Slyke or the Hal-
dane-Barcroft apparatus. The Van Slyke method is as follows:
The apparatus is filled to the top of the graduated tube with mercury
(Fig. 11) by raising the mercury reservoir F, care being taken that
D and E are also filled. One c.c. of the CO2-saturated plasma is then de-
ACIDOSIS 45
livered into A (which has been rinsed out with C02-free ammonia water),
and the stopcock / turned so that by cautiously lowering the level of the
reservoir F, the plasma runs into B (but no trace of air). The same
procedure is repeated with 1 c.c. water, so as to wash in all of the plasma,
and finally 0.5 c.c of 5 per cent H2S04 is sucked in, after which stopcock 7
is turned off. The reservoir F is then lowered sufficiently to allow all
of the mercury, but none of the blood, to run out of B and C. A vacuum
is thus produced in B and C.
As the level of the mercury falls in B and C, the plasma effervesces vio-
lently,* because it is exposed to a vacuum. To be certain that all traces of
C02 have been dislodged from the solution, the apparatus is inverted
several times. To ascertain how much C02 has been liberated, stopcock //
is now turned so as to bring C and E into communication, and by cautiously
lowering the reservoir the fluid in C is allowed to run into the bulb E.
Stopcock II is thereafter turned so as to connect C and D, and the reser-
voir raised so that the mercury runs into C as far as the C02 that has col-
lected in the burette will permit it to go. After bringing the level of the
mercury in F to correspond to that in the burette, the graduation at which
this stands is read. It gives the c.c. of C02 liberated from the plasma.
Under the above conditions normal plasma binds about 75 per cent of
its' volume of C02 ; therefore, since the total capacity of the pipette is 50
c.c., the mercury should stand at 0.375 c.c. on the burette. For accurate
measurement it is necessary to allow for the C02 that remains dissolved
in the water, etc., as well as for barometric pressure and temperature.
This is best done by the use of a table based on the known solubility of
C02 under the various conditions obtaining, which is given in Van
Slyke's paper.12
The Haldane-Barcroft apparatus that is most suitable for the above
analysis is shown in Fig. 136, page 382. t One c.c. of C02-free ammonia
water is placed in the bottle and the 1 c.c. of plasma delivered beneath it.
*This may be prevented by adding a small drop of caprylic alcohol.
tThis form of Haldane-Barcroft apparatus is not quite the same as the differential manometer
that is used for measurement of the (^-combining power of hemoglobin (page 382). In the form
used for the present purpose, a side tube at the bend of the U-tube is connected with a small rub-
ber bag, which Ann be compressed by a screw. When the gas is evolved in the bottle, it presses
down the fluid in the proximal limb of the manometer correspondingly and raises that in the distal
limb. Since the calculation of the amount of gas evolved depends on finding the pressure produced
without any change in volume, it is necessary after the gas has been evolved to compress the rubber
bag until the meniscus of fluid in the proximal limb of the manometer is brought back to its original
level. The height at which the fluid stands in the distal limb then obviously corresponds to the
pressure created by the evolved gas.
The equation for determining the amount of gas evolved depends on the gas law, which states
that the pressure of a gas is inversely proportional to its volume (page 336). Suppose that the
volume of gas evolved was equal to the volume of the bottle, then, since the volume has been
kept constant, the pressure would be doubled — that is, the fluid in the distal limb would equal that
of 1 atmosphere, or 10,400 mm. of water or 10,000 of clove oil, which is the fluid actually used to
fill the manometer. Any other observed pressure would therefore correspond to the volume of
evolved gas according to the equation,
vol. of bottle (and tubing to meniscus)
10,000 (when clove oil is used)
_ In using the apparatus in the above manner, only one of the bottles is employed, and the tartaric
acid is added from a pocket in the stopper by a simple manipulation.
46 PHYSICOCH^MlCAL BASIS OF PHYSIOLOGICAL PROCESSES
The bottle is then connected with the manometer with the precautions
described elsewhere in this volume. "When temperature conditions have
been alloAved for, saturated tartaric acid is mixed with the plasma solu-
tion and the gas evolved measured by the displacement of the fluid in the
manometer. The apparatus may also be used with blood in place of
plasma. In this case, however, it is necessary that the oxygen be removed
before adding the tartaric acid. This precaution is necessary, since acid
can dislodge some of the 02 from hemoglobin. The blood is therefore first
of all laked with ammonia containing some saponin, then shaken with
0.25 c.c. saturated potassium ferricyanide solution, and finally with the
saturated acid solution. If blood is used, the estimations must be made
on strictly fresh blood, since on standing the C02-combining power
greatly deteriorates.
3. Indirect Methods
There are several other methods by which the alkaline reserve may be
measured. These include:
1. Determination of the Tension of C02 in Alveolar Air (page 344).—
Since this method is employed more particularly in investigating the
hormone control of the respiratory center, we shall defer a description
of it until later. The alveolar C02 tension corresponds to the C02 ten-
sion in arterial blood and this is proportional to the alkaline reserve as
determined by Van Slyke's method as is proved by the fact that the ratio,
plasma C02 . , . ,, .,
. — -.. : — , is satisfactorily constant.
alveolar C02 tension
2. The Measurement of the Acid Excretion by the Kidney. — As might
be expected, the acid-base equilibrium of the body may also be gauged by
measurement of the acid excretion of the urine, in which the acids are
contained partly in combination with ammonia or a fixed base, and partly
in a free state. We shall first of all consider the methods 'of acid
excretion and then examine the evidence showing that the total acid
excretion is proportional to the alkaline reserve as measured by the
above described methods.
EXCRETION OF ACID. IN COMBINATION WITH AMMONIA. — The production
of ammonia is essentially an endogenous process, and when excessive
quantities of acid make their appearance in the organism, the fixed alkali
may not be sufficient to neutralize it all, so that ammonia, derived from
the breakdown of amino acids (page 616), instead of being converted
into urea is employed to neutralize the excess of acid. Most workers
have in this way explained the very large ammonia excretion that has
long been known to occur in such conditions as diabetic acidosis. Some
recent workers are, however, inclined to question the significance of
ammonia in this connection, believing that the increased ammonia ex-
cretion is, like the acetone bodies themselves, a product of perverted
metabolism. Be this as it may, it is no doubt true that ammonia is used
for neutralizing acid in disease, although it may not be an important
factor in the maintenance of neutrality under normal conditions. It is
a factor of safety, in that it helps to care for an increase in acid when
the normal mechanism of the body is overtaxed.
EXCRETION OF PHOSPHATES. — The more permanent control of neutrality
depends on the excretion of phosphates by the kidney. The principle
governing this process is exactly the same as that already discussed in
connection with carbonic acid. In the one case it is the volatile acid
C02, and in the other, the fixed phosphoric acid that is concerned in the
reaction. The ratio between the acid salts of phosphoric acid, MH2P04,
and the alkaline salts, M2HP04, in blood is approximately 1 to 5, but in
the urine this ratio varies according to the amount of H ion that must
be eliminated from the blood. In other words, a definite amount of phos-
phoric acid is enabled to carry variable amounts of H ion out of the body
by causing the amount of alkali excreted in combination with it to be-
come altered. For example, in the form of MH2P04 a given amount of
P04 obviously carries out more H ion than when it is excreted as
M2HP04. The adjustment between these two salts is a function of the
kidney. We may accordingly measure the amount of alkali retained by
the organism by finding how much standardized alkali must be added
to a given quantity of urine until the reaction of the blood is obtained.
Since the latter value is constant, the titration can be done simply by
titrating the urine with an indicator such as sulphonephenolphthalein,
which changes tint at about PH of blood.
A more serviceable indicator to use, however, is phenolphthalein, be-
cause its end point is such that when human urine just reacts neutral
to it — that is, when the titrable acid approaches zero — the C02-absorb-
ing power of the plasma is at its maximum of 80 vols. per cent and the
ammonia excretion by the urine is zero (Van Slyke). It is advantageous,
therefore, to use this indicator, because it happens to have its turning
point situated for a reaction which is well to the alkaline side of neu-
trality, and which is reached in urine when the blood is at its maximal
acid-combining power and no ammonia is being used for neutralization
purposes. As the C02-combining power of the blood decreases, there
should, therefore, be a proportionate increase in ammonia and in the
titrable acidity of the urine.
Although a general parallelism exists between these values in cases of
diabetes, etc., there is no strict proportionality. The expedient has
therefore been tried of comparing the alkaline reserve of the blood with
48 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
the excretion rate of acid as determined by an application of Ambard's
equation for chlorides and urea, and with curiously satisfactory results
(Fitz and Van Slyke). This equation is:
Blood concentration = constant x V^VC; where D is the excretion
rate, W the body weight, and C the concentration of excretory prod-
uct in the urine. For the present purpose D is therefore the number of
c.c. of N/10 alkali (or acid) required to bring the urine to the neutral
point of phenolphthalein plus the NH3 expressed as c.c. of an N/10 solution,
for the twenty-four hours, and C is c.c. of N/10 alkali and of N/10 NH3
per liter of urine. If we assume that the acid accumulation in the blood
is proportional to the fall of the plasma C02 figure below the maximal
figure of 80, the above equation becomes:
/ L) _
Retained acid = 80 - plasma C02 = constant x y \\fVC.
For practical purposes it is best to make the necessary analysis on a
sample of urine collected over a period of one to four hours, and to col-
lect the blood for determination of its reserve alkalinity in the middle
of this period. The twenty-four-hour rate of excretion is then computed
(D) from the analysis.
The value calculated by the above equation has been found to agree
with that of the C02-combining power of the plasma to within 10 vol-
umes per cent, except when bicarbonate is being taken by the person,
when the blood bicarbonate is much higher than indicated by the urine.
3. Determination of Alkali Retention. — Another valuable criterion of
the alkaline reserve is the amount of alkali required to change the re-
action of the urine. In health the CH of the urine varies from
0.000,016 N (PH = 4.8) to about 0.000,000,035 N (PH = 7.46) with a mean
of about 0.000,001 N (PH = 6). These extremes are rarely overstepped
in disease, but frequently the average is considerably different. In car-
dio-renal disease, for example, the mean acidity may be approximately
0.000,005 N (PH = 5.3), or five times the normal value. A certain de-
gree of acidosis is therefore common enough in this condition — a fact
which has indicated the advisability of administering sodium bicarbon-
ate. It has been found that 5 grams or less of soda, given by mouth to
a normal person, causes a distinct diminution in the CH of the urine,
whereas in pathologic cases it may be necessary to give more than 100
grams before a similar effect is observed (L. J. Henderson and Palmer15
and Sellards16).
For this very large holding back of alkali, the organism and not the
kidney is responsible. This is indicated by the fact that, when the
administration of alkali is discontinued, the acidity of the urine soon
ACIDOSIS 49
regains its old level, although now if a smaller dose of alkali is given,
the CH of the urine will immediately be lowered. These facts indicate
that for the moderate degrees of acidosis common in chronic disease, the
properly controlled administration of soda is very probably a most advan-
tageous treatment.
CHAPTER VII
COLLOIDS
Substances which can be obtained in the crystalline state and which,
when "in solution, are capable of readily diffusing through membranes,
are designated as crystalloids, and are to be distinguished from another,
larger group of substances not having these characteristics or having
them only in very minor degree — the colloids. In every field of chem-
istry the properties of colloids have been studied extensively during
recent years, but in no field more than in that which covers the chem-
istry of biological fluids and tissues, into whose composition colloids
enter much more extensively than crystalloids. The subject of colloidal
chemistry has indeed become so extensive that an attempt to do more
than indicate some of the most important characteristics of colloids
would take us far beyond the limitations of this book. The far-reaching
applications of the subject in physiology and medicine are only begin-
ning to be realized.
The term "colloid," or "colloidal," does not refer to a class of chemical
substances, but rather to a state of matter which is quite independent
of the chemical composition of the substance. We are familiar with
more colloids in the organic than in the inorganic world, yet they are
plentiful in both, and the same substance may at one time be colloidal
and at another noncolloidal. Indeed, under appropriate conditions prob-
ably all substances may assume the colloidal state — not solids and liq-
uids alone, but gases as well. It is mainly with liquids, however, that
we are concerned in biochemistry.
CHARACTERISTIC PROPERTIES
The distinction between molecular* and colloidal solutions is a rela-
tive one. Suppose, for example, that we take a piece of gold in water
and divide it up into smaller and smaller parts. At a certain stage, the
particles will be so fine that they will remain in suspension and be in-
visible, by ordinary means. They are then said to be in the colloidal
state. If we divide 'them further until they become molecules of gold,
a molecular solution will be obtained. In the colloidal state, there are
*Molecular solutions include those of nonelectrolytes, such as sugar, and electrolytes, such as
inorganic salts.
50
COLLOIDS 51
two distinct phases in the solution, one solid and the other liquid, and
between the two, because of the great subdivision of the original par-
ticle, is an enormous surface of contact. The solution is heterogeneous,
and at the interface between the two ' ' phases ' ' the physical forces which
depend on surface — e. g., surface tension (see page 64) — are enormously
developed, and are responsible for the peculiar properties of colloidal
solutions as compared with those of molecular solutions, which may,
therefore, be styled homogeneous. The solutions of crystalline substances
which we have hitherto been concerned with, are homogeneous.
Between these two groups of solutions is an intermediate one — namely,
suspensions (as suspensions of quartz or carbon, or oil emulsions). Be-
sides being turbid in transmitted light, the solutions may be seen by
means of the ultramicroscope to contain particles. These can be sepa-
rated by filtration from the fluid they are suspended in, except in the
case of many emulsions in wrhich the particles can squeeze their way
through the filter pores by changing their shape. On standing or being
centrifuged suspensions may also separate into their constituents, al-
though this can be greatly hindered by the addition of a suspending
substance such as gelatin or certain bodies having a so-called protec-
tive action (as peptone, proteose, etc.).
True Colloidal Solutions
1. The Solution Is More or Less Turbid. — Frequently this can be recog-
nized by holding the solution in a thin-walled glass vessel against a
dark background, but the turbidity may be so slight that it requires
for its detection the use of the Tyndall phenomenon. This is familiar
to all in the effect of a beam of sunlight let in through a small aperture
into an otherwise darkened room. In the course of the beam suspended
dust particles, which are invisible in an equally illuminated room, be-
come visible, and thus render very distinct the pathway of the beam.
If a colloidal solution contained in a glass vessel, preferably with paral-
lel sides, is held in the course of such a beam, the Tyndall phenomenon
will be seen in the liquid, which is not the case with molecular solutions.
Focused artificial light may be employed for intensifying the effect.
The light that is sent out at right angles to the beam is plane-polarized,
which means that the particles reflecting the light must be smaller than
the mean wave length of the light forming the beam. It should be under-
stood that the individual particles themselves may not be rendered
visible to the naked eye by the beam, although in such cases they can
often be seen by using intense illumination and a dark-field (ultramicro-
scope) combined with suitable magnification (Fig. 12).
2. Colloids Do Not Readily Diffuse. — To demonstrate this, test tubes
52 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
are half filled with a 5 per cent solution of pure gelatin or a 1 per cent
solution of pure agar, and, after the jelly is set, the solution under
examination is poured on the surface; or, when it is of high spe-
cific gravity, the tube of gelatin, etc., is placed mouth downwards in
the solution. In the case of colloidal solutions very little if any diffu-
sion into the gelatin or agar will occur, even after several days; whereas
true molecular solutions will diffuse for. a considerable distance. When
colored solutions are used, the diffusion can readily be recognized by
visual inspection (see Fig. 13), but when they are colorless, the presence
or absence of diffusion must be determined by removing the column
of gelatin or agar and dividing it into slices of equal size, which are
then examined chemically for the substance in question.
A further test is afforded by the failure of colloids to diffuse through
membranes (dialysis). This was the method originally used by Thomas
Graham to distinguish between molecular and colloidal solutions. The
solution under examination is placed in a dialyzer, which is then im-
mersed in a wide vessel containing the pure solvent. The older forms
Fig. 12. — Ultramicroscope (slit type) for the examination of colloidal solutions. The arrange-
ment of diaphragms, etc., in this form removes the absorptive effects of the surfaces of the glass
vessel or slide used to contain the colloidal solutions.
of dialyzer consisted in general of a bell-shaped glass vessel closed be-
low with parchment paper, but more recently so-called diffusion sacs
have been adopted. These consist of pig or fish bladders or of col-
lodion sacs. The latter are made by placing some collodion dissolved
in ether in a test tube, which is then tilted so that the collodion runs
out except for a thin layer which remains adherent to the walls. When
the collodion has set, the sac can be removed after loosening it by allow-
ing a little water to flow between the sac and the walls of the test tube.
The sac containing the colloidal solution is then suspended in water
or some of the solvent used in preparing the colloidal solution, care
being taken that the menisci of the fluids inside and outside of the sac
stand at the same level. Sometimes, especially when collodion sacs are
used, some colloid may at first diffuse through, but if the outer fluid
(the dialysate) is renewed and the dialysis allowed to proceed, this
ceases.
COLLOIDS 53
When a fluid solution exhibits both of the above properties (i. e., the
Tyndall phenomenon and indiffusibility) , there can be no doubt as to its
being in a true colloidal state, but there are substances, such as congo
red or protein solutions of certain strengths, which may exhibit a very
slight diffusibility in a dialyzer but not show the Tyndall phenomenon.
Substances of this group constitute transitional types between molecular
and colloidal solutions, and to determine their true nature it is neces-
Fig. 13. — To show diffusion into gelatin of a crystalloid stain in b and the nondiffusion of a
colloid stain in a. (From W. Ostwald.)
sary to employ refined methods such as those of ultramicroscopy, ultra-
filtration, etc., which can not be described here.
3. The Size of Colloidal Particles. — It will be apparent that the essential
property upon which the above-mentioned phenomena depend is the size
of the particle. Particles which can still be seen under the microscope
are called microns. They have been computed to have a dimension of
0.1 /u, (0.001 mm.) or more, and they form suspensions. Particles which
are invisible microscopically under the ordinary conditions of illumina-
54
PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
tion, but are still visible when the ultramicroscopic illumination is
used, are called submicrons. They have a dimension between 0.1 p. and
1 fjLfjt. (0.000,001 mm.),* and they constitute the colloids. Particles smaller
than 1 fjifj, are called amicrons, this term being used to include the mol-
ecules and ions present in molecular solutions. (The amicron of hydro-
gen is, for example, computed to be 0.067 to 0.159 p,^, and that of water
vapor, 0.113 /x/u,.) This classification of dissolved substances according
to the size of the particles and molecules shows the relationship of one
Fig. 14. — Diagram from W. Ostwald showing the relative size of various particles and colloidal
dispersoids compared with a red blood corpuscle and an anthrax bacillus.
class of substances to others. An idea of the relative sizes of colloidal
particles and molecules in comparison with. such familiar objects as a
blood corpuscle and an anthrax bacillus is given in Fig. 14. The fluid
in which the "particle" is suspended is called the dispersion medium, or
external phase, and the particle itself the dispersoid, or internal phase.
It is the enormous development of surface which determines the dif-
*H — 0.001 mm., and up, = 0.000,001 mm.
COLLOIDS 55
ference in the properties of a colloidal solution from those of a suspen-
sion of the same substance. Thus, the difference between a colloidal
solution of platinum (prepared by allowing an electric arc to form be-
tween platinum electrodes in water) and pieces of platinum in water
depends on the fact that the surface of the platinum in the former case
has been increased many million times. When the subdivision becomes
still greater and the particles gain the size of molecules, the phenomena
due to surface development become suppressed and those due to con-
centration in unit volume become accentuated. The properties depend-
ent on osmotic pressure, diffusibility, etc., are exhibited by all dispersoids,
whether ions, molecules or particles, but some of these properties are
much more pronounced when the dispersoids are of large dimensions,
and others when they are small. In other words, the phenomena due to
surface, such as those of surface tension (see page 64), become apparent
only when the dispersoids have the properties of matter in mass; when
the dispersoids become molecular in size, they manifest the properties
characteristic of true solutions.
4. Electric Properties of Colloids. — Most colloids carry a charge, which
may be either positive or negative toward the dispersion medium. Both
crystalloids and colloids therefore .carry electric charges; in the former
case, however, the charge does not reveal itself until the molecules in
solution have become dissociated, when each ion carries a charge of
opposite sign (see page 16), whereas1 in the case of colloids, each col-
loid particle usually carries a charge which is always of one sign, either
positive or negative. Colloids may therefore be grouped into positive
and negative, according to the charges which they carry, and there is
a third group in which the charge may be either positive or negative ac-
cording to the nature of the dispersion medium.
A colloid not carrying a charge to begin with can be caused to assume
one by the action of electrolytes, for the electrical properties of colloids,
as well as those of inert powders suspended in water, are readily in-
fluenced by the charges present in the ions of the dispersion medium.
The II • and OH' ions are especially liable to exert this influence. The
particles of inert powders in suspensions (kaolin, sulphur, etc.) carry
a positive charge when the water- in which they are suspended is acidi-
fied, and a negative charge \vhen it is made alkaline. In general, it may
be said that suspensions of most powders and of insoluble organic acids
in water (e. g., charcoal, cellulose, kaolin, caseinogen, mastic, free acid
of congo red, etc.) are electro-negative. Of true colloids ferric hydrox-
ide (ferrum dialysatum) and serum globulin are positive in acid solu-
tions; arsenious sulphide and serum globulin are negative in alkaline
solution, and serum globulin in neutral solutions has no charge.
56 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
To ascertain the nature of the charge various methods may be em-
ployed, of which the following are important:
1. The method of electrophoresis. The colloid solution is placed in a
U-tube, each side of which carries a platinum electrode dipping into the
solution. After a strong continuous electric current has been allowed
to pass for some time through the solution, it will be found that the
colloid collects at the anode (where the current enters) when it is a
negative colloid (since unlike electric charges attract each other), and
at the cathode when it is positive. In the case of colored solutions, the
migration can be readily seen, but otherwise it may be necessary to ana-
lyze the solution at the two poles.
Fig. IS. — Capillary analysis of colloids. Strips of filter paper, after being suspended with
the lower ends dipping into colloidal solutions. Those on the right hand were positive colloids,
Which did not rise in the strips, but formed a sharp line of demarcation at • the lower end on
account of precipitation. Those on the left hand were negative • colloids. (From W. Ostwald.)
2. The method of capillary analysis. For this purpose a long strip of
filter paper is arranged vertically over the solution, with its lower end
dipping into it. In the case of negative colloids the colloid, as well as
the dispersion medium, rises uniformly on the strip of paper (it may be
to a height of 20 cm.) ; whereas with positive colloids the dispersion
medium alone rises, the colloid itself doing so only to a very, slight ex-
tent, but becoming so highly concentrated at the interface between the
solution and the paper that it coagulates on the end of the strip of paper,
where it forms a sharp line of demarcation (Fig. 15).
3. The method of mutual precipitation of colloids. When a positive
COLLOIDS 57
and a negative colloid are mixed in such proportions that the electric
charges are neutralized, precipitation usually occurs. When it does so,
we can tell the nature of the electric charge of an unknown colloid by
its behavior when a colloid of known electric sign is added, to it. For
example, if ferric hydroxide (positive) causes a precipitate to form
when it is added to an unknown colloidal solution, the electric charge
of the latter must be negative; if it does not precipitate with ferric
hydroxide, but does so with arsenious sulphide (negative), it must be
positive.
5. Brownian Movement. — Like the particles in fine mechanical suspen-
sions, those of colloidal solutions, especially when examined ultra-
microscopically, exhibit the so-called Brownian movements, which have
been described as "dancing, hopping and skipping." These movements
occur in straight lines, which are suddenly changed in direction and
are quite independent of external sources of energy, such as change in
temperature (although they become quicker as the temperature of the
solution is raised), earth vibrations, chemical changes, or the electric
charge of the colloid. The movements become more rapid the smaller the
particles, and they become sluggish as the viscosity of the solution in-
creases. Addition of electrolytes decreases the movement by causing the
particles to clump together. The density and viscosity of the disper-
sion medium, the electric charge of the dispersoid and the presence of
Brownian movements, are the forces which operate together to prevent
sedimentation of the particles in a colloidal solution.
6. Osmotic Pressure. — As one of the distinguishing properties of col-
loids we have seen that their diffusibility, as into gelatin or agar jel-
lies, is extremely slow when compared with that of a molecular solution.
This does not mean, however, that colloids are possessed of no power of
diffusibility if left long enough. Indeed the existence of the Brownian
movement indicates that such diffusion must occur, and therefore it
should be possible, by the application of the same principles as those
which govern molecular solutions (e. g., by using a semipermeable mem-
brane), to measure the osmotic pressure.
Many studies of the osmotic properties of colloidal solutions have been
undertaken, especially by those who are interested in the possibility
that the colloids of blood serum (serum albumin and globulin) may cre-
ate an osmotic pressure. If this should prove to be the case, it would
be necessary for the osmotic pressure to be overcome by mechanical
pressure such as that supplied by the heart (i. e., the blood pressure) in
the various physiologic processes of filtration and diffusion taking place
through cell membranes (as in the formation of urine in the kidney).
For measuring the osmotic pressure of colloids, osmometers similar
58 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
to those already described (page 4) can be employed. Most of the
recent work has been done either with collodion sacs, or with unglazed
clay cups impregnated with some gel, such as silica cr gelatin. When
such an osmometer, filled with some colloidal solution (like a solution of
pure albumin) and provided with a vertical glass tube, is placed in an
outer vessel containing water, the fluid will be seen to rise in the ver-
tical tube, the height to which it rises being proportional to the osmotic
pressure.
But the observed pressure does not necessarily give us the osmotic
pressure of the pure colloid, for to this, even when highly purified, there
is almost certain to be attached a considerable amount of inorganic
salt, which may be responsible for the osmosis. It has indeed been
maintained by some observers that electrolytes form an integral part
of certain colloids, being bound to them perhaps by adsorption (see
page 65), and that they are essential to the maintenance of the colloidal
state. In any case, since electrolytes are always present, the osmotic
pressure of the pure colloid can be measured only when means are
taken to discount their influence. Several devices have been used, of
which the following may be mentioned:
1. Addition to the fluid outside the osmometer of a percentage of
salt equal to that found by chemical analysis to be present in the col-
loid. (This method is untrustworthy.)
2. The use of a limited quantity of fluid on the outside of the osmom-
eter so that equality of saline content soon becomes established, by
diffusion, in the fluids on the two sides of the membrane.
3. The use of a membrane which is permeable to electrolytes but
not to colloids.
Even when the greatest care is taken in its measurement, the osmotic
pressure of a given colloid has been found to vary considerably not
only according to the method used in its preparation, but also accord-
ing to the amount of mechanical agitation (shaking, stirring, etc.) to
which the colloid solution has been subjected. Regarding the influ-
ence of the method of preparation, it was found in one series of experi-
ments that albumin that had been repeatedly washed (but still con-
tained considerable ash) gave no osmotic pressure, whereas another
preparation that had been purified by crystallization tAvice (and con-
tained much less ash) had a pressure of 3.38 mm. Hg. According to
these results the ash content of the colloid is not fundamentally re-
sponsible for its osmotic pressure. As to the influence of mechanical
agitation, the osmotic pressure of a gelatin solution is increased by
shaking, while that of a solution of egg albumin is. decreased.
The property upon which the osmotic pressure depends is undoubtedly
COLLOIDS 59
the state of dispersion of the colloid particles, and until we know all of
the factors which may influence this, measurements of osmotic pressures
of colloids can scarcely be of very much value. Nevertheless, that this
property has some physiologic bearing is clear from the effect which col-
loids have in restoring the blood pressure after hemorrhage (page 141).
Further evidence that the osmotic pressure of colloids has not the
significance that it has in the case of molecular solutions is furnished by
the fact that the osmotic pressure is only approximately proportional
to the concentration of the solution; it may either increase or decrease
relatively to 'the strength of the solution. Temperature also has quite
a different influence on the osmotic pressure of colloids from that which
it has on the osmotic pressure of molecular solutions, and it frequently
has an influence which persists after the solution is brought back to its
original level.
The influence of added substances on the osmotic pressure of colloidal
solutions is of considerable interest to the biologist, for, whereas in the
case of molecular solutions this is purely additive, in the case of col-
loids the added substance may at one time cause the osmotic pressure to
increase, at another, to decrease. It has been found that the osmotic
pressure of gelatin solutions at first decreases, then rapidly increases as
the H-ion concentration is raised. The addition of alkali increases the
osmotic pressure until a maximum is reached, beyond which it begins to
fall. Both acids and alkalies lessen the osmotic pressure of egg albu-
min. Electrolytes always decrease the osmotic pressure of gelatin and
albumin solutions, and the degree to which they exert this influence
depends on the nature of the cation and anion composing the electrolyte.
In the order of their depressing influence the cations arrange them-
selves:
Heavy metals > alkaline earths > alkalies;
and the anions:
S04 > Cl > N02 > Br > I > CNS.
The influence of a given electrolyte varies extraordinarily with the reac-
tion of the colloid, a fact which must be carefully regarded in all work
in this field.
! CHAPTER VIII
COLLOIDS (Cont'd)
SUSPENSOIDS AND EMULSOIDS
According to whether colloids form solutions that are more or less
viscid than the suspension medium, they are divided into emulsoids and
suspensoids. Examples of the former class are silicates and gelatin, and
of the latter, dialyzed iron and arsenious sulphide. The following char-
acteristics are used to distinguish between suspensoids and emulsoids:
1. Measuring the time it takes, at a standard temperature, for a given
volume of the fluid to flow out of a standard pipette (10 c.c.) shows the
viscosity to be, roughly, inversely proportional to the time of outflow. In
the case of suspensoids the viscosity is no different from that of the
dispersion medium alone, and does not vary much when the solution is
cooled. The viscosity of emulsoids even in very dilute solutions is, on
the other hand, considerably greater than that of the dispersion medium
itself, and it becomes greatly increased by cooling.
2. Suspensoids are much more readily coagulated by the addition of
electrolytes than emulsoids. This is particularly true when water is
the dispersion medium (so-called hydrosols), and when electrolytes hav-
ing a polyvalent ion (such as Al or Mg.) are employed. Thus, practically
all suspensoids are coagulated in the presence of 1 per cent of alum,
which has no influence on emulsoids. We shall return to this phase of
our subject later on.
The division of colloids into emulsoids and suspensoids is more or less
arbitrary, since one class may be changed into the other, the determining
factor being the water content of the dispersoid. The water content of
suspensoids is low (lyophobe), while that of emulsoids is high. By
changing the relative amounts of water and solid of which a colloidal
solution is composed, the nature of the dispersoid may be changed. If
the water is diminished, the dispersoid behaves as a suspensoid and be-
comes readily precipitated. The practical importance of this fact is
that it explains the salting out of proteins — a process extensively used
in their separation. Ordinarily these behave as emulsoids, but the addi-
tion of salt raises the osmotic pressure of the dispersion medium, and
thus attracts water from the dispersoids, with the result that they come
60
COLLOIDS
61
to behave as suspensoids, and are accordingly precipitated by the elec-
trolytes.
Another property of emulsoids of biological' importance is the pro-
tection which they can afford against the precipitating influence of
electrolytes on suspensoids. If a colloidal solution of gold is mixed with
a trace of gelatin, the subsequent addition of salts will be found to
produce no precipitation. The explanation of this is that the emulsoid
becomes distributed as a film on the suspensoid particles, thus practically
converting them into emulsoids.
Gelatinization
One of the best known properties of emulsoids is that of gelatiniza-
tion, which has an interesting bearing on many problems of biology.
After the gel has set, an enormous pressure is required to squeeze out
any water from it, indicating that the water no longer forms the con-
tinuous phase but must be enclosed in vesicles formed of more solid
material.
Fig. 16.
As a gelatin solution cools, the gel at first forms a polarized cone of
light, but the very fine particles which are responsible for this effect
soon increase in number and size so that they obstruct one another in
their Brownian movements and adhere, giving an appearance of fine
felt-like threads throughout the solution. A sort of impervious sponge
work of the more solid phase is therefore formed, the more fluid phase
being inclosed in the meshes.
If, as in the accompanying diagram, the dispersion medium is repre-
sented by white and the dispersoid in black, the relationship between
the two in a suspensoid is as in A, and that in a gel as in B. To express
any of the dispersion medium in B, it will require a pressure sufficient to
62 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
cause the more fluid phase to penetrate the more solid. If the gel is
treated with reagents like formaldehyde, the liquid can be readily pressed
out. This occurs during fixation for histological purposes.
Imbibition
Closely related to gel formation is the process of imbibition — the
power of taking up large quantities of water without actually forming
liquid solutions. Besides gelatin the dried tissues of plants and animals
exhibit the phenomenon, and it is undoubtedly of importance in many
physiologic processes such as growth and the passage of water into
cells, etc. The materials present as vacuoles in plant cells attract water
from the environment of the cell by imbibition, and thus exert on the
cell wall a pressure which, acting along with the osmotic pressure,
maintains the turgor of the cell. The initial growth of pollen is also
dependent upon imbibition, and important observations on this process
under varying conditions, are likely to furnish us with useful informa-
tion concerning the significance of imbibition in connection with growth
of cells in general.
By measuring the rate of increase in length of long, narrow strips of
gelatin placed in Petri dishes containing solutions of varying composi-
tion, the factors that influence the imbibition process can be -quantita-
tively investigated. Working in this way, F. H. Lloyd17 has found that
for all acids there is a certain concentration (about N/320 H2S04) which
Induces a maximum rate of swelling, and another, much weaker
(N/2800 H2S04), in which the rate of swelling is even less than in pure
\vater. In higher concentrations of acid than N/320, the gelatin at first
swells very quickly, but the rate slows off so that it soon comes to be
less than that with intermediate concentrations. Regarding alkalies,
at high concentrations the effect is similar to that of acids. Salts alone
seem to repress the swelling below that of water. It should be pointed
out that the concentrations of acid and alkali in the above observations
are much greater than those that could occur in the animal body. The
experiments recall the attempts made some years ago by Martin Fischer
to explain edema as due to excessive imbibition of water by the pro-
teins of the tissues because of increased acidity of the blood and tis-
sue fluids. That imbibition might possibly play some role in such
processes is not denied, but Fischer disregards entirely the now well-estab-
lished facts that hydrogen-ion concentration is one of the most constant
properties of the blood, that very low concentrations of acid may dimin-
ish rather than increase imbibition, and that it is manifested only in
the absence of inorganic salts.* Moreover, the fluid in edema can often
'Determinations of the hydrogen-ion concentration of the blood recently published from Fischer's
laboratory do not inspire confidence.
COLLOIDS 63
be drained off by hollow needles, and it passes by gravity from one part
of the blood to another, neither of which processes would be possible
if imbibition were the essential factor concerned. If further evidence
against this hypothesis should be demanded, it might be found in the
utter failure of the therapeutic measures — alkali administration — that
are recommended to combat the edema.
Action of Electrolytes on Colloids (apart from their effect on osmotic
pressure). — It has been stated above that the charge which a colloidal
particle assumes may be neutralized by a charge of opposite sign car-
ried by an ion present in the dispersion medium. The neutralization
of the electric charge causes coagulation of the suspensoids but not of
the emulsoids. Of the positive and negative ions into which the elec-
trolytes dissociate, the one producing the coagulation is that which is
opposite in sign to the electric charge of the colloidal particle.
A quantity of electrolyte which is capable of producing complete pre-
cipitation when added all at once to suspenroids will be ineffective when
added in small quantities at a time. This phenomenon, which is also
known to be exhibited when toxins and antitoxins are mixed together, is
probably owing to the fact that precipitation depends on inequality and
irregular distribution of electric charges, a condition which becomes
established when the electrolyte is suddenly added, but not so when it
is gradually added. The particles in the latter case become, as it Avere,
acclimated to the electric charges introduced by the addition of the
electrolyte.
Proteins as Colloids. — The most prominent colloids in the field of bio-
chemistry are the proteins. On account of complexity of structure,
however, certain factors intervene which render the investigation of
their behavior very difficult. As we shall see later, proteins are made
up of combinations of amino acids, each of which contains basic (NH2)
and acid groups (COOH). The various amino acids are linked together
in protein by the COOH of one uniting with the NH2 of another, with
the elimination of -water — thus, CO j OH + H ; HN — but some NH2 and
COOH groups are left uncombined. According to the relative number
of these uncombined radicles, the protein (or polypeptid, page 601)
will exhibit faintly acid or basic or neutral properties. With acids, for
example, a salt will be formed by union with the NH2 groups, Avhich will
dissociate into the anion of the acid and a large organic cation; whereas
with alkalies union will occur with the COOH group, and the salt on
dissociating will form a small cation of the metal of the salt and a large
complex anion. We may therefore obtain the protein with either a
positive or a negative electric charge by altering the chemical nature of
64 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
the fluid in which it is dissolved, so that the reaction towards other
colloids and towards electrolytes will vary.
One feature of proteins of importance in this connection is that known
as the isoelectric point, at which the protein exists with a maximum of
electrically neutral molecules. This point is reached by adding acid to
a protein solution. The acid represses the dissociation of the protein
acting as an acid, and therefore diminishes the number of free hydrogen
ions ; and at the same time it combines with the NH2 groups and neutral-
izes the basic characteristics. The alteration in electric charge thus in-
duced alters the water-absorbing powers of the protein and therefore
all of the properties which we have seen to be associated therewith
(page 63).
SURFACE TENSION
Before we consider a very important property of colloids known as
adsorption, by means of which they are able to perform many reactions
that do not conform with the laws of mass action, it will be well to
A.
Fig. 17. — Diagram to illustrate surface tension. The rings A and B inclose soap films in
which a very fine loop of silk is suspended. In A it is loose but in B, where the film inclosed
in the loop has been broken, it is drawn into a circle by the tension of the soap film. (From
Bayliss.)
say a few words concerning the physical phenomenon upon which this
depends — namely, surface tension. The creation of this force is due
to the fact that, whereas the molecules within a liquid are subjected to
equal forces of attraction on all sides, at the surface these forces act on
one side of the molecules only, and therefore tend to pull them inwards.
This causes the surface to pull itself together so as to occupy the least
possible area, and it is this force which constitutes surface tension.
The surface behaves as if stretched. There are various simple experi-
ments that reveal the presence of surface tension. If a film is made on
a loop of wire by dipping it in soap solution, a fine silk thread can be
floated in the film, so that it forms a loop that is quite loose. If the
portion of the film inside the loop is destroyed by touching it with filter
paper, the film will break in the loop, which will now be pulled into a
circular shape by the tension of the film around it (Fig. 17).
For the measurement of surface tension, various methods are used.
COLLOIDS
65
The size of drops of liquid falling from an orifice is dependent on sur-
face tension; the larger the drops, the greater the surface tension. If
the number of drops obtained by allowing a liquid to drop from a stand-
ard orifice in a given time is counted, we have a measure of the surface
tension. Account must of course also be taken of the specific gravity
of the liquid. The instrument used for this purpose is called a
stalagmometer (Fig. 18). Another method depends on the fact that
the height to which a fluid rises in a capillary tube is dependent on
surface tension (and inversely on the diameter of the capillary). The
difference in the heights to which two liquids rise in capillary tubes of
known bore permits us to compare their surface tensions, and if this
is known for one of the solutions, it can be determined for the other.
Besides existing between liquid and air, surface tension also exists at
the interface between two immiscible liquids, and at that between sus-
Fib. 18.- — Traube's stalagmometer. The surface tension is proportional to the number of
drops formed in a given time. The right-angled tubes are for thin liquids, and the straight
one for blood and other viscous fluids.
pended solid particles and liquid, as in colloidal solutions. Since, as
we have seen, the surface area (interface) is enormously increased in
these solutions, a very great surface energy is present, for this is equal
to the surface tension multiplied by the surface area.
ADSORPTION
The surface tension between liquid and air is lowered when organic
substances are dissolved in the liquid, but is slightly raised when inor-
ganic salts are dissolved. The degree of lowering varies markedly ac-
cording to the organic substance dissolved, being very pronounced with
bile salts, upon which fact the well-known (Hay) test for the presence
of bile in urine is based. Between liquid and liquid, as well as between
66 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
solid and liquid, the surface tension is always lowered ~by dissolving sub-
stances in the liquid. Now, at the interfaces between solid particles and
liquid there must be a local accumulation of free surface energy, which
will be equal to the surface tension multiplied by the surface (inter-
face) area. A constant tendency exists for such free energy to be de-
creased and, since dissolved substances have this effect, they will become
concentrated at the interface. This is known as the principle of Willard
Gibbs, and it is of fundamental importance to the biochemist, because
on it depends the phenomenon known as adsorption, which in the case
of colloidal solutions may therefore be denned as the local concentra-
tion or condensation of dissolved substances at the interface between
the two phases. The amount of substance concentrated at the interface
can be calculated by a formula which takes into account the concentra-
tion of the dissolved substance, the temperature, and the surface tension
at the interface (the Gibbs formula). After absorption has occurred, vari-
ous reactions of a chemical, electrical or purely physical nature (e. g., dif-
fusion) may follow at a rate which depends on the amount of the
condensation.
Every-day Reactions Which Depend on Adsorption
1. Decolorization of liquids by charcoal. That no chemical reaction oc-
curs in such a case is readily shown by the ease with which the pigment
can be extracted from the charcoal.
2. Adsorption of gases by such solids as charcoal and spongy platinum.
In these cases there must be great condensation, even a liquefaction of the
gas, during which heat must be evolved. By adsorbing oxygen and hydro-
gen, spongy platinum causes these gases to combine and form water. The
hemoglobin of blood may take up oxygen by a similar process.
3. Formation of solid surface films on solutions of protein, saponin, etc.
The condensation may lead to coagulation, which explains why, if the
froth produced by beating the white of an egg is allowed to stand, it can
not be again beaten into a froth, the albumin having gone out of solution
by surface coagulation.
An interesting phenomenon depending on the surface tension occurs
when the protoplasmic contents of a ciliated infusorian is pressed out in
water. A new membrane forms on the protoplasm because of surface con-
centration of all constituents which lower surface energy. By application
of the principle of Willard Gibbs, A. B. Macallum18 concludes that not only
adsorption, as exhibited in a colloidal solution, but also the local accumula-
tions of material often seen in cells, are associated with changes in sur-
face energy. His conclusions are based largely on microscopic studies
of various forms of cell exhibiting different degrees and types of activity,
COLLOIDS 67
and ingeniously stained for potassium by cobalt hexanitrite. By such
a means the potassium stains intense black. In vegetable cells, local
accumulations of potassium occur either near the interface between the
clear and the chlorophyl-containing parts of the cell (spirogyra) or
under a portion of the cell wall from which later a protrusion grows out
to form the first stage in conjugation. The outgrowth from the cell,
as well as the accumulation of potassium, may be the result of a low
surface tension. In unicellular animal organisms, such as Vorticella,
much less potassium is present, being confined to the base of the cilia,
which Macallum believes indicates that the structures are produced as
an outcome of low surface tension.
In the cells of higher animals, deposits of potassium are also- localized ;
in striated muscle, for example, they occur in a zone at each end of the
doubly refractive band and immediately adjacent to the singly refrac-
tive band. Changes in surface tension, associated with changes in the
distribution of potassium, are believed by many to be responsible for
muscular contraction. In nerves and nerve cells, potassium is concen-
trated at the axon and at the surfaces of the cells. Interesting sugges-
tions are offered to explain the relationship among changes in surface
tension at the terminations of axons (synapses, terminations in gland and
muscle cells) brought about by the nerve impulse acting as a change in
electric potential. Surface condensation of potassium has also been
observed at the lumen border of gland cells (pancreas), and on the lu-
men surface of the cells of the renal tubules. Such observations indicate
in what way surface tension may be called into play to control cellular
activities. The field is new and almost unexplored, but there is already
much to indicate that surface energy plays a most important role in the
performance of many cellular activities.
Conditions That Influence or Are Influenced by Adsorption
Electric Changes.— Besides mere concentration, other forces come
into play to assist or retard adsorption. One of the most important of
these is electrical. Most solids when present as particles in a fluid carry
a negative charge of electricity, some a positive one. In conformity with
the AVillard Gibbs law, a constant tendency will exist for this free energy
to be diminished by the neutralization of the electric charge. This can
occur by deposition on the interface of other particles carrying an
electric charge of opposite sign or by the action of that present on ions.
Charcoal in suspension in water, for instance, has a negative charge.
If colloidal iron, which has a positive charge, is added to the solution, it
will become deposited on the charcoal, as will also the cations of an
inorganic salt. On account of electric adsorption, dyestuffs and bile
68 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
salts are adsorbed much more freely than they would be if the process
depended solely on surface condensation ; that is, if the Gibbs formula is
used to calculate the adsorption, it will give values that are much below
those actually found.
If the dissolved substance and the particles both have the same electric
sign, adsorption will not occur. Filter paper, for example, has a nega-
tive charge and can not therefore adsorb a negative dye such as congo
red (as shown by the depth to which it becomes stained) ; whereas it
readily adsorbs night blue, which is positively charged. If the negative
charge of the paper is lowered, it becomes capable of adsorbing some of
the negative congo red. This can be effected either by placing the paper
in alcohol or by adding inorganic salts (NaCl) to the water with which
it is in contact. The positive-charged ions of Na, produced by dissocia-
tion, neutralize some of the negative charge on the paper, and allow a
certain amount of adsorption of the negative-charged congo red to oc-
cur. As would be expected, acids and alkalies are capable of greatly
altering the electric charges by the H and OH ions which they contribute.
Chemical Forces. — If the nature of the phase at the surface of which
adsorption occurs is such that it can enter into chemical combination
with the substance adsorbed, reactions will occur that do not obey the
laws of mass action. By adsorption, reactions of a certain type may be
encouraged over other reactions, even although the necessary reacting
substances may be present in the solution (specific adsorption). The
adsorbing substance itself is not, however, usually susceptible of chem-
ical change even when it exists as very minute particles, as in the case of
colloidal solutions. Nevertheless, adsorption may accelerate chemical
reactions by bringing together in concentrated form substances of high
chemical reactivity. In such cases the adsorbing substance itself does
not enter into the chemical reaction, and can be recovered at the end
in an unchanged condition. It acts as a catalyst (page 72). As we
shall see later, enzymes act in this way — i. e., their rate of reaction is
controlled by adsorption.*
The distinguishing feature of such adsorption phenomena is that a
curve of the reaction (drawn by plotting amount of chemical change
'Another instance of the influence of surface energy on the course of chemical reactions is seen
in the accelerative influence of charcoal on such reactions as the oxidation of formic acid, glycerol,
etc. Surface tension may also cause retardation of chemical reactions, as is- seen in the turbidity
(due to the separation of chloroform) which gradually develops when a —^ — Na2CO» solution is
M
mixed with a -^r— chloral hydrate solution. The surface remains clear, because surface energy has
prevented the reaction.
An important effect of surface tension on chemical reactions is also seen in the relationship
between it and the absorption coefficient of gases (volume of gas dissolved by unit volume of
liquid). The lower the surface tension, the greater the solubility of the gas. Oxygen and nitrogen
are, for example, much more soluble in alcohol, hydrocarbons or oil than in water. This shows
the futility of attempting to prevent the loss of gases from fluids such as blood by covering them
with oils or hydrocarbons.
COLLOIDS 69
against concentration of reacting substances) is a parabola, indicating
that the laws of mass action (page 23) are no longer followed. In
order to be able to determine whether some particular process — as, for
example, a fermentation process, or the absorption of oxygen by blood —
is caused by adsorption, we must compare its curves, constructed ac-
cording to the same principles, with the typical adsorption curve. A
formula may be used in constructing the curves. In arriving at this
formula, two facts have to be remembered: (1) As adsorption proceeds
and less and less of the free energy on the adsorbing surface remains
to be neutralized, the reaction slows off, until equilibrium is reached.
The more dilute the solution, the greater is the proportion of its con-
tents to be adsorbed, which means that if a is the amount of substance
adsorbed from a certain solution, then, from a solution of twice that
strength, somewhat less than 2 a will be adsorbed — i. e., a multiplied
by some root of 2. Although the formula is one belonging to the class
known as parabolic, it must not be assumed that every reaction which
happens to give such a parabolic curve (such as the combination of 0.,
with hemoglobin under certain conditions) (see page 383) must be one
dependent on adsorption.
It must be understood that although the substance that is removed
from a solution by adsorption is no longer capable of contributing to the
conductivity or the osmotic pressure of the solution, it is nevertheless
not so firmly fixed that it can not be set free again by purely mechanical
means, as by constant dilution of the fluid. If charcoal which has ad-
sorbed sugar is placed in a dialyzer made of membrane the pores of
which allow sugar but not charcoal to pass through, the sugar will
gradually be removed if the dialyzer is immersed in running water. A
certain equilibrium exists between the substance adsorbed and the same
substance still remaining in solution. If the latter is constantly dimin-
ishing by dialysis, the adsorption compound must break down to main-
tain the equilibrium. It is clear, however, that the process of removal
will be extremely slow. The ability of adsorbed substances to withstand
removal by washing is taken advantage of by nature in holding back
foodstuffs in the soil.
Physiologic Processes Depending on Adsorption
Instances in which adsorption undoubtedly plays a most important
part in physiologic processes are as follows:
1. The action of enzymes (see page 71).
2. The combination of toxin with antitoxin occurs according to the laws
of adsorption rather than those of mass action. In this case it is im-
portant to note that when the toxin of diphtheria is added in small sue-
70 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
cessive quantities to diphtheria antitoxin, more toxin is neutralized than
when the toxin is all added at once. A similar phenomenon can also be
observed by adding filter paper to congo red, more of the pigment being
adsorbed when the paper is added in small quantities than when added
all at once. The explanation is that relatively more adsorption of a
given substance occurs from a dilute than from a strong solution (cf.
page 69).
3. The sensitizing of leucocytes by opsonins, as well as the subsequent
ingestion of bacilli by the sensitized leucocytes, both of which follow the
course of an adsorption reaction.
4. The formation of adsorption compounds, such as the inorganic salts
and proteins and the complex lecithin compounds that can be extracted
from egg yolk or brain tissue. In such compounds the laws of chemical
proportion no longer hold, and properties may be exhibited that are quite
different from those of either one of its components. When yolk of egg
is extracted with ether, for example, a compound of lecithin with vitellin
goes into solution, although vitellin itself is quite insoluble in ether.*
There can be no doubt that adsorption compounds of this character are
very abundant in living cells, and that they are constantly being formed
and broken down.
CHAPTER IX
FERMENTS, OR ENZYMES
One of the most striking developments of modern research in biochem-
istry concerns the nature of enzyme action. So remarkable are many of
the facts that have been brought to light that it can not fail to interest
every one engaged in the study of life phenomena — whatever the nature
of that study may be — -to know something of the main questions at
present occupying the attention of investigators in this field. In this
chapter a brief survey will be given of some of these questions; no at-
tempt will be made at completeness, and only where necessary for the
sake of example will reference be made to individual types of enzyme
action.
The discovery by Buchner that an enzyme can be expressed from yeast
cells which is capable of instantly bringing about the alcoholic fermen-
tation of dextrose solutions has been responsible for a great deal of the
modern advance. Formerly, yeast cells were believed to bring about
alcoholic fermentation as a result of their growth: it was believed to be
a life phenomenon, or " vital process." Now we know that yeast cells
produce an intracellular ferment or endo-enzyme* to which its sucroclastic
properties are due and which can act apart from the cells that produce it.
It is no great stretch of imagination to think of all chemical reactions
mediated by cellular activity as due to a similar mechanism, and this thought
has led to the hypothesis that all processes of intermediary metabolism in
the animal and plant are caused by enzyme action. Before Buchner 's
day we knew only of the extracellular enzymes (such, for example, as
the digestive ferments), that is to say, of enzymes, produced indeed by
cells, but secreted from them and acting outside their protoplasm ; now
we must recognize intracellular enzymes acting where they are produced,
in the protoplasm of the cell. But we must not permit this conception to
carry us too far. Without further investigation we must not imagine
that the riddle of life is thus solved.
As an example of the role which extra- and intracellular enzymes are
supposed to play in the animal economy may be cited the metabolism of
protein. Proteolytic enzymes are very widely distributed in the active
tissues of the animal and plant. By their agency in animal life, the com-
*The terms "ferment" and "enzyme" are synonymous, but the latter is preferable as the noun,
leaving the former to be used as the verb.
71
. 72 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
plex protein molecule is split up to render it absorbable from the intes-
tine, and the tissues appropriate from the blood those of the degradation
products that they require for the construction of protoplasm, which,
later, they decompose so as to utilize the energy which the organism
demands. All these processes are believed to be the work of enzymes.
The Nature of Enzyme Action
The changes brought about by enzymes can also be accomplished by
ordinary chemical means, but these have often to be of a very energetic
nature to accomplish what the enzyme can so quickly and quietly
perform.
It is the custom to regard "enzymes as catalysts. A catalyst is a sub-
stance which accelerates (or retards) a chemical reaction which in its
absence could proceed at a different (usually slower) pace. The action
of catalysts has been aptly likened to that of a lubricant. A weight
placed at the top of an inclined plane, so held that the weight only slowly
slips down, has its velocity greatly increased if its under surface be
oiled. The oil accelerates the action but does not affect the ultimate
result. Catalysts do not combine with the final products of the reaction,
these being, as a rule, the same as they would have been had no catalyst
been added. Another characteristic is the tremendous amount of chem-
ical change which even a trace of catalyst can induce. There are many
examples of catalysts in the inorganic world, among which may be cited
the action of spongy platinum on hydrogen peroxide. This substance
normally tends to decompose into water and oxygen, but if a small
amount of spongy platinum is added to it, the decomposition is greatly
accelerated: H202 = H20 + 0.
A very good example of the action of an inorganic catalyst is that of
the hydrogen ion on cane sugar, or other disaccharides, in the presence
of water. It accelerates the hydrolysis. Cane sugar solution at room
temperature does not indeed, in sterile solution, undergo any appreciable
hydrolysis, but at 100° C. it does, which leads us to believe that, though
inappreciable, the action also occurs at room temperature. By adding
a little hydrochloric acid, or other acid not having an oxidizing effect
on sugar, we greatly accelerate the hydrolysis because of the hydrogen
ions present in the acid solution. Within certain limits the rate of hy-
drolysis is proportional to the amount of catalyst present.
Enzymes, like other catalysts, produce their action when present in
very small amounts (e. g., sucrase can hydrolyze 200,000 times its weight
of cane sugar; diastase can convert starch to sugar in a dilution of
1-1,000,000) and there is a distinct relationship between the amount of
enzyme present and the rate of the reaction. The final product of the
FERMENTS, OR ENZYMES 73
reaction is, however, the same at whatever rate it proceeds, and the
enzyme does not appear in the final products. Many enzymes such as
diastase can be found unaltered in amount after they have completed
their action. This is determined by adding a fresh supply of substrate
(that is, of material to be acted on), when the enzymic action proceeds
again in the usual way. The same is no doubt true for all enzymes,
though as yet it can actually be proved for only a few of them. Enzymes,
therefore, may be defined as catalysts produced by living organisms.
The Properties of Enzymes
Although enzymes are examples of catalysts, they exhibit many proper-
ties that appear to differ from those of inorganic catalysts. It will,
therefore, be advisable in considering each quality to compare it in
catalysts and enzymes, for by this method a much clearer conception of
the nature of enzyme action can be gained (Bayliss19). Those properties
that are strictly peculiar to enzymes we shall consider later.
1. Most enzymes are remarkably specific in their action, whereas inor-
ganic catalysts are very much less so. Thus, in the case of the enzymes
which bring about inversion of disaccharides, this specificity is clearly
shown. There is a special enzyme for each of the three disaccharides —
maltose, lactose and cane sugar — :and one of these can not replace
another.
Still more strikingly is this specificity of enzyme action demonstrated
in the fact that certain enzymes, such as zymase (expressed from yeast),
will act only on bodies having a certain configuration, that is, having
their side chains arranged in a certain way. Thus, there are two varie-
ties of dextrose (a and /?), which differ from each other solely in the
fact that the side chains are arranged in different positions with rela-
tion to the central chain of carbon atoms. This form of isomerism is
called stereoisomerism because the two bodies rotate the plane of polar-
ized light to an equal degree in opposite directions. Zymase acts on one
of these but not on the other, and there are innumerable examples of the
same kind. Indeed, of all bodies that exist in two stereoisomers only
one is found in living cells and it is on this variety alone that the enzymes
in animals can act. A similar specificity exists between certain drugs and
their pharmacologic action.
Specificity of action is explained by supposing that a union occurs
between the substrate and the enzyme, and for this union to take
place the enzyme must possess a configuration which corresponds accu-
rately with that of the substrate. The process has been compared to a
lock and key; the key must be shaped to fit the lock, or it can not
operate. The specificity does not, however, in itself disprove the close
74 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
relationship between enzymes and inorganic catalysts, for on the one
hand there are several enzymes which do not exhibit this property, and
on the other, there are inorganic catalysts which do. For example,
lipase, the fat-splitting enzyme of pancreatic juice, decomposes not only
fats but to a greater or less degree a number of bodies of the same gen-
eral build (esters), and tyrosinase can decompose, not ty rosin alone,
but all phenol compounds. Conversely, the hydrogen ion — to the pres-
ence of which acids owe their catalytic powers — can decompose the ordi-
nary esters (that is, of acids containing the carboxyl or COOH group)
but it has no action on the sulphonic esters. However, enzymes are cer-
tainly much more specific in their action than inorganic catalysts.
2. Temperature does not influence catalysis and enzyme action in the
same way. As the temperature is raised in the case of inorganic catalysts,
the reaction becomes about doubled in rapidity for each rise of 10° C.,
whereas in the case of enzymes it becomes increased up to a certain opti-
mum temperature, beyond which, as the temperature rises, the reaction is
first slowed and then disappears altogether.
This peculiarity of enzymes as compared with inorganic catalysts need
not in itself disprove the analogy between the two, because enzymes do
not form true, but colloidal solutions. Colloidal solutions, as we have
seen, are really fine suspensions of ultramicroscopic particles ; there is no
splitting into ions of the dissolved substance, as is the case with true
(molecular) solutions, but the colloid is suspended in the water or other
solvent to form a heterogeneous system (page 51), on which account
the surface area of the menstruum is enormously increased. Rise in
temperature alters the extent of the surface area, and thereby intro-
duces an influence which progressively opposes catalysis.
Although inorganic catalysts in molecular solution show no optimum
temperature but increase in activity in proportion as the temperature is
raised, inorganic colloidal catalysts may show an optimum temperature.
Thus, spongy platinum shows an optimum temperature in its action on a
mixture of hydrogen and oxygen. It has therefore been suggested that
it is because they are colloids that enzymes exhibit this property.
3. Inorganic catalysts frequently carry the reaction to a further stage
than that attained by the action of enzymes. For example, acid breaks
down the protein molecule much more completely than do the proteolytic
enzymes. This difference is perhaps explained by the fact that enzymes
are retarded in their activities when there comes to be a certain accumu-
lation of the products of the reaction present. The final stages in the
reaction may become so slow as to be almost inappreciable. This de-
crease in activity is partly due to a union between the enzyme and the
products of its activity.
FERMENTS, OR ENZYMES 75
4. The velocity constant in the case of inorganic catalysts remains un-
changed throughout the reaction, whereas in the case of enzymes it 'be-
comes either less or greater as the process proceeds. When a substance is
changed by catalytic action, it is, of course, constantly being diminished
in concentration so that less and less of it remains to be acted on. This
implies that there are fewer molecules present for the same amount of
catalyst to act on and consequently that the amount changed in a unit
of time is progressively less. At any moment, therefore, the rate of
catalysis will be proportional to the amount of substance (substrate)
left. To understand this we must refer back to what we have learned about
mass action. If we suppose that two substances A and B react to form
two other substances C and D, then, by the law of mass action, the reac-
tion will not go on to completion but will stop when a certain equilibrium
is reached. The reaction can be represented by the equation
A + B +± C + D, which means that it proceeds at a rate proportional to
the reacting molecules. In some cases this reaction goes on until either
A or B has practically disappeared (that is, the equilibrium point is very
near the right of the equation), as is the case in the inversion of cane
sugar:
C12 H22 On + H20 = C0 H12 06 + CG H12 06
Taking place as it does in an excess of water, and there being very
little tendency for this reaction to go in the opposite direction (cf. re-
versible action) (page 25), the only thing Avhich will influence its
velocity is the concentration of cane sugar ; in other words, the velocity
of the reaction at any moment will depend solely on the concentration,
C, of the material still left undecomposed. This can be determined by
means of an equation.*
The value of such an equation is that it gives us a figure K, represent-
ing the amount of inversion that would occur in each unit of time if the
cane sugar were kept in constant concentration. When, for example,
it is stated that K for a particular strength of acid acting on cane sugar
solution is 0.002, this means that when volume, concentration of acid and
*If x be the amount of sugar inverted in time *, and if we use a figure called a constant (K) to
express the fundamental rate of the reaction (which will therefore be different for different reac-
tions), then — — = KC. But C can not be the same at any two consecutive periods of time, because
the reaction is going on continuously. This renders it necessary to use the notation of the differential
calculus, and we have— — = KC. The sign 5 indicates that the reaction is a constantly changing
ot
one so that 5x and 5t represent such infinitely small amounts that they can not be measured. By
methods of integration, however, it can be shown that the above equation may be written:
K = 1 • log. nat. -^-,
1 2 1 *-2
thus permitting us to find the value of K (Ci C2 being the concentrations of cane sugar at the
times TI T2).
Any two determinations during; the course of the reaction can be used for calculating K. These
equations apply only to cases in which but one substance is changing (monomolecular reaction).
When two substances are involved, the equation is more complicated.
76 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
temperature are constant in a gram-molecular solution of sugar, 0.002
gram-molecule of sugar would be inverted the first minute and 0.002
gram each succeeding minute, provided we could keep the solution con-
stantly a gram-molecular one, that is, provided we could add sugar just
as quickly as it becomes inverted.
At first sight it may appear of little practical importance to determine
K. In our present discussion concerning the nature of enzyme action,
it is however of great value for, whereas with inorganic catalysis K is
really of constant value, with enzyme action it is not so. Thus, when
cane sugar is inverted by sucrase — an enzyme present in the intestine
and in yeast — the constant gradually rises; for most other unimolecular
reactions mediated by enzymes it gradually falls ; for example, the action
of trypsin on proteins.
Where there is a great excess of substance to be acted on, in compari-
son with the amount of enzyme present, it will be found that a more
constant value than K is obtained when we compute the absolute amount
of substance decomposed in a given time. In such a case, too, the
amount of change in a given time will be proportional to the amount of
enzyme present, indicating that some sort of combination between en-
zyme and substrate must be the first step in the fermentative process.
This fact has been noticed by us in connection with the hydrolysis of
glyeogen in the liver. When there is an excess of glycogen present, the
amounts which disappear in equal intervals of time after death are the
same; when, on the contrary, there is not much glycogen, the amount
which disappears gradually declines, but, if K be computed by the above
equation, it is constant.
To make these facts clear it may be well to pause for a moment to
consider an illustration. The conditions obtaining when there is a large
excess of substrate over enzyme may be compared to those governing
the removal of a pile of bricks from one place to another by a number of
men. The pile of bricks represents the substrate ; the men, the enzyme.
If each man works up to his capacity, it is plain that the number of
bricks transferred in a given time will not depend at all on the size of
the pile to be transferred. When, however, the pile of bricks gets small,
though the same number of men continue to work the number of bricks
transferred in a given time falls off, because the men interfere Math one
another's activities in securing their loads from the pile. When a similar
stage is arrived at in enzyme processes, we have to use the velocity con-
stant to show how much work could be done by the enzyme if the amount
of substrate were maintained of constant amount.
In the large volume of recent work which has been done with the
object of discovering the cause of these variations in the velocity con-
FERMENTS, OR ENZYMES 77
stant in the case of enzymes, four important conditions have been recog-
nized: (1) reversibility; (2) gradual destruction of the enzyme; (3) com-
bination of the enzyme with products of the reaction; (4) autocatalysis.
Of these four influences the only one which could be held accountable
for an increase in the activity of the enzyme is autocatalysis; in this
process the enzyme by its action produces substances which intensify
its own activity. In some cases at least — for example, the action of
invertase on cane sugar — these are acid bodies, a moderate increase in
acidity favoring the action of this enzyme.
The other influences all tend to retard the reaction and progressively
lower the value of K. Negative autocatalysis occurs when the enzyme
produces products which interfere with its activity. Gradual destruc-
tion of the enzyme and its union with the products of its activity will
manifestly also decrease its power. There is plenty of evidence that
both of these processes may occur.
Reversibility of Enzyme Action
But the most important of all the causes of retardation of enzyme
activity is undoubtedly reversibility of action, which is an application of
the law of mass action (page 25). If we take the saponification of an
ester, the equation is:
CH3CH2CH2COOC2HS + H2O ±5 CH3CH2CH2COOH + C,HBOH.
(ethyl buty rate) (butyric acid) (ethyl alcohol)
The equilibrium point is not so near the position of complete hydrol-
ysis as in the case of the inversion of saccharose; in other words, the
tendency for the bodies produced by the hydrolysis to reunite and form
the original substances is quite marked, so that the reaction comes to an
end before all the ethyl butyrate has been decomposed. For some time
before the equilibrium point is reached, there will have existed a progres-
sively increasing opposition to the breakdown of the ester, as a conse-
quence of which, when enzymes are used to accelerate the reaction, the
velocity constant, as determined by the above equation, will gradually
fall as the reaction proceeds. Conversely, in a mixture of ethyl alcohol
and butyric acid there is very slow synthesis to ethyl butyrate, and here
again lipase accelerates the process; it induces a recognizable synthesis
within a short time. Ethyl butyrate is usually employed for these ex-
periments because, on account of its odor, the ester is readily recognized.
Thus, if the alcohol and acid be mixed alone, no ester will be detectable,
but if some lipase be added, it will soon become so. Similar synthetic
action of lipase has also been demonstrated for mono- and tri-olein.
78 PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES
It should be clearly understood that pure catalysts, such as the hydro-
gen ion, in accelerating a reaction like the above, do so equally in both
directions, so that the position of equilibrium remains unchanged. En-
zymes may, however, cause this position to change because of their form-
ing intermediate combinations.
The reverse phase of certain reactions is probably the cause of at least
some of the synthetic processes which occur in the animal body. A great
difficulty in accepting such a view, however, is the fact that the equilib-
rium point of all hydrolytic reactions, in the presence of an excess of
water, is so near complete hydrolysis that very little synthesis can be
possible. That is true so long as the substance synthesized is soluble,
but if it is nearly insoluble in water, or if it is immediately removed
from the site of the reaction by diffusion, or in any other way,, then it is
obvious that it will go on being synthesized by the reaction. Thus, in the
intestine neutral fat is hydrolyzed by pancreatic lipase into fatty acid
and glycerin, which are absorbed into the epithelium, where they again
come under the influence of intracellular lipase. This latter will tend to
accelerate the synthesis of neutral fat from the fatty acid and glycerin
until the equilibrium point of the system (fat acid + glycerin ^± neutral
fat + H20) is again reached; but this point, although it is near the right
hand of the equation, will really never be reached for the reason that the
neutral fat, as quickly as it is formed, will become deposited in insoluble
globules in the protoplasm and thus be removed from the equation. In
support of this view it has been found that lipase is present in intestinal
mucosa after all traces of adherent pancreatic juice have been washed
away. By similar reactions the fat of the tissues becomes decomposed to
fatty acid and glycerin and passes out of the blood when the concentra-
tion of fat in this fluid falls below a certain level. Provided one of the
substances synthesized is insoluble or can in some other way be removed
from the reaction, it is plain that, even though the equilibrium point is
very near to that of complete hydrolysis, yet the reversion will be suf-
ficient to do all that is required of it.
Results such as the above have prompted many to conclude that it is
by such reversible action that all synthetic processes occur in the living
organism. But the demonstrable synthesis of an ester must not be taken
as evidence that all other syntheses are explainable on the same basis.
For example, we have seen above that in the case of cane sugar the equi-
librium point in the equation is so near that of complete hydrolysis, that no
measurable amount of cane sugar is formed when dextrose and levulose are
allowed to act on each other, and that cane sugar does not appear
when sucrase is added to the mixture. If instead of sucrase we take
another of the sugar enzymes — namely, maltase, which accelerates the
FERMENTS, OR ENZYMES 79
decomposition of maltose into two molecules of glucose— there is, how-
ever, evidence of synthesis as a result of the acceleration of a reversible
reaction. To understand these results we must remember that ordinary
dextrose is a mixture of two stereoisomers designated a and /?. When
two molecules of a dextrose condense (that is, fuse togther with the
loss of a molecule of water) maltose is formed, but when two molecules
of /? dextrose condense isomaltose results. There is some controversy
as to whether maltose is really responsible for the synthesis of a dextrose
molecules to maltose, it being claimed by some that this is accomplished
by another enzyme, emulsine. If this were true it would materially
minimize the importance of reversible action as a factor in cellular syn-
thesis. The latest evidence goes to show, however, that it is maltase
and not emulsine that is responsible in the above case (cf. Bayliss).
Evidence, both direct and indirect, is also steadily accumulating to
show that enzymes may accelerate the synthesis of proteins. As pieces
of direct evidence we have: (1) the retardation of the digestive action
of trypsin, etc., which sets in after the process has gone on for a time,
and (2) the recommencement of a digestive process apparently at an
end, if the products of the digestion are removed by dialysis or other
means. As direct evidence may be cited the formation of synthetic
products when pepsin is added to concentrated solutions of peptone,
and the diminution in the number of small molecules, as judged by meas-
urements of electrical conductivity, when trypsin is added to the prod-
ucts of tryptic digestion of caseinogen. Protamine — a simple form of pro-
tein— has also been found to be produced when trypsin — obtained from
a mollusc — was added to a tryptic digest of the same protamine. The
significance of these facts in connection with the metabolism of the
amino aids will be evident \vhen we come to study this subject (page
598).*
Specificity of Enzyme Action
Although in all of the above features of enzyme action there is nothing
to contradict the vieAv that they are catalytic agents, there remains one
peculiarity which at first sight seems uninterpretable on such a basis.
This is with regard to their often remarkable specificity of action. Thus,
as we have seen, maltase can hydrolyze maltose alone (which is com-
posed of two a-dextrose molecules), but not isomaltose (composed of
/^-dextrose). This means that mere difference in the configuration of
molecules is sufficient to alter the influence of enzymes on them. Since
such differences could not influence that of inorganic catalysts we must
*We have been unable in this laboratory to demonstrate any synthesis of glycogen when gly-
cogenase is added to a hydrolysis mixture of dextrine, maltose and glucose produced by the prolonged
action of glycogenase on pure glycogen.
80 PHYSICOCHEMICAL BASIS OP PHYSIOLOGICAL PROCESSES
explain the cause of the difference. This has been done on the basis
either that enzymes are colloids or that the active (catalytic) group of
the enzyme is attached to a colloid molecule. Before a substance can
be acted on, it must combine with the colloid, which it does by the proc-
ess of adsorption (see page 65). Thisi can occur, however, only when
there is a harmony between the adsorbing substance and the substance
adsorbed. Instances of the specificity of adsorption have already been
given.
In support of this view it has been found that of the two proteases,
a and ft, in the spleen, one is adsorbed but not the other when a solu-
tion containing them is shaken with Kieselguhr. Furthermore, when
solutions of invertase are shaken with certain inert powders, the in-
vertase is adsorbed by some of them but not by others. In strong sup-
port of the adsorption hypothesis is also the fact that the same mathe-
matical laws as apply in the process of adsorption are obeyed in the
ratio which exists between the activity of an enzyme and its concen-
tration in the solution.
To sum up, then, catalysis as exhibited by enzymes involves three
processes: (1) contact between the enzyme and the substrate, which will be
dependent on their rates of diffusion; (2) adsorption between them, which
will depend on their configurations (cf. the lock and key simile) ; and
(3) the chemical change which itself probably takes place in two stages.
In connection with the third process, it is probable that an initial com-
pound of a definite chemical nature is first formed, followed by the
hydrolytic or other chemical change, after which the enzyme group
becomes free.
It is very significant in this connection to. note that in their solubil-
ities there exists a distinct relationship between the ferments and the
substrates on which they react. Thus, trypsin is very soluble in water
and acts on water-soluble proteins; lipase is soluble in fat solvents.
Certain Peculiarities of Enzymes
Notwithstanding the very strong case that is made out for the cata-
lytic hypothesis, there are certain facts which many find it difficult to
make conform with such a view. One of these is that dextrose can
undergo three distinct and separate types of decomposition according
to the enzyme allowed to act on it. These are alcoholic fermentation,
butyric acid fermentation and lactic acid fermentation. It is difficult
to see how simple catalytic action can be responsible for all three results.
The enzyme must not only initiate the changes but also direct their
course.
Another peculiarity is that when certain enzymes — e. g., rennin, pep-
FERMENTS, OR ENZYMES 81
sin, etc. — are inoculated in animals, they cause specific antienzymes to
appear in the blood of the inoculated animal. Thus, when antirennin
serum is added to milk it greatly hinders clotting on the subsequent
addition of rennin. It is probable that powerful antienzymes are pro-
duced in the animal body for the purpose of protecting the tissues from
attack by enzymes. It is on account of the presence of antienzymes
that intestinal parasites can exist in the intestine, and the immunity
from digestion which the mucosa of the gastrointestinal tract enjoys,
is believed to be due to the same cause. But there is considerable doubt
regarding this claim. Fresh pancreatic juice when injected into the
empty intestine digests its walls. When food is present in the intes-
tine it evidently prevents digestion of the walls by diverting the enzyme
to itself.
Types of Enzyme
Having learned something about the general nature of enzyme action,
we may now turn our attention to certain details that have a practical
importance. In the first place, with regard to nomenclature, in the
earlier work each newly discovered enzyme received a name which was
often quite inappropriate. Many of these names are retained, such as
pepsin, trypsin, ptyalin, etc., but it is now customary to name the
enzyme according to the substance on which it acts. This is done either
by replacing the last part of the name of the substance acted on by the
termination -ase (for example, the enzyme which inverts maltose is called
maltase), or by merely adding -ase to the name of the substance acted
upon (thus, the enzyme which hydrolyzes glycogen is called glycogenase).
Most of the enzymes in the animal body accelerate hydrolytic proc-
esses and are classified according to the chemical nature of the sub-
strate on which they work. Thus, we have:
1. The amylases — accelerating the hydrolysis of polysaccharides, e. g.,
ptyalin (in saliva), amylopsin (in pancreatic juice), glycogenase (in
liver), diastase (in malt).
2. The invertases — accelerating hydrolysis of disaccharides, e. g., malt-
ase, lactase and sucrase (in succus entericus).
3. The proteinases — accelerating hydrolysis of proteins, e. g., pepsin
(in gastric juice), trypsin (in pancreatic juice), erepsin, intracellular
proteinases.
4. The Upases— accelerating disruption of neutral fats, e. g., steapsin
(in pancreatic juice), intracellular lipases.
5. Arginase — accelerating hydrolysis of arginin into urea and or-
nithin, (intracellular).
82 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
6. Urease — accelerating hydrolysis of urea to ammonium carbonate
(in many microorganisms and in the soy bean).
7. Glyoxylase — converting glyoxals into lactic acid (page 666).
Other enzymes accelerate oxidative processes and are called oxidases
and peroxidases. Others bring about the displacement of an ammo
group by hydroxyl (desamidases) . Others cause coagulation (coagula-
tive ferments], e.g., thrombin, rennin. One of the enzymes present in
succus entericus acts by converting the zymogen (trypsinogen) into the
enzyme (trypsin).
Enzyme Preparations
So far it has been impossible to prepare enzymes in a pure state al-
though, being colloidal in nature, they are readily precipitated, or ad-
sorbed along with other colloids.
Since most enzymes exist in cells, it is necessary to break up the cells
in order to isolate the enzyme. This is done in various ways. By one
method the cells are ground in a mortar with fine sand, then made into
a paste with infusorial earth (Kieselguhr), the paste enclosed in stout
canvas and placed under an hydraulic press at about 300 atmospheres
pressure; a clear fluid separates and this contains the enzymes. An-
other way is to freeze the tissue with liquid air and grind it in a steel
mortar by means of a machine. Still another and less expensive method,
and one which we have found most useful for organs and tissues, con-
sists in reducing the tissue to a pulp and, after sieving it to get rid of
connective tissue, etc., spreading the pulp on glass plates and drying
in a slightly warmed, dry air current. The scales of dried material are
then ground in a paint mill with toluene, and the resulting suspension
filtered ; the powder which remains on the filter, after thorough washing
with toluene, is dried and kept for future use. The toluene removes all
the fatty substances, so that when shaken with water, etc., the enzymes
dissolve.
Conditions for Enzymic Activity
Reactions brought about by intracellular enzymes are very readily
inhibited when there comes to be a certain accumulation of their prod-
ucts of action. Thus, yeast ceases to ferment sugar when the alcohol
has accumulated to a certain percentage. This action is partially due
to a toxic action of the alcohol on the cell, which paralyzes its power of
absorbing the substance to be acted on by the intracellular enzyme. If
these products be not in some way removed, they will ultimately kill
the cell and stop the fermentation. We have seen above how the ac-
cumulation of products may interfere with the activities of enzymes in
FERMENTS, OR ENZYMES 83
other ways in which the enzyme does not suffer destruction, as is shown
by the fact that it resumes its original activities on removal of the
products.
Enzymes, both intracellular and extracellular, are very sensitive to-
wards the inorganic composition of the medium in which they are act-
ing. For the intracellular enzymes this is what we should expect when
we bear in mind the profound influence of inorganic salts on the heart
beat and on cell growth and division. This influence of salts and of
reaction (acidity, etc.) on the life of the cell is so pronounced as to lead
some observers to believe that abnormal cell multiplication in the body,
as in the case of tumor formation, is due to changes in the inorganic
composition of the tissue fluids. Extracellular enzymes are also very
susceptible to the influence of inorganic salts but more especially so
towards the reaction of the solution. In terms of modern chemistry
we may say that the concentration of H- and OH' ions has a profound
influence on the activities of enzymes. Most of the enzymes of the an-
imal body perform their action normally in the presence of a slight ex-
cess of OH' ions, that is, in faintly alkaline reaction. Indeed the only
exception of importance to this is the pepsin of gastric juice, which nor-
mally acts in an acid medium. An excess of either OH' or H- ions
inhibits the activity of the enzyme and usually destroys it permanently.
The activities of enzymes are also influenced by light, many of them
being destroyed by sunlight; cells such as microorganisms are similarly
affected.
Before being secreted the digestive enzymes exist in the cells which
produce them as inactive precursors called zymogens. The granules seen
in resting gland cells are of this nature. The activation of the zymogen,
or its conversion into the enzyme, occurs after it has left the cell, and
this has been considered as another safeguard to digestion of the cell.
Sometimes the activation does not occur until the zymogen has travelled
some distance along the gland duct, as in the case of the proteolytic
enzyme of pancreatic juice. Till it reaches the intestine, this exists as
trypsinogen (the zymogen), but it is here acted on by another enzyme-
like body produced by the intestinal epithelium and called enterokinase.
PHYSICOCHEMICAL REFERENCES
(Monographs and Original Papers)
^Bayliss, W. M.: Principles of General Physiology, Longmans, Green & Co., 1915.
2Philip, J. C.: Physical Chemistry, Its Bearing on Biology and Medicine, Arnold,
ed. 2, 1914.
3McClendon, J. S.: Physical Chemistry of Vital Phenomena, Princeton University
Press, 1917.
••Starling, E. H.: Principles of Human Physiology, ed. 2, 1915, Lea and Febiger.
84 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
sKahlenberg, L.: Jour. Physical Chem., .1906, x, 141. ,
eReid, E., Weymouth: Jour. Physiol., 1898, xxii, Ivi.
7 Wilson, T. M.: Am. Jour. Physiol, 1905, xiii, 150.
siialdane, J. S., and Priestley, J. G.: Jour. Physiol., 1916, 1, 296; Priestley, J. G :
Ibid., p. 304.
9Clark, W. M., and Lubs, H. A.: Jour. Bacteriology, 1917, ii, 1 and 109.
loHenderson, L. J.: The Excretion of Acid in Health and Disease, Harvey Lectures,
J. B. Lippincott Co., 1915, x, 132.
"Henderson, L. J.: The Fitness of the Environment, Macmillan, N. Y., 1913.
i2Van Slyke, D. D.: Jour. Biol. Chem., 1917, xxx, 289, 347.
isLevy, K. L., and Eowntree, L. G.: Arch. Int. Med., 1916, xvii, 525.
i4Cull'en, G. E.: Jour. Biol. Chem., 1917, xxx, 369.
ispalmer, W. W., and Henderson, L. J.: Arch, Int. Med., 1913, xii, 153.
16Sellards, A. W.: The Principles of Acidosis and Clinical Methods for Its Study,
Harvard University Press, Cambridge, 1917.
i^Lloyd, F. H.: Private communication.
isMacallum, A. B, : Surface Tension and Vital Phenomena. University of Toronto
Studies, No. 8, 1912; also Ergebnisse der Physiologic, 1911, ii, 598.
isBayliss, W. M.: Enzymic Action, ed. 2. Monographs in Biochemistry, Longmans,
Green & Co.
PART II
THE BLOOD AND THE LYMPH
CHAPTER X
BLOOD: ITS GENERAL PROPERTIES
BY R. G. PEARCE, B.A., M.D.
The blood, being the carrier of the nutritive and waste substances of
the body's metabolism, must at one time or another contain all the ma-
terials which compose the tissues in addition to those which are peculiar
to the blood itself. It is a very complex fluid, and all of its constituents
are not fully known. Structurally it is composed of water in which are
dissolved various gases and organic and inorganic bodies, the corpuscles
and platelets.
The most accurate method of determining the volume of blood in
the body is by bleeding and subsequently washing out the blood from
the vessels and then estimating the amount of hemoglobin in the total
fluid (Welcher's method). This method employed in the case of two
criminals who had been decapitated gave the weight of the blood as
7.7 and 7.2 per cent of the body weight. Bloodless methods for deter-
mining the total volume of blood are based upon the principle of add-
ing a definite quantity of a knoAvn substance to the circulation and then
estimating its concentration in a sample of blood withdrawn from the
body shortly afterward. If the substance can not leave the blood vessels
and does not cause fluid to be withdrawn from the tissues, the total quantity
of blood in the body can be calculated from the concentration of the
injected substance in the blood. The most accurate methods based on
this principle are Haldane and Smith's, in which carbon monoxide gas
is inhaled in a given amount and the carbon monoxide hemoglobin sub-
sequently determined colorimetriealjy ; and Keith, Rowntree and Ger-
aghty's, which employs vital red, a dye of low diffusibility. The dye
remains long enough in the body to be thoroughly mixed with the
blood, and its concentration in the plasma is determined colorimetrically
85
86 THE BLOOD AND THE LYMPH
by comparing with a suitable standard mixture of dye and serum. These
methods give the total amount of blood in the body as from 5 to 8.8 per
cent of its weight. Meek has recently developed a method in which gum
acacia is used. After mixing with the blood, the concentration of this
substance is determined from the calcium content. Being colloid, none
of the gum leaves the blood vessels.
The newer methods have shown that the volume of the circulating
fluid is maintained fairly constant in spite of influences tending to alter
it. The body accomplishes this by drawing upon the reserve fluid in
the tissues and by varying the rate of water excretion, particularly
through the kidneys. Years ago the doctrine of an increased amount of
blood in the body (plethora) gave rise to the therapeutic use of bleeding.
Especially was this thought to be useful in conditions which we now
recognize as chronic hypertension, and which show no increase in blood
volume. Indeed variation in blood volume is not common, although
plethora may occur in polycythemia, chlorosis, and anemias, and there
may be a temporary reduction in the amount of blood in diseases in
which there is a great depletion of water, as in Asiatic cholera, and fol-
lowing very severe hemorrhage.
While the total quantity of the blood in the body does not vary greatly,
the concentration of its various constituents is subject to distinct change.
The volume percentages of the corpuscles and the plasma can be approx-
imately determined by allowing oxalated blood to sediment or by cen-
trifuging in a graduated cylinder by the use of the hematocrit. Such
methods are not very reliable, but may yield some important information.
Normally 45 to 50 per cent of the volume of blood is composed of cor-
puscles. It varies more or less directly with the number of red blood
cells.
THE WATER CONTENT OF THE BLOOD
Since the blood plasma is essentially a watery solution, some idea of
its water content can be obtained by a determination of the specific
gravity. The most accurate method for accomplishing this is to deter-
mine directly the weight of a given volume of blood and compare it
with the weight of the same volume of water. Since this method re-
quires a rather large amount of blood, indirect methods using smaller
amounts have been devised. One of these (Hammerschlag's) uses a.
solution of chloroform and benzol of a specific gravity of about 1.050,
in which a drop of blood is suspended by delivering it cautiously from
a pipette bent at right angles near its tip. If the drop sinks, chloroform
is added; if it rises, benzol is added until the drop remains suspended.
BLOOD: ITS GENERAL PROPERTIES 87
The specific gravity of the benzol-chloroform mixture is then determined,
and this value is supposed to give the specific gravity of the blood.
The specific gravity of the blood determined in this way varies be-
tween 1.040 and 1.065. It is somewhat less after eating and increases
after exercise; it is slightly lower during the day than at night, and
the variation in .individuals. is considerable. The changes which occur
in the specific gravity of the blood in disease are chiefly due to variation
in the percentage of protein, since the salt content of the blood is rela-
tively fixed. It is only when great changes occur in the concentration
of the noncolloidal salts that they markedly affect the specific gravity.
From 90 to 92 per cent of the plasma and from 59.2 to 68.7 per cent of
the corpuscles consist of water. Of the whole blood, from 60 to 70 per cent
by volume or about 55 per cent by weight consists of plasma; and from
40 to 30 per cent by volume or 45 per cent by weight consists of cor-
puscles.
THE PROTEINS OF THE BLOOD
The plasma obtained by centrifuging the blood rendered noncoagula-
ble by oxalates, hirudin or other means (see page 99), contains 5 to 8
per cent of coagulable proteins. These proteins are serum albumin,
serum globulin, and fibrinogen. They can be separated from each other
by the use of acids and neutral salts. Their proportion varies under dif-
ferent conditions, but is approximately as follows:
Fibrinogen 0.15-0.6%
Serum globulin : 3.8%
Serum albumin 2.5%
The amount of fibrinogen is subject to the greatest variation (Mathews).
Fibrinogen
The least soluble of the blood proteins is fibrinogen. The plasma is
almost freed of it by half-saturation with sodium chloride, or with a
small amount of acetic acid. It is precipitated as fibrin in the process
of blood coagulation (see page 99), and is estimated by weighing the
amount of fibrin which it produces.
Serum Globulin and Serum Albumin
Globulins are ordinarily defined as being insoluble in distilled water,
and albumins as being soluble. It is, however, impossible to separate
serum globulin and albumin satisfactorily in this manner. The globu-
lin obtained by dialysis can be returned to solution by the addition of
88 . THE BLOOD AND THE LYMPH
a suitable amount of water, Avhich makes the salt adherent to the pre-
cipitate a weak saline solution. In neutral or acid solutions it is coag-
ulated by heat at about 75° C. But it does not act as an individual pro-
tein, since a portion of it is precipitated by dialysis or by carbon diox-
ide. Probably serum globulin really consists of two or more proteins.
The serum albumin remaining in solution after saturation with am-
monium sulphate likewise does not represent a chemical entity. It is
possible by carefully heating the solution of serum albumin to distin-
guish three separate coagulation temperatures. This fact has been in-
terpreted as meaning that the serum albumin consists of at least three
closely related proteins.
Since the refractive index of the Mood depends primarily upon the
amount of protein present, it has been taken as a means of determining
variations in the concentration of the proteins. It has been found that
the concentration of the blood proteins varies somewhat; during ex-
ercise it is increased probably because of the taking up of water by
the tissues, and during profuse bleeding it is diminished because
large amounts of fluid are being added to the blood from the lymph,
which is relatively poor in proteins. The ingestion of considerable
amounts of salts has been found to reduce the concentration of the blood
proteins for a short time. In pathologic conditions, as in diabetes, when
rapid changes in the body weight due to alterations in the diet are oc-
curring, changes in the fluid content of the blood are often observed.
Likewise in edema caused by faulty renal function, there may be a re-
tention of fluid in the blood before there is any indication of edema. The
hydremic condition of the blood can therefore be considered as a useful
diagnostic aid in determining the water metabolism.
The relative concentration of the proteins of the blood is also of some
interest, especially since in some diseases a considerable amount of
blood protein is lost. By refractrometric methods it is possible to sep-
arate the globulin and albumin fractions. Normally the total proteins
range between 6.7 and 8.7 per cent, of which the albumins lie between
4.95 and 7.7 per cent, and the globulins between 1 and 2.54 per cent. In
some diseases, as in chronic nephritis, pneumonia, and syphilis, the
total proteins of the blood are decreased and the relative amount of
serum globulin is increased On the other hand, in many mild infections
and chronic septic conditions the globulin fraction may be increased
with no change occurring in the total protein content.3
Our knowledge of the origin and the function of the Hood proteins is
quite unsatisfactory. Previous to the discovery of amino acids, the
building stones of the proteins, in the blood it was thought that the
nitrogenous nutrients were converted somehow into blood proteins dur-
BLOOD: ITS GENERAL PROPERTIES 89
ing or immediately following their absorption from the alimentary
canal, and that the tissue cells were nourished from this common pro-
tein. It is now known that the amino acids are not immediately syn-
thetized into blood proteins after their absorption from the digestive
system. The blood proteins are radically different from the tissue pro-
teins. Substances which retard or accelerate nitrogen metabolism do
not alter the relationship existing between the protein bodies of the
blood. This fact indicates that the. serum proteins have a function quite
independent of the nitrogenous metabolism of the body. They un-
doubtedly maintain the viscosity of the blood and assist in preserving
its neutrality. Attempts to localize the site of formation of the blood
proteins have not been successful. There is some evidence that fibrin-
ogen is formed for the most part in the tissues of the splanchnic area
(liver). It is quite possible that the blood forms its own protejns, just
as do other tissues, from the amino acids it contains.
THE FERMENTS AND ANTIFERMENTS OF THE BLOOD
The blood plasma contains many of the ferments present in the tissues.
The nature of these ferments has been the subject of many investiga-
tions in recent years, primarily because it has been found that they are
intimately connected with the problems of immunity.
Among the ferments the following have been demonstrated in the
blood:
Proteases are probably present normally in the human blood serum
in small amounts, but they are found in large amounts in the white
blood corpuscles. A protein foreign to the body if injected into the
blood ordinarily produces no untoward symptoms, but a second injec-
tion following the first by some days will produce symptoms of poison-
ing known as anaphylaxis. This fact has led to the assumption that
the injection of any foreign protein into the blood promptly leads to
the appearance therein of specific proteolytic enzymes which will digest
the strange protein into its derivatives, which are poisonous. This
power of the body to produce specific proteases has been the subject
of much research and debate, and Aberhalden proposed a test for preg-
nancy, for cancer, and for other conditions in which he made use of this
phenomenon. He believes the presence of placenta or tumor tissue to
cause the presence of proteins that bring about the production of specific
ferments whose duty it is to rid the system of these substances. Other
investigators fail to find the specificity in proteolytic action claimed by
Abderhalden, and believe that proteolytic ferments which are" capable
of digesting foreign proteins are absorbed from the alimentary canal
9Q THE BLOOD AND THE LYMPH
from the digestive juices (Boldyreff). Some investigators fail to confirm
the claim that the proteolytic activity of the blood serum is increased under
the above conditions.
Blood contains an antiferment known as antitrypsin. This can be
removed from the blood serum by several substances, among which are
kaolin, colloidal iron and starch. Serum thus treated shows strong pro-
teolytic activity and autodigestion will occur. In this case there can be
no question of the specific origin of proteases. Abderhalden believes
that the ferments of the blood of the pregnant woman are able to digest
the placental tissue. Human placental tissue has the ability of absorb-
ing antitrypsin and it is very questionable as to whether the test pro-
posed by Abderhalden is due to the new formation of ferments or to
the removal of the antitrypsin and the action of the protease normally
present in the blood.
Nuclein ferments are capable of decomposing nucleic acid and purins
into the. simpler bodies.
Lipases have been demonstrated in the blood.
Amylase. — The presence of starch-splitting ferments in the blood was
first shown by Magendie in 1841, and later Bernard showed that gly-
cogen or starch injected into a vein produced glycosuria. Since then
it has been proved conclusively that diastatic enzymes are normally
present in the blood and lymph. The source of these enzymes has given
rise to much speculation. Some observers believe that they are derived
from the amylopsin of the pancreatic secretion, while others believe that
they are manufactured by the liver. Ligature of the pancreatic ducts
is said to inccease the amount of amylase, while removal of the pan-
creas may (Carlson and Luckhart) or may not (Schlesinger) increase
the amylase of the blood. In some forms of experimental diabetes the
amylase of the blood has been found increased, and this is the case in
human diabetes (Myers and Killian). If this is true, a cause for the
inability of the diabetic to store up glycogen is found. In impairment
of renal function, there is usually an increase in the blood amylase and
a decrease in the urine amylase. This has been suggested as being of
diagnostic value.
The blood contains a feeble glycolytic enzyme capable of destroying
glucose. It is claimed that this power is reduced in diabetics (Lepine).
Catalase is found in the blood and tissues generally. It has the power
of liberating oxygen from hydrogen peroxide without any accompany-
ing oxidation process. Its physiologic significance is not known. It
is said that the amount of catalase is increased during excitement and
exercise,* and is decreased in conditions where the body's activity is
lowered. Its determination is clinically unimportant at present.
CHAPTER XI
BLOOD: THE BLOOD CELL
BY R. G. PEARCE, -B.A., M.D.
THE RED BLOOD CORPUSCLES, OR ERYTHROCYTES
The most prominent function of the blood is to carry oxygen to the
tissues. It owes this property chiefly to the red blood cells which are
present in large numbers (5,000,000 per c.mm. of blood). These cells
are biconcave discs, having a diameter of about 7.7 /*. They are con-
structed out of a framework composed largely of lipoidal material, in
the meshes of which is deposited a substance called hemoglobin, to
which the remarkable oxygen-carrying power of the blood is due. Nei-
ther the manner by which the red cell carries its hemoglobin nor the
intimate structure of the cell itself is accurately known. It is com-
monly believed that the hemoglobin is held enmeshed in a framework
or stroma, or encased in the cell membrane. One thing is certain, how-
ever, that the union of hemoglobin with the stroma of the red cell is
a fairly strong one, since mere fragmentation of the corpuscle fails to
liberate the hemoglobin.. The fact that the framework contains a large
amount of lipoidal substances enables the corpuscles to maintain their
shape and is responsible for their characteristic permeability.
Hemoglobin is a very complex substance belonging to the group of
conjugated proteins. By chemical means it can be broken up into a
simple globulin and a pigment hematin, containing iron. "When com-
pletely saturated, oxygen is present in hemoglobin in the proportion
of two atoms of oxygen to one atom of iron (Peters) ; or 401 c.c. of
oxygen can be carried by hemoglobin containing one gram of iron, the
molecular weight of the molecule being about 16.669, or some multiple
thereof (Barcroft and Peters) (see also p. 397). At this figure the
iron in the molecule would represent 0.34 per cent of the total weight
of the molecule. The corpuscular surface area has been estimated to
be 3200 square meters. There is therefore a very large surface avail-
able for the absorption of oxygen from the alveolar air, as the blood
corpuscles pass in single file through the capillaries of the lungs.
Since the amount of oxygen which the blood can carry depends upon
its hemoglobin content, it is of some importance clinically to have
91
92 THE BLOOD AND THE LYMPH'
methods of determining the approximate amount present. The amount
of hemoglobin present in a quantity of blood is usually determined
colorimetrically by comparing the color of the blood with standard col-
ors which correspond to known strengths of hemoglobin. In normal
persons the amount of hemoglobin varies greatly at different ages, and
in order to determine whether or not a given blood contains more or
less hemoglobin than normal, it is imperative to consider the age. The
greatest variations occur between birth and the sixteenth year. After
the sixteenth year the blood in males usually contains a larger amount
than that in females (Williamson4). Instruments used in determining
the amount of hemoglobin should be standardized to give the value in
grams hemoglobin per 100 c.c. of fluid.
The amount of hemoglobin which is present in each corpuscle in
terms of normal is therefore of some clinical interest. This relation of
the number of red cells to the amount of hemoglobin is known as the
color index and is computed as follows: The average red count in man
is 5,000,000 to the c.mm., and the average minimal amount of hemo-
globin is taken as 13.88 grams in 100 c.c. of blood (=80, Sahli; =90,
Miescher; =86, Plesch; and 110, Tallquist methods). These relative
values give a color index of one. The percentage of normal red cells
divided by the percentage of normal hemoglobin present gives the
color index.
The Origin of the Red Blood Cells
In fetal life the spleen and the liver are generally believed to be re-
sponsible for the formation of the red blood cells. In extrauterine life
this function is taken over by the -red bone marrow. In the primitive
condition all red blood cells are supposed to be nucleated. In extra-
uterine life the nuclei of the red cells are lost, and nonnucleated forms
are alone present in the blood stream. In fetal life and in certain path-
ologic conditions, the rate of blood formation is so rapid that some
nucleated cells appear in the blood. The normal response of the body
to a loss of red blood corpuscles consists in an increased activity of the
blood-forming cells of the red bone marrow. It is not easy to follow
the course of the regeneration of the red corpuscles or to discover the
mechanism of their formation in the bone marrow, since this tissue pre-
sents a mixture of cells which are precursors of the varied corpuscles
found in the blood and the identity of which can not be determined.
Recently new methods of staining blood for microscopic examina-
tion have allowed more detailed study to be made on the site and
method of blood cell formation. When fresh unfixed blood is treated
with solutions of various dyes, such as brilliant cresyl blue, polychrome
THE BLOOD CELL 93
methylene blue or neutral red, an otherwise invisible structure appears
in some cells in the form of coarse granular particles or threads, which
give a reticulated appearance to the corpuscles. These reticulated cells
are more abundant in infants' blood and in patients suffering with se-
vere anemia or hemolytic jaundice than in normal blood, and may be
taken as evidence of the youth of the red cell and not as a degenera-
tive process. Since the number of the reticulated cells that are present
in the blood is more or less directly proportional to the hemopoietic
activities of the bone marrow, enumeration of the reticulated cells is
of clinical importance in anemias. In conditions in which animals have
been made plethoric by the transfusion of blood,- it has been found that
the number of reticulated cells is decreased; the bone marrow of these
animals also shows a marked reduction in reticulated erythroblasts.
The diminished rate of blood cell formation sometimes noted after blood
transfusions may be explained by assuming that the stimulus which
awakens the formation of red cells in the bone marrow is absent or
made subnormal on the injection of red cells into the blood, and thus
the formation of red cells is depressed. Small transfusions are there-
fore preferable to large ones in cases in which the rate of blood forma-
tion is greatly impaired. By means of living cultures of red bone mar-
row the different stages of the development of the normoblasts into
true red corpuscles may be studied (Tower and Herm5). Some evidence
has been gathered from such studies which points to the conclusion that
in place of the red cells being cells which have lost their nucleus, as is
the current teaching, they are rather cells which develop as a nuclear
bud and escape into the circulation as true red cells.. The nucleated
red cell and the red nucleated corpuscle of the bird are the product of
intranuclear activity and are morphologically identical.
Rates of Regeneration of Erythrocytes
Microscopic examination of the blood during rapid regeneration of
red cells shows the presence of nucleated forms. Nucleated red cells
in the blood have therefore been taken as an inevitable feature of rapid
blood regeneration. The evidence upon which this belief depends,
however, is hardly complete, since changes in the manner of red blood
cell formation may be responsible for the nucleated forms. The red
bone marrow is considered the seat of red cell formation, and it is true
that an abnormal increase in the red bone marrow usually accompanies
increased red cell formation. The nature of the stimulus which brings
about the new. formation of red cells is not understood. Oxygen wrant
may be an important factor, since we find the presence of an abnormally
large number of red cells in conditions where there is a scarcity of
94 THE BLOOD AND THE LYMPH
oxygen in the inspired air, as in life at high, altitudes, or a difficulty in
its absorption through the lungs, as in congenital heart disease.
The red cells produced following hemorrhage and in simple anemia
contain less than the normal amount of hemoglobin, but their shape and
size are approximately normal, and few nucleated cells are present. In
the regeneration of red cells which is found in pernicious anemia, we
find the cells containing an unusually large amount of hemoglobin.
The red cells in this disease have abnormal forms, many being large,
with or without a nucleus, and containing basic staining granules.
This type of blood cell formation is due to degenerative changes.
The Fate of the Erythrocytes
The length of life of the red blood cell is unknown. Estimates based
upon the daily excretion of bile pigments are not reliable, since Hooper
and Whipple have shown that the pigments, in part at least, arise from
pigments which the liver has made in excess of its needs for the manu-
facture of hemoglobin, and which, not being needed, are excreted.5
There is no question however that every erythrocyte sooner or later
undergoes disintegration, a process formerly thought to be ushered in
by the ingestion of the red blood cell by a phagocyte in the spleen or
in a hemolymph gland, the hemoglobin of the disintegrated cell being set
free and carried to the liver, where it is broken up into hematin, which
the body stores for future use, and into bile pigments, which are ex-
creted. Rous and Robertson6 fail to find evidence that this process
occurs in man to an extent sufficient to account for the normal destruc-
tion of the blood cells. However they have recently found another and
unsuspected method for blood destruction in all animals thus far
studied — namely, the disintegration of the blood cells by fragmentation
while they are circulating, without loss of their hemoglobin. These
fragmented cells are found most frequently in the spleen. They believe
that the small ill-formed cells, known as microcytes and poikilocytes,
observed in severe experimental anemias, are due not to the fact that
they are produced by the bone marrow, but rather to the fact that the
marrow in its anemic condition is not able to produce a resistant ery-
throcyte, and fragmentation therefore takes place too readily.- A sim-
ilar condition may exist in the severe anemias of man and account for
the general high resistance of the red cells found in the blood of these
patients, inasmuch as the weak cells are generally fragmented very soon
after they are formed. Long ago Ehrlich stated that the microcytes
and poikilocytes of anemia are the result of fragmentation of the cells
in the circulating blood, but he believed that this fragmentation was a
THE BLOOD CELL 95
purposeful division in order to increase the total surface of the red
cells. The ultimate fate of the red cell fragments is not known. It is
reasonable to suppose that the fragmented bits containing hemoglobin
are carried to the liver, where the hemoglobin is transformed into
hematin and bile pigments.
Hemolysis
Another method of red blood cell destruction, which, however, does
not take place normally, is by hemolysis. The nature of the combina-
tion of the hemoglobin with the stroma of the red cell, as already re-
marked, is not definitely known. That it is not merely contained in a
sac is shown by the fact that the cell may be cut into bits without the
hemoglobin being set free. In some manner the hemoglobin is chem-
ically bound with the stroma of the red cell, from which it can be
freed by a number of physicochemical and chemical agents. This proc-
ess is known as hemolysis, and the substances which bring it about are
known as hemolytic agents. The manner in which these agents effect
the release of hemoglobin from the blood is quite varied.
If the osmotic pressure of the plasma is lowered by dilution, the pres-
sure within the corpuscle remains high, and water is absorbed by the
cell. If this absorption is sufficient, the cell ruptures and the hemoglobin
is discharged. For this reason it is necessary in diluting the blood to
use solutions of salt having an osmotic pressure equal to that of the
blood to protect the red cell from hemolysis. This is obtained by using
a 0.9 per cent solution of sodium chloride. Better results are had,
however, by using either Ringer's solution (0.9 per cent NaCl, 0.026
per cent CaCl2, and 0.03 per cent KC1) or Locke's solution (0.9 per cent
NaCl, 0.024 per cent CaCl2, 0.042 per cent KC1, 0.01-0.03 per cent
NaHC02 and 0.1 per cent glucose).
In normal corpuscles hemolysis occurs to a small extent in solu-
tions containing about 0.42 per cent of sodium chloride. In certain
diseases the fragility of the corpuscles may be increased (Butler7).
The membrane and stroma of the erythrocyte contain lipoidal ma-
terial which is soluble in alcohol, ether, fatty acids, and bile salts.
Addition of these agents to the blood brings about hemolysis, presum-
ably by dissolving the lipoidal material present. The hemolysis which
occurs with saponin is similar in type, since saponins combine with
lipoids, the compound being soluble in water.
The hemolytic properties of serum, whether they are found to be
normally present when the bloods of certain animals are mixed or to
be produced artificially by the injection of foreign red cells, furnish a
subject of great interest from the standpoint both of immunology and
96 THE BLOOD AND THE LYMPH
of clinical medicine. The hemolytic serum produced by the injection
of foreign corpuscles owes its activity to two substances. The one
called the amboceptor, or immune body, is specific against the type
of cell injected and is increased during immunization. The second
body is the complement; it is nonspecific, and is not increased dur-
ing immunization. Complement is destroyed by heating the serum for
one hour at 55° C., leaving the amboceptor alone present. Corpuscles
placed in such serum are not hemolyzed until complement either from
fresh immune or from nonimmune serum is added.
The serum of animals possessing natural hemolytic properties towards
the corpuscles of other animals likewise owes its effect to the joint action
of amboceptors and complement.
Ordinarily the serum from animals of one species does not exhibit
hemolytic properties to blood from another animal of the same species.
In unusual cases, however, the serum of an animal will produce hemol-
ysis of the corpuscles of an animal of the same species. Such sera are
said to possess isohemolysins. The fact is of great importance in the
transfusion of blood from one individual to another.
The cause of the acute hemolysis which occurs in the disease parox-
ysmal hemoglobinuria is not known. It is probably due to the presence
of a hemolytic substance Avhich unites with the blood corpuscles at
temperatures below the normal body temperature, since the attack fol-
lows exposure to cold, and blood from patients subject to the condition
may be hemolyzed in vitro by cooling and subsequently heating it.
LEUCOCYTES
There are a number of varieties of white cells in the blood. These are
differentiated from one another by their shape, staining properties, and
the granules in their protoplasm. We may divide them into two main
groups — nongranular mononuclear cells and granular polynuclear cells.
The nongranular mononuclear cells are termed lymphocytes. Two va-
rieties are differentiated, the small and the large.
The small mononuclear leucocyte makes up from 23 to 28 per cent
of the total leucocytes and the large mononuclear, from 2 to 4 per cent.
The polynuclear leucocytes are divided into three groups according
to whether their granules stain with basic, neutral or acid stains. The
leucocytes that stain with basic dyes, or the basophile cells, are very
few, making up less than one per cent of the total count. Likewise the
acid-staining granular cells, acidophile, are few, comprising from 2 to
4 per cent of the total count. The most numerous are the neutrophiles,
THE BLOOD CELL 97
or the polynuclear leucocytes, with neutral-staining granules. These
comprise from 65 to 75 per cent of the total count.
Another type of white cell is known as the transitional cell, because
it was supposed to represent an intermediate form between the mono-
and polynuclear cells. Probably such transitions do not occur, and the
transitional leucocyte is related to the mononuclear cells.
The polynuclear cells originate in the bone marrow, and for this
reason .have been termed myeloid cells. They develop from cells in
the bone marrow termed myeloblasts, wrhich are nongranular and con-
tain a large nucleus. In the course of development the characteristic
granules appear, and the nucleus remains round and later becomes
lobulated. These intermediate forms are called myelocytes. The mono-
nuclear cells originate in the lymphatic tissues of the body.
The leucocytes possess the ability to make ameboid movement and
to ingest foreign particles which may be presented to them. On ac-
count of this latter ability they are commonly called phagocytes. In
the process of inflammation the leucocytes assemble at the spot which
is the seat of the injury or infection, and remove the foreign organism
or necrotic tissue by ingesting and digesting it.
It is not definitely known whether or not the lymphocytes func-
tion as phagocytes. Other functions besides those as phagocytes have
been ascribed to the white cells, but they are not universally ac-
cepted. The number of leucocytes in the blood is subject to con-
siderable variation. They normally number between 6,000 and 8,000
per c.mm. At the height of digestion and after strenuous exercise
there is usually a small increase, and under pathologic conditions,
especially in infectious diseases, this becomes quite marked. Some
infections increase the polymorphonuclear cells, while others add to
the lymphocytes. The factors governing the type of increase are not
fully known, nor are the functions of the various forms differentiated.
The Blood Platelets
These are small oval particles about 3 p in diameter, which are found
in large numbers (250,000 to the c.mm.) in the blood. They are sup-
posed to be formed from particles of protoplasm which are pinched
off from the large blood cells in the bone marrow. Their biological
and chemical properties are not understood. They probably play a
very important role in the coagulation of the blood (see page 103).
CHAPTER XII
BLOOD: BLOOD CLOTTING
On leaving the blood vessels, the blood clots so as to form "a plug,
which assists in preventing further hemorrhage. The clotting must
therefore be considered as a protective mechanism against excessive
draining of blood out of the organism. When the wounded vessels
are small, the clotting, along with constriction of the damaged vessels
and the formation in them of thrombi containing large numbers of
platelets, serves to effect complete stoppage of the hemorrhage even
though the blood pressure may not have become materially reduced.
The greater loss of blood from larger vessels causes the arterial pressure
to fall, and this enables the clot to stiffen and seal the wound before
the pressure again rises. When the clotting power of the blood is
subnormal, life is endangered by even trivial wounds; under these
conditions the smallest surface scratch may continue to bleed exces-
sively in spite of whatever local treatment is applied. The most ex-
treme degree of this condition occurs in hemophilia, a disease which
is characterized by a most interesting family history — namely, that
although it affects only certain of -the male members of a family,
yet it is transmitted from generation to generation by the female side
alone. The disease has existed in certain of the royal families of
Europe for many generations, which has made it possible by con-
sulting the genealogic trees to demonstrate the infallibility of this
law of inheritance.
The clotting of the blood is also either depressed or increased in a
variety of physiologic and pathologic conditions. We shall, however,
defer further consideration of these until we have learned something
of the nature of the factors which are responsible for the process itself.
The Visible Changes in the Blood During Clotting
In a few minutes after it leaves the blood vessels, the blood forms a
jelly-like clot, which adheres to the walls of the container in which the
blood is collected and soon becomes so solid that the vessel may be
inverted without spilling any of the blood. Clotting is now said to be
complete. The clot soon begins to contract, and as it does so, drops of
clear fluid or serum become expressed and float on the surface of the
98
BLOOD CLOTTING 99
clot or collect between it and the walls of the container, so that after
some time the clot breaks away from the container and comes to float
in the serum. The latter may be perfectly clear, but usually is more or
less opalescent, partly because of the presence of fat, and partly be-
cause of leucocytes which have migrated out of the clot on account of
their power of diapedesis.
If a drop of freshly shed blood is examined under the microscope, it
will be observed that the first step in clotting consists in the formation
of fine threads radiating from foci, which are undoubtedly the blood
platelets. The fine threads are called fibrin. They multiply rapidly,
so as to form an interlacing meshrwork which entangles the red blood
corpuscles and leucocytes. By the use of the ultramicroscope (page 52),
How ell1 and others have observed that the fibrin (produced by adding
thrombin to oxalated plasma) is really deposited in the form of fine
crystalline needles — "fibrin needles" — which become packed together
as they increase rapidly in numbers. Although the process of clotting
consists therefore in the conversion of a hydrosol into a hydrogel (see
page 60), it is a unique process; a solution of the blood protein which
is responsible for the formation of the fibrin (fibrinogen) may, like other
colloidal solutions, be precipitated in a variety of ways, but it is only
when the conditions are favorable for blood clotting that fibrin needles,
and therefore fibrin threads, are formed. The blood of invertebrates
forms a structureless gel when it clots (Ho well).
Methods of Retarding Clotting of Drawn Blood
To understand the nature of the clotting process and the factors that
are responsible for its occurrence, it is advantageous to simplify the
conditions somewhat by getting rid of the red corpuscles and most of
the other formed elements of the blood and then using the fluid in
which these are suspended in living blood — namely, the plasma. This
separation of blood into corpuscles and plasma is readily effected either
by sedimentation or by centrifuging after measures have been taken to
inhibit or greatly delay the clotting process. The methods used for this
purpose are numerous. A few of the most important are as follows:
(1) Keeping the blood at a temperature very slightly above freezing
point. This method is, however, not very effective unless the blood is
immediately received into narrow vessels placed in ice and the tempera-
ture kept most strictly at the low level. In the case of horses' blood and
other slo\vly clotting bloods, the method succeeds without these precau-
tions. (2) Receiving the blood through a strictly clean and smooth can-
nula, coated with a layer of paraffin or vaseline, into a vessel similarly
coated. This method is of practical importance when it is necessary to
100 THE BLOOD AND THE LYMPH
transfuse blood without making a vessel-to-vessel anastomosis. (3) Mix-
ing the blood with chemicals that are capable of removing the calcium
from solution. Such reagents are potassium or sodium oxalate (in a con-
centration of 0.1 per cent after mixing), and sodium fluoride and sodium
citrate (2 per cent solution, with one part of the solution to four parts
of blood). (4) Mixing the blood with certain neutral salts, particularly
the sulphates of sodium and magnesium (one part of 27 per cent solution
of magnesium sulphate mixed with four parts of blood). Blood thus
treated is known as "salted blood," and the plasma separated by centri-
fuging, as "salted plasma." Clotting is readily induced by adding water
to the salted blood or plasma, and in this way diminishing the concen-
tration of the salts. (5) The addition to blood of one of a class of sub-
stances known as antithrombins. Leech extract or the purified substance
separated from it, knowrn under the trade name of "hirudin," and sub-
stances present in blood removed from animals after they have been
injected with peptone solutions, are examples.
The methods which have just been described are those applied to blood
after it has left the blood vessels. Another interesting group of anti-
coagulants prevent clotting only when injected into the blood vessels of
the living animal. The most powerful example of this group is snake
venom, certain varieties of which can prevent clotting in the dosage of
%QO °f a milligram for each kilogram of body weight. Similar but much
less potent effects are produced by the injection of several proteolytic
enzymes, but most attention has been paid to the effect of commercial
peptone injected in solution intravenously in the proportion of 0.3 gram
to each kilogram of body weight. Blood subsequently removed up to about
half an hour or more does not clot, and as we have already seen, if added
to blood from another animal, materially retards clotting. This group of
intra vitam anticoagulants is particularly interesting, since none of the
substances belonging to it is capable of preventing clotting of blood
when mixed with this after it has been shed. Their action therefore
obviously depends on the production of some substance in the body,
probably, as we shall see later, in the liver, since they fail to act after
the removal of this organ from the circulation (see page 111).
The time of clotting varies greatly according to the conditions under
which the blood is collected and the animal from which it is derived.
Human blood, for example, received into a test tube from a puncture
through the skin may clot at any time within three or ten minutes, five
minutes being taken as an average time for blood kept at a temperature
of about 20° C. This time may be considerably shortened by increasing
the extent of foreign material with which the blood comes into contact,
and more particularly by whipping the blood with a bunch of twigs or
BLOOD CLOTTING 101
wires. In this latter case, however, the clot does not form in the usual
manner, but the fine threads of fibrin collect on the twigs or wires, leav-
ing behind the blood serum with the corpuscles still suspended in it.
The fibrin removed in this way may then be washed free of. adherent
serum. The serum and corpuscles now form defibrinated blood, which
is used for many physiologic purposes. Clotting is also greatly acceler-
ated by allowing the blood to flow over exposed tissues. Something is
evidently added to it from the tissues which accelerates the clotting
process, this influence being particularly marked in the case of blood
of the lower vertebrates. When the blood of the bird, for example, is
received through a cannula inserted directly into a vessel with as little
injury to the walls as possible, it very slowly clots if at all, but soon
does so if the blood is allowed to come into contact with excoriated
tissues, or if it is mixed with tissue extract, such as that of muscle.
Clotting is considerably accelerated by warming the blood. The ap-
plication of a cloth or tampon well wrung out with hot physiological
saline to a wounded surface is a most efficient means of allaying hem-
orrhage from vessels too small to ligate.
The Nature of the Clotting Process
Plasma obtained by centrifuging blood that has been prevented from
clotting by one of the foregoing methods can be made to clot by removing
the inhibiting influence ; for example, in cooled plasma by warming the
blood to room temperature, in salted plasma by diluting it with at least
an equal volume of water, and in decalcified plasma by adding a suffi-
cient amount of soluble calcium salts to combine with all the added
oxalate and leave a small trace of calcium salts in excess.
The first question concerns the source of the fibrin, and the answer to
it is furnished by comparing the composition of bood plasma with that
of serum. Though both of these fluids contain the proteins, albumin
and globulin, in approximately the same concentrations, the plasma also
contains another protein not unlike globulin in most of its reactions,
but distinguished from typical globulin in that it is precipitated by
half-saturation writh sodium chloride, in which typical globulin is solu-
ble, and is more readily coagulated by heat. To produce half-saturation
of the plasma with sodium chloride, equal volumes of plasma and satu-
rated sodium-chloride solution are mixed together. The precipitate of
fibrinogen, as the substance is called, is then collected at the bottom of
the tube by centrifuging and is washed several times by decantation with
half-saturated sodium-chloride solution. The washed precipitate, dis-
solved in weak saline solution (preferably containing a trace of bicar-
bonate), will then be found to clot under certain conditions.
102 THE BLOOD AND THE LYMPH
The next question concerns the nature of the conditions that cause the
fibrinogen to clot. When a fibrinogen solution is mixed with a few drops
of blood serum, a clot usually forms, which however is not the case when
plasma is added or when the serum is heated before adding it. Because
a small quantity of serum is capable of causing the clotting of a large
quantity of fibrinogen solution or plasma, it is supposed that the active
substance present in it is of the nature of a ferment — fibrin ferment or
thrombin. It must be pointed out, however, that there is considerable
doubt whether this active body is really of the nature of a ferment or
enzyme. For example, although heated serum does not cause clotting,
thrombin, prepared from serum by the method about to be described, in
the absence of inorganic salts can withstand even a boiling temperature.
Moreover, true enzymes are characterized by the fact that, like other
catalytic agents, a very minute quantity can effect a change in an indef-
inite amount of substance without the enzyme becoming used up in the
process (page 72). When thrombin is allowed to act upon a fibrinogen
solution, on the other hand, it is said that only a fixed amount of fibrin
can be formed when a small amount of thrombin is added. Neither does
this amount increase when the time of reaction is prolonged.
Whatever may be the significance of the foregoing facts, it is impor-
tant to know that the clotting substance, thrombin, can be isolated from
blood serum in a tolerably pure condition. For this purpose blood
serum is allowed to stand under a large volume of alcohol for a week or
two ; the precipitate is then collected and rubbed up with water, which
extracts the thrombin from it, leaving the serum protein in a coagulated
state. The resulting watery solution of thrombin may be further pre-
cipitated by alcohol, the precipitate washed in alcohol and redissolved
in water, yielding ultimately a solution which exhibits very marked co-
agulating powers Avhen added to plasma or fibrinogen solution. Throm-
bin shows most of the protein reactions but it is not coagulated by heat.
As would be expected, a considerable quantity of thrombin remains
adherent to the fibrin formed in the process of clotting, and Howell8
describes a very useful method by which it can be separated from fibrin
and preserved in a dry condition. Briefly stated, this method consists
in allowing washed fibrin to stand overnight under eight per cent
sodium-chloride solution, which dissolves the thrombin. The resulting
extract is then mixed with an equal volume of acetone, which throws
down a precipitate containing the thrombin. To preserve it, the precip-
itate is collected on a number of small filter papers, which are subse-
quently opened out and dried by exposure to a current of cold air before
an electric fan. When the thrombin solutions are desired, the dried pre-
cipitates are extracted with a little water.
BLOOD CLOTTING 103
Thrombin does not exist in blood plasma, for if a clean and paraffined
glass tube is inserted into an artery and the blood collected under al-
cohol, the precipitate after standing a few weeks will yield no thrombin
when triturated with water. Quite clearly, therefore, the thrombin is
produced at the time the blood clots, and the question arises, What is
it produced from? It will be remembered that, when the blood is ex-
amined under the microscope during the clotting process, the fibrin
threads are seen to- start from foci which correspond to the blood plate-
lets. It would appear therefore that the thrombin must be derived from
some substance that is shed forth from the platelets during the disin-
tegration which they undergo shortly after the blood is shed. The sub-
stance is called prothrombin. The platelets or their precursors, the
megacaryocytes of red bone marrow, are probably not its only source,
for clotting may occur in the complete absence of platelets, when it
appears to come from the leucocytes. Prothrombin appears plentifully
in the fluid used to perfuse red bone marrow outside the body (Drinker
and Drinker9).
To sum up what we have so far learned, it may be stated that the
process of clotting starts with the disintegration of blood platelets and
probably of leucocytes, as a result of which there is shed forth into the
plasma a substance called prothrombin, which immediately afterward
becomes activated or converted into thrombin. The thrombin then at-
tacks a protein present in plasma called fibrinogen, producing from it in
thread-like form the insoluble protein, fibrin. But this does not com-
plete the history, for at least two other important factors come into
play; the one is the presence of soluble calcium salts, and the other that
of peculiar substances derived from the tissues outside the blood vessels
and called thromboplastic substances or thromboplastin (Howell). We
must now consider the action of these two factors.
The Influence of Calcium Salts. — As already explained, the proof that
soluble calcium salts are necessary for clotting is furnished by the ob-
servation that the process is entirely prevented when the freshly drawn
blood is mixed with soluble oxalate. To this proof, however, objection
might be made on the score that the oxalate per se inhibited the clotting.
That such is not the case is indicated by the fact that, if the oxalated
blood or plasma is dialyzed against physiologic saline solution till all
the soluble oxalate has been removed from it, clotting is still absent but
immediately supervenes if some soluble calcium salts are added. The
question arises as to how the calcium ion acts. Two possibilities exist:
(1) that it 'is concerned in the conversion of fibrinogen to fibrin, and
(2) that it is necessary for converting prothrombin into thrombin. It
can quite readily be shown that it is by the second of these processes
104 THE BLOOD AND THE LYMPH
that the calcium acts; for example, clotting occurs when purified throm-
bin is added to dialyzed oxalate blood or plasma or to a pure solution of
fibrinogen. Citrates prevent clotting by forming calcium citrate, which
although soluble does not ionize in solution. It is the free calcium ions
that are important. The action of the fluoride is somewhat mysterious,
for it has been found that to produce clotting in fluoride plasma the sim-
ple addition of calcium chloride will not suffice; thrombin itself must be
added as well. Some authors assert, however, that if the calcium chlo-
ride is added cautiously to " fluoride" blood, it will induce clotting
(Rettger).- . In any case it appears that the fluoride does something more
than precipitate the calcium; possibly it prevents the breaking up of
platelets and leucocytes.
The Influence of the Tissues. — As already stated, when slowly clotting
blood, like that of a bird, is collected through a sterile glass tube into a
thoroughly clean vessel and immediately centrifuged, the plasma will
often remain indefinitely unclotted. If an extract of some tissue, such
as muscle, is added, however, the plasma immediately clots. To a much
less degree, the same phenomenon is exhibited by mammalian plasma
when it is collected in a similar manner. From these observations the
conclusions may be drawn that the tissues furnish some substance as-
sisting in the clotting process, and that this substance is also formed
from certain elements present in mammalian but not present in avian
blood. The absence of platelets from the latter blood suggests that
these must be the source of the activating substance in mammalian blood.
It is plain that this tissue factor in clotting is of importance in hasten-
ing the process when an animal is wounded.
Before attempting to formulate an hypothesis that will explain the
process of clotting, it is necessary to call attention to one other impor-
tant fact. This refers to the presence in blood of a substance that pre-
vents clotting and is hence called antithromfoin. Antithrombin.is pres-
ent in normal blood, for a given specimen of pure fibrinogen will clot
less rapidly when mixed with serum to which some oxalated plasma has
been added than with an equal amount of the same serum correspond-
ingly diluted with a solution of soluble oxalate. A striking increase
in the concentration of antithrombin in blood can be brought about by
rapidly injecting a solution of commercial peptone into the blood ves-
sels fifteen to thirty minutes before bleeding. The peptonized blood or
plasma will remain fluid for many hours, if not indefinitely. That the
failure of this blood to clot depends on the presence of some anticlotting
substance, and not upon the absence of one of the necessary clotting sub-
stances (fibrinogen, thrombin, etc.), is evidenced by the fact that the
addition of some of it to a mixture of thrombin and fibrinogen inhibits
BLOOD CLOTTING 105
the coagulation, which it does not do, however, if it is first of all heated
to 80° C. and filtered free of the coagulated protein. Moreover, the
antagonistic action is quantitative in the sense that a fixed amount of
the peptone-plasma inhibits the action of a fixed amount of thrombin.
The source of antithrombin in the body appears to be mainly at least
the liver, for it has been found: (1) that peptone injection into an animal
from which the liver has been removed does not cause antithrombin to
be formed (Denney and Minot) ;10 (2) that peptone injections into the
portal vein cause antithrombin to appear in the blood much more rap-
idly than when the injection is made into a systemic vessel; and (3) that,
when the liver is perfused outside the body with a perfusion fluid con-
taining peptone, antithrombin accumulates in the perfusion fluid.
A fluid containing a high concentration of antithrombin is secreted
by the so-called salivary gland at the head end of the leech. The func-
tion of the fluid is to prevent clotting of the blood, so that the animal
may continue to suck it without interference by clotting. After apply-
ing leeches for medicinal purposes it is therefore necessary to wash the
wound thoroughly with water so that all traces of .the antithrombin may
be removed ; otherwise the bleeding may continue for a considerable time.
Practical use is made of this effect of the leech to prevent clotting of blood
outside the body, or it may be used to inhibit coagulation intra vitam in
experiments where clotting would otherwise interfere with 'their prog-
ress; for example, in crossed circulation experiments (page 365) and in
experiments in vividiffusion (page 607). For such purposes the leech
head is cut off and extracted either with saline or by treatment with
chloroform, which removes other proteins from the saline solution leav-
ing a strong antithrombin, known under the trade name of "hirudin."
At temperatures about that of the body the action of antithrombin is
greatly augmented. In animals like the mammals in which the content
of antithrombin is small, this may be important in maintaining the flu-
idity of the blood (Howell). Blood containing antithrombin can be
made to clot by the addition of thrombin, and therefore of blood serum.
CHAPTER XIII
BLOOD: BLOOD CLOTTING (Cont'd)
THEORIES OF BLOOD CLOTTING
Attempts to link all the foregoing facts together in the form of a
simple theory have not so far been entirely successful. All agree that
the fibrin is derived from fibrinogen by the action of thrombin, the points
in dispute being those which concern the origin of the thrombin and
the mode of action of the calcium and thromboplastic substances. The
theory most widely accepted in Europe is that of Morawitz, according
to which the thrombin exists in living blood in an inactive state called
thrombogen (prothrombin), which becomes converted into thrombin by
the simultaneous action on it of soluble calcium salts and of thrombo-
plastic substances furnished by the tissue cells in general and by the
cellular elements of the blood platelets and leucocytes. According to
this view the thromboplastic substance, aided by the presence of calcium
ions, converts thrombogen (prothrombin) to thrombin. It acts there-
fore as a kinase and is called thrombokinase. The fundamental fact of
this theory, then, is that kinase is necessary for the union of the cal-
cium Avith prothrombin — a fact, however, which is challenged by Howell,
who states that prothrombin may be converted to thrombin by the action
of calcium ions alone. This investigator believes that the thrombo-
plastic substance acts not as a kinase but because it neutralizes anti-
thrombin, which is constantly present in the blood, and the function of
which is to prevent the calcium from uniting with the prothrombin to
form thrombin. Howell's theory in his own words is as follows: "In
the circulating blood we find as constant constituents fibrinogen, pro-
thrombin, calcium salts and antithrombin. The last named substance
holds the prothrombin in combination and thus prevents its conversion
or activation to thrombin. When the blood is shed, the disintegration
of the corpuscles (platelets) furnishes material (thromboplastin) which
combines with the antithrombin and" at the same time liberates more
" prothrombin; the latter is then activated by the calcium and acts on
the fibrinogen." Antithrombin can also prevent the action of thrombin
on fibrinogen. As already pointed out, the thromboplastin can be de-
rived from the blood itself in the mammals, but only from the tissues
jn the lower vertebrates. It is interesting to note that the thromboplastin
100
BLOOD CLOTTING 107
can be extracted from the tissues by fat-solvents, and that it appears to
belong to the class of phosphatids, being indeed closely related to, if
not identical with, kephalin (Howell).
Intravascular Clotting
The practical application of the theory of blood clotting concerns the
manner in which the blood is maintained in a fluid condition in the blood
vessels, and the disturbance of this function causing intravascular clot-
ting. According to the one theory, the blood is maintained fluid by the
absence from it of any considerable quantity of kinase, and according
to .the other, by the presence in it of an amount of antithrombin suffi-
cient to prevent the union of calcium with prothrombin. The fluidity
is maintained even when large amounts of thrombin or of blood serum,
which contains this substance, are injected into the living animal. We
can best explain the immunity of the blood to the action of thrombin un-
der these circumstances as being due to the instantaneous appearance in it
of antithrombin in amounts sufficient to prevent the action of thrombin
on fibrinogen, for, as stated above, it is claimed by Howell that anti-
thrombin has this influence as well as that of preventing the conversion
of prothrombin into thrombin.
Intravascular clotting may be brought about by a variety of means:
(1) Mechanical damage to the lining of the blood vessels; after the ap-
plication of a ligature, for example, the damaged endothelium is soon
covered by a clot, which gradually becomes firmer and firmer, and may
spread up the vessel to the next branch. (2) The presence of foreign
substances in the blood. Emboli, for example, are apt to cause clots
to form at the places where they stick, namely, in the smaller vessels.
Clotting is also a frequent occurrence when there are local dilatations of
the cardiovascular tube, and it may occur under imperfectly understood
conditions causing the condition known as thrombosis. (3) An inter-
esting variety of intravascular clotting is that caused by the intrave-
nous injection of saline extracts of cell-rich tissues, such as the thymus,
lymph glands or testes (Wooldridge). By precipitation with acetic
acid and digestion with peptone, a residue can be obtained from these
extracts which, when dissolved in alkali, has a very pronounced intra-
vascular clotting effect. Since these precipitates are very rich in phos-
phorus, it is probable that they are of the nature of phosphoprotein
(nucleoalbumin). Their action must depend on neutralization of anti-
thrombin, according to Howell 's theory, or because they serve as throm-
bokinases (according to Morawitz' theory).
As a matter of fact, however, the foregoing observation is not com-
pletely explained by either theory. If in place of making one injection
108 THE BLOOD AND THE LYMPH
frequent injections of small amounts of the above material are made,
instead of intravascular clotting, a delay in the coagulation time is
likely to occur. Indeed, repeated injections of small amounts may en-
tirely remove the clotting power of the blood. The readiness with which
this so-called "negative phase" appears, seems to depend on the nutri-
tive condition of the animal at the time of injection. If a large dose is
injected into a fasting dog, for example, thrombosis is confined to the
portal area, w-hereas if it is injected into a recently fed animal, the
thrombosis is universal throughout the vascular system. The develop-
ment of the negative phase is undoubtedly dependent upon some reac-
tion on the part of the living cells of the organism, since it does not occur
on the addition of similar substances to blood outside the body. The
reaction is, indeed, akin to that by which immune bodies in general are
produced. For example, a toxin injected in large amount has a cer-
tain toxic effect, but in repeated small doses with intervening intervals
it leads to the production of an antitoxin. So with the substance in
question ; a large dose injected at one time causes a positive effect — clot-
ting— but smaller doses frequently injected, the opposite effect — want of
clotting. It is probable, as suggested by Starling, that more intensive
study of the conditions causing intravascular clotting will throw con-
siderable light on the general question of the production of immunity.
Measurement of the Clotting Time
To measure the clotting time of drawn samples of blood, several con-
ditions must be observed. These have been tabulated by Addis11 as
follows:
1. The specimens of blood must always be obtained by exactly the
same technic. It would introduce serious errors to compare the clot-
ting time of one specimen of blood received from an incision of the
skin (ear lobe) with that of another collected in a syringe by veni-
puncture.
2. The temperature conditions must always be the same. Probably
25° C. is the best temperature to use. Higher temperatures are unsuit-
able for two reasons: first, because during its collection the blood will
have become cooled to about or below this point, and time would be con-
sumed in raising it higher; and second, because the time of coagulation
is more and more shortened for each degree that the temperature is
raised, this acceleration becoming especially pronounced for tempera-
tures above 25° C. Quite apart from the liability to incur errors inci-
dent to measurement of shorter periods of time, observations at higher
temperatures necessitate most rigorous adherence to a fixed temperature
of the water-bath. Temperatures much below 25° C. are unsuitable, be-.
BLOOD CLOTTING
109
cause the clotting sets in gradually and it is difficult to tell precisely
when it occurs.
3. The blood must always be collected in the same sort of vessel and
come in contact with the same kind and amount of foreign material.
To this it may be added that the receiving vessel must be scrupulously
clean ; any trace of old blood clot or of serum is especially to be guarded
against.
4. The end point must be sharp. It is here that the greatest technical
difficulties are met with in making precise measurements, and it is
greatly to be desired that different investigators should adopt some uni-
form method. For experimental purposes the method of Cannon and
Mendenhall12 is no doubt the best, and it has the added advantage of
giving a graphic record of the observations. The accompanying figure
(Fig. 19) shows the principle of the method. The blood is received
through a standard cannula (C) into a tube (T) 5 cm. long and of 5 mm.
t
R1
Fig. 19. — Diagram of the graphic coagulometer. The cannula at the right rested in a water
bath not shown in this diagram. For further description see text. (From Cannon and Men-
denhall.)*
internal diameter; and a loop (of 2 mm. diameter) at the end of a
copper wire (Z>), which is 8 cm. long and 0.6 mm. in diameter, is al-
lowed to fall gently into the blood at. regular intervals. The upper end
of the wire is articulated with the short arm of a light lever so counter-
poised that when the stop (R), which ordinarily holds it in a horizontal
position, is released, the wire, now having a net weight of 30 mg., falls
on the blood in the tube. The long arm of the lever is provided with
a 'writing point, which is made to inscribe its movements on a drum.
So long as the blood is unclotted the loop sinks into it when the lever
is released and a vertical line is traced, but whenever clotting occurs
the loop sticks on the blood and the writing point does not rise.
For clinical purposes where blood collected in a syringe by venipuncture
is used, the method of Howell13 is most accurate. It consists in placing,
*Am. Jour. Physiol., May 1, 1914, xxxiv, No. 2.
110 THE BLOOD AND THE LYMPH
2 or 4 c.c. of the blood in a wide tube (of 21 mm. diameter) that has
been cleaned by a bichromate-acid mixture. The period that elapses
between the moment of the entry of fluid into the syringe and that at
which the clot has become firm enough so that the tube can be inverted
without spilling any blood, is taken as the clotting time. Since the blood
does not come in contact with exposed tissues, it takes from 20 to 60
minutes to clot by this method.
For routine clinical examination of blood taken from a skin wound
Brodie and Russel's method14 is most satisfactory. This consists in
principle in observing a drop of blood, under the low power of the
microscope, while a fine current of air is gently blown against it at
regular intervals in a tangential direction. Until clotting sets in, the
individual corpuscles move freely in a circular direction, but as soon
as clotting begins they move in masses which soon tend to become fixed
so that, although they move somewhat when the air impinges on them,
they immediately return to their original position when the current
is discontinued. When clotting is complete, the air current merely
Fig. 20. — Coaguloniett- r. The drop of blood is placed on the lower end of the glass cone and
the air stream is directed against it from the side tube shown by the black dot. The apparatus
is placed on the stage of the microscope and the drop observed by the low power.
presses on the corpuscles at one point. By this method the clotting
time averages five minutes. A convenient apparatus for this method is
that of Boggs, which is shown in Fig. 20. It consists of a truncated
cone of glass, projecting into a moist chamber provided with a tube on
the side so arranged that when air is blown into the chamber, it strikes
the drop of blood placed on the end of the cone tangentially.
Blood Clotting1 in Certain Physiologic Conditions
Besides the experimental conditions already enumerated as changing
the clotting time in the blood of laboratory animals, special mention
must be made of the influence of epinephrine injections, of conditions
supposed to cause a hypersecretion of this hormone, of the emotions,
and of hemorrhage.
Epinephrine added to drawn blood does not affect the clotting time,
but if small amounts are injected intravenously or even subcutaneously,
a marked decrease occurs (Cannon and Gray; cf. Cannon, loc. cit.).
Larger injections may have the opposite effect, and intermediate amounts
BLOOD CLOTTING 111
may cause at first a prolongation and later a shortening of the time.
These results with larger doses are related to Howell's observation that
repeated doses of relatively large amounts of epinephrine in dogs may
so greatly retard coagulation as to make the animals practically hemo-
philic. It was further found by Cannon and his coworkers that epineph-
rine does not influence the clotting time when injected into animals
from which the abdominal viscera have been removed from the circulation
by ligation of the inferior vena cava and the abdominal aorta. In the light
of the influence which destruction of liver cells (by phosphorus, chloro-
form, etc.) is known to have in lengthening clotting time, it would seem
reasonable to conclude that it must be through this organ that epineph-
rine develops its clotting effects.
Stimulation of the splanchnic nerves also shortens the clotting time,
and it would appear that this action depends on the resulting hyperse-
cretion of epinephrine (page 746), for it is not observed following stimula-
tion of the nerves in animals from which the adrenal glands have been
excised (Cannon and Mendenhall). The interesting suggestion is made
by Cannon that the shorter clotting time observed in animals showing
strong emotions of fright or fear may also be due to the hypersecretion
of epinephrine which this worker believes accompanies such states.
Blood Clotting in Disease
With the factors concerned in the process so wrapped in mystery, it is
not surprising that the underlying causes responsible for delayed or de-
ficient clotting of blood in diseased conditions or for the formation of
intravascular clots (thrombi) are little understood. According to How-
ell 's theory of the nature of the process, which is the most satisfactory at
the present time, abnormal clotting might be due to the following
causes: (1) A diminished amount of fibrinogen. This occurs when the
hepatic cells are greatly damaged, as in poisoning by chloroform or
phosphorus and in such diseases as acute yellow atrophy and yellow
fever. In many cases of chronic cirrhosis of the liver, as Whipple, etc.,15
have shown, the blood also clots feebly because of deficient fibrinogen.
It should be pointed out that it is not so much the clotting time that is
increased in such cases, as the firmness or consistency of the clot that
is affected.
2. A deficiency in prothrombin. In the condition known as "melena
neonatorum," undoubted benefit is derived from intravenous injections
of blood serum or by direct blood transfusions, probably because throm-
bin or prothrombin is thus furnished.
3. A deficiency of thromboplastin. Since this substance is derived from
both blood cells and tissue cells, it does not seem likely that a deficiency
112 THE BLOOD AND THE LYMPH
could ever occur. Certain observers, however — Morawitz, for example —
lay great stress on this as an important factor in hemorrhagic diseases.
4. An excess of cmtithrombin. The undoubted increase in this substance
that can be brought about experimentally by injecting hirudin or pep-
tone into animals, has stimulated careful search for a similar increase in
the blood in clinical conditions in which abnormal blood clotting is one
of the symptoms (Whipple10). Antithrombin is said to be increased in
septicemia, pneumonia, miliary tuberculosis, etc.
5. A deficiency of calcium ions. Although at one time it was supposed
that this might be responsible for the feeble clotting in hemophilia, it
has not been found, after very extensive trials, that the exhibition of
Ca salts in any way relieves the condition. It is said, however, that the
slow coagulation seen in obstructive jaundice is decidedly shortened by
treatment with calcium salts.
One thing stands out prominently in connection with the whole problem,
and that is the close relationship of the blood platelets to the clotting
process. From these cells are derived, according to Howell, not only the
prothrombiri but also, as from other cells, thromboplastin. It is not sur-
prising therefore to find that decided alterations in the platelet count
occur in cases of faulty blood clotting, and that local accumulations of
these elements within the blood vessels, produced by their clumping to-
gether or agglutinating, is followed by a formation of local clots, as in
thrombosis.
Hemorrhagic Diseases
In many of the so-called hemorrhagic diseases (acute leucemia and
aspastic anemia) and in the hemorrhagic varieties of diphtheria and
smallpox, the platelet count drops from its normal of between 200,000
and 800,000 per cubic millimeter to well below 100,000, and indeed in
these conditions it is frequently difficult to find any platelets. Samples of
blood clot outside the body within the normal time, but the clot is soft
and usually fails to retract in the normal manner. It is on account of
this, rather than slow clotting that the hemorrhage continues, so that in
appraising the gravity of the symptom it is best to measure not the clot-
ting time but the time that it takes for bleeding to cease from a small
skin wound, as in the lobe of the ear. This can be very accurately done
by applying blotting paper at regular intervals to the puncture (Duke17).
The most interesting and at the same time the most mysterious of all
conditions in which blood clotting is interfered with is hemophilia. The
clotting time is longer than normal, but even after the clot forms, bleed-
ing is likely to continue because the clots are very readily displaced. Both
clotting time and bleeding time are increased. So far no change in the
BLOOD CLOTTING 113
clotting factors of the blood has been demonstrated in this disease ; the
corpuscles and the platelets are normal in numbers, fibrinogen and cal-
cium salts are normal, and, as Howell has shown, there is no excess of
antithrombin. One significant fact, however, is that the addition of
thromboplastin or of its active ingredient, kephalin, greatly shortens the
clotting time of the blood when it is removed by venipuncture. In agree-
ment with this observation it has been found that hemophilic blood clots
much more rapidly, indeed sometimes in the usual time, if it is allowed to
flow over cut or damaged tissue and so become mixed with thromboplas-
tin. These facts taken together would seem to indicate that the fault
must lie in a deficiency in prothrombin, and since this is derived mainly
from the platelets, which however are not decreased in number, we must
further assume that these elements have undergone some qualitative
change preventing their disintegration. An accompanying defect in
their agglutinating properties would at the same time explain their fail-
ure in hemophilia to clump together at the' site of the hemorrhage so as
to block the smaller vessels with thrombi; hence the prolonged bleeding
time even after clotting has occurred.
Thrombus Formation
The first formed portion of a thrombus is paler than those formed later,
because it contains excessive numbers of platelets; and it seems clear
that it is by agglutination of these into m&sses, which then stick in the
blood vessels and by disintegrating shed forth prothrombin and thrombo-
plastin, that the clotting starts. This platelet agglutination may result
from stagnation in the bloodflow, or from roughening and damage to the
vessel walls. Stagnation may be due either to failure of the circulation
as a whole as in heart disease, or to local physical alterations in the vas-
cular tube, setting up conditions in which eddy currents with stagnant
pools of blood are formed, such as will occur at places where the vessels
suddenly become wider, as in varicose veins, in aneurisms and at the
sudden bend of large veins. The first formed (platelet) thrombus is fol-
lowed by one of a darker color, which fills the vessel up to the next
anastomotic branch. Similar stagnation may also follow the obstruc- .
tion caused by lodgment of emboli in the smaller vessels (air, foreign
bodies in fine suspension, bacteria, etc.). The thrombi in such cases are
very small and occur particularly in the capillaries of the liver, spleen,
and lungs. The small thrombi often serve as foci from which clotting
spreads into the larger vessels, this being often encouraged by an increase
in the coagulability of the blood. When the intima is inflamed, it is pos-
sible that excessive amounts of thromboplastin are produced and that
this neutralizes the antithrombin in blood moving so slowly that it is not
114 THE BLOOD AND THE LYMPH
replaced by fresh blood before clotting ensues, or it may be that sub-
stances derived from the inflamed tissue cause the platelets to aggluti-
nate. The increased clotting often observed after the injection of hemo-
lytic agencies (foreign sera, snake venom, etc.) may also be due to
platelet agglutination. Like the thrombosis following embolism, the
clotting occurs at first in the capillaries, the initial thrombi containing
masses of platelets along Avith skeletons of blood corpuscles and cells
from the blood-forming organs.
CHAPTER XIV
LYMPH FORMATION AND CIRCULATION
GENERAL CONSIDERATIONS
Lymphatics are modified veins. They grow from the veins in embry-
onic life as buds of endothelium, which are first visible in the human
embryo in the sixth week of development. The earliest outgrowth oc-
curs from the internal jugular vein, and the endothelial buds soon be-
come hollow and join together, forming first a plexus and subsequently
a sac, from which again lymphatic vessels made of endothelium grow
out to invade the skin of the head, neck, thorax and arm, and partly
the deep structures of the head. The sac is ultimately transformed into
groups of lymph glands. At a later stage similar nodes develop from
certain of the abdominal veins, forming a retroperitoneal sac, from which
grow out the lymphatics of the abdominal and, to a certain extent, of
the thoracic viscera. A similar pair of sacs also develops from the iliac
veins supplying the lymphatics for the skin of the legs and abdominal
walls. The retroperitoneal and iliac sacs then become connected with
the jugular sac by means of the thoracic duct. In the embryo there are
no valves in the lymphatic vessels, so that the whole system can be in-
jected either from the thoracic duct or from the skin, showing clearly
that the superficial and deep lymphatics are parts of one closed system
of vessels.
Anatomists have succeeded in tracing the course of the lymphatics in
many parts of the body. This knowledge is of great importance in
connection with the spread of infections, etc. Lymphatics are abun-
dant in the skin, the intestine, and connective tissues, but are absent
from the muscle bundles, from the hepatic lobules (though present in
the connective tissue between them), from the substance of the spleen,
and from the central nervous system.
The lymphatics have the same functions as blood capillaries, namely,
to absorb substances from the tissue spaces. There is some evidence to
show that this absorption may be selective. When injections are made
into the peritoneal cavity, the pathway of absorption may be either the
blood vessels or the lymphatics, according to the nature of the sub-
stance injected. True solutions are absorbed by the blood, but granules
115
116 THE BLOOD AND THE LYMPH
are taken up by special large cells showing phagocytic powers, and trans-
ferred to the lymphatics — for example, those of the diaphragm. A sim-
ilar selective absorption is well known in the case of the villi of the in-
testine, where fat passes into the lacteals and carbohydrates into the
blood. It appears as if lymphatic adsorption, both of solid materials
and of solutions, requires the cooperation of phagocytic cells.
The newer conception of the lymphatics as a closed system is at vari-
ance with the older one, in which they were supposed to get smaller and
smaller, and their walls less and less complete until ultimately they
faded off into the tissue spaces. These, however, bear no closer relation-
ship to lymphatics than they do to blood capillaries. The tissue spaces
include all the minute spaces between the fibers and cells of the con-
nective tissues and between the parenchyma of the organs and the great
serous cavities of the body (pleural, peritoneal), as well as specially
developed tissue spaces, forming the subarachnoid spaces of the brain,
the scala vestibuli and tympani of the cochlea and the anterior chamber
of the eye. The fluids in these spaces — the tissue fluids — are quite dif-
ferent from the lymph in the lymphatics both in composition and in
function. Indeed, the tissue fluids are among the most varied of all
the fluids of the body. The spaces may themselves become linked to-
gether so as to form a circulatory system, M'hich is quite independent of
the lymphatics. This is particularly the case in the brain, where the tis-
sue spaces surrounding every individual nerve cell extend into the sub-
arachnoid area, where they drain into the cerebral sinuses through the
arachnoidal villi, which exist as lace-like projections of the arachnoid
into the dural sinuses, being covered by a layer of mesothelial cells spe-
cially abundant at the tips of the villi, where they form cell nests. Ob-
servations of the passage of substances in solution by these pathways
have been made by injecting potassium ferrocyanide and citrate. of iron
into the subarachnoid and subdural spaces and afterwards detecting
the presence of the salts by mounting sections in acid media, so as to
permit prussian blue to develop. Ordinarily the precipitate is found in
or near the villi, but after cerebral anemia it forms in the tissue spaces
that surround the nerve cells.
There are therefore three fluids concerned in the transference of food
materials and gases between the gastrointestinal apparatus and lungs
and the tissue cells — namely, the blood plasma, the tissue fluids, and the
lymph. The tissue fluid, being in contact with the tissue elements, serves
as their immediate nutritive fluid, and it is the function of the blood and
lymph to maintain it of proper composition. Everything must be trans-
ferred to and from the tissue cells through the tissue, fluid, making it
LYMPH FORMATION AND CIRCULATION 117
therefore in many ways the most important of the fluids of the body.
In the tissue cells themselves there is also the fluid in which the various
colloids and crystalloids that enter into the composition of protoplasm
are dissolved. This can be removed from cells only by mechanical means,
such as grinding with fine sand in a mortar and subjecting the mass
to a pressure of several thousand atmospheres in a hydraulic (Buchiier)
press. This is known as the tissue juice. The ultimate exchange of
foodstuffs occurs between the tissue fluids and the tissue juices across
the cell membrane. The extent and character of this exchange depend
on many circumstances, some affecting the cell wall, others, the osmotic
and other properties of the two fluids. Obviously, the function of the
circulation is to maintain the tissue fluids of correct composition, the
blood plasma serving to carry food materials and dissolved oxygen to
them (see page 380), but being assisted in the opposite function of re-
moval of effete products by the lymph. The lymph is purely a scavenger ;
the blood is both purveyor and scavenger.
The above description of the lymphatics is not universally accepted
by anatomists, certain of whom believe that the lymphatics are developed
from tissue spaces and are consequently much more extensive than they
appear to be from injected specimens. The above conclusions are based on
reconstruction models, made from serial sections of embryonic tissues,
in which the lymphatics frequently appear as isolated vesicles without
visible connections. The failure of injections to penetrate into the re-
moter parts of such a lymphatic system in the embryo is attributed to
the discontinuity of spaces, which is, however, removed at later stages
of development.
The manner of absorption of injected fluids does not, however, sup-
port the view that the lymphatics are directly connected with the tis-
sue spaces. When all the structures of a part are ligated except the
main artery or vein, injected poisons which affect central structures,
such as the nerve centers, develop their action as quickly as in the in-
tact animal (e.g., strychnine). Similarly, when pigments such as meth-
ylene blue are injected into the pleural cavity or subcutaneously, they
appear in the urine long before the lymph of the thoracic duct. Such
results indicate the pathway of absorption to be the blood rather than
the lymph vessels. Through this latter channel absorption proceeds
more slowly, but can be greatly assisted by massaging the site of injec-
tion. When colored solutions, such as India ink or carmine, are injected
subcutaneously, however, a very perfect injection of the neighboring
lymphatics may ultimately occur, and through the same pathways mi-
croorganisms spread from an infected area.
118 THE BLOOD AND THE LYMPH
EXPERIMENTAL INVESTIGATIONS
It has proved a most difficult problem to gain any exact knowledge
of the production of lymph by experimental means. Starling, some years
ago, in repeating many of the experiments of older physiologists in the
light of the newer facts of physical chemistry, added much that is of
interest, and it is chiefly with his work that we will concern ourselves
here.
The unequal lymph supply of different regions of the body is strik-
ingly demonstrated by comparing the lymph flow from the lymphatics
of the leg with that from the thoracic duct. No lymph flows from the
former unless the muscles are thrown into activity or the blood is pre-
vented from, leaving the limb by ligaturing all the veins. Changes in the
arterial blood pressure do not affect the flow. On the other hand, a great
increase in the flow from the thoracic duct can readily be induced by
disturbances in the blood supply. Obstruction of the portal vein, for
example, immediately increases the lymph flow four or five times because of
venous congestion in the intestinal capillaries, whilst a still greater
increase — perhaps tenfold — is induced by obstruction to the inferior vena
cava, which raises the capillary pressure in both the liver and the intes-
tines. After ligation of the hepatic lymphatics (at the hepatic pedicle),
obstruction of the vena cava no longer causes the outflow of lymph to
increase, indicating that the lymph in the last mentioned experiment
must have come from the hepatic lymphatics.
These results, so far as they go, could be satisfactorily explained on
the basis that lymph formation is a filtration process, that is, a process
dependent upon difference in mechanical pressure between the blood
capillaries and the tissue spaces. The lymphatics would then serve as
channels to return this fluid to the blood vessels through the thoracic
duct. The difference in the magnitude of the increased lymph flow from
increase in capillary pressure in different regions would be dependent
on the permeability of the filter, the capillaries of the limbs being much
less permeable than those of the intestine, and particularly of the liver.
Another fact in conformity with this view concerns the composition of
the lymph from the two regions, that from the limb lymphatics being
poor in protein, whereas that from the thoracic duct does not fall far
behind the blood plasma in this regard.
Although filtration may explain the considerable increase in lymph
flow produced by extreme changes in capillary pressure, it by no means
suffices to explain lymph formation under less abnormal conditions.
When a muscle or a gland is at rest, it produces practically no lymph,
LYMPH FORMATION AND CIRCULATION 119
but during activity the flow becomes marked. This can not be explained
by filtration, but may be accounted for by a physico-chemical process —
namely, osmosis. The energy required for the activity of the tissue
cell is produced by chemical changes, whereby large molecules become
broken down into numerous smaller ones. These smaller molecules are
then discharged into the surrounding tissue fluids, the osmotic pressure
of which they increase, with the consequence that water is attracted
by osmosas from the plasma in the blood capillaries (see page 4). This
increases the volume of tissue fluid, which is then drained away by the
lymphatics. The increase in molar concentration will also affect the
tissue juices, tending to make the cell swell up by absorbing water.
In gland cells this extra water is immediately extruded to form the
water of the secretion (see page 421).
An analogous method of lymph formation is not confined to situations
where the capillaries are relatively impermeable, for it also occurs in
the liver, the lymph flow from which is greatly increased by the injec-
tion of bile salts. A similar process no doubt results from muscular
activity, although in this case the tissue spaces must form a continuous
system of their own, there being, according to most authorities, no
lymphatics.
Considerable interest has been taken in the stimulating effect which
certain chemical substances have on the secretion of lymph from the
thoracic duct. These so-called lymphagogues belong to two classes —
crystalline and colloidal. Of the former, glucose, urea, and sodium
chloride in hypertonic solution, are the best known. Starling explains
their action as dependent upon an increase in the osmotic pressure of
the blood. This attracts water into the blood from the tissue juices,
and leads to an hydremic plethora, with a consequent increase in capil-
lary pressure. If the blood pressure is lowered by hemorrhage before
the hypertonic solution is injected, very little stimulation of lymph flow
occurs, because there is no available fluid in the tissue to produce the
plethora. This observation does not, however, very strongly support
the explanation, because so many other disturbances may result from
hemorrhage.
The colloidal lymphagogues include watery extracts of the dried tis-
sues of leeches, crayfish, and mussels, as well as commercial peptone.
They probably act by damaging the endothelium of the capillaries, so
that filtration occurs more readily. Although their action is displayed
more particularly on the lymphatics of the liver and intestines, it is also
apparent on the skin capillaries, producing cutaneous edema and the
formation of blisters (nettle rash).
THE BLOOD AND THE LYMPH
EDEMA
With such an imperfect knowledge concerning the physiology of
lymph formation, it is not surprising that the causes of excessive accu-
mulation of fluid in and between the tissue elements should be little un-
derstood. All of the conditions which have been mentioned as capable
of causing an increased secretion of lymph — such as increase in capillary
pressure, hydremic plethora, action of poisons on the endothelium — are
likely to cause edema if the lymphatics of the part are simultaneously
obstructed. To produce in animals edema of the subcutaneous tissues
like that observed clinically, it is, however, necessary that the vascular
disturbance be accompanied either by local damage to the capillary
endothelium, such as is produced by arsenic or uranium; or by a gen-
eral toxemic condition, such as is set up by nephritis. When large
amounts of saline solution are injected intravenously, extensive ex-
travasation of fluid may occur into the liver, peritoneum and intestinal
lumen, without any subcutaneous edema.
Clinical edemas are of at least three types:
1. The inflammatory edemas, in which the fluid permeates the cells of
the inflamed area and does not shift to other parts of the body under
the influence of gravity.
2. The nephritic edemas, in which the fluid is more or -less loose in the
subcutaneous tissues and readily changes its position, and which is
accompanied by excess of water in the blood with a corresponding in-
crease of sodium chloride ; the percentage concentration of sodium chlo-
ride in the blood remains unchanged, but that the other constituents
diminished.
3. Cardiac edemas, which are also hypostatic, but are unaccompanied
by changes in the relative amount of water and sodium chloride in the
blood.
The second and third varieties of edema may of course be more or
less present together, for the kidneys are likely to become secondarily
affected during venous stasis.
The salt retention in nephritic edema is very significant. As ex-
plained elsewhere, it is revealed by comparing the daily output of so-
dium chloride by the urine with the concentration of this salt in the
blood. Less salt is eliminated than would be the case in a normal in-
dividual with the same percentage of salt in the blood. In many cases
also edema can be diminished by withholding salt from the food. Widal
and Javal have conclusively shown the relationship of retention of water
in the body, as judged by variations in body weight, to the hydremic
condition, as judged by the refractive index of the blood serum, and
LYMPH FORMATION AND CIRCULATION 121
to the amount of salt in the diet. A very considerable retention of
water usually occurs before there is any evidence of edema; indeed, as
a result of giving salt, the body weight may increase from five to seven
kilograms (10 to 15 pounds) within a day or two without the appear-
ance of puffiness.
The cause of the edema during salt retention is no doubt closely re-
lated to the action of lymphagogues. In a normal person excessive in-
gestion of salt is immediately followed by excretion of the excess through
the kidney. Where the kidneys are diseased, this excess of salt is re-
tained in the blood, raising its osmotic pressure and attracting water
from the tissue fluids. This leads to excessive thirst, the imbibed water
being used to replace that lost from the tissues. But all the crystalline
lymphagogues do not, when present in excess in the blood of nephritic
patients, necessarily cause edema; urea, for example, may accumulate
considerably without any such effect. The different action is usually
attributed to inequality in the diffusibility of the two crystalloids through
animal membranes, sodium chloride diffusing much less readily than
urea.
It is most important to note that the fluid in edema is loose in the
tissues and can be drained away by the insertion of tubes. There is
absolutely no evidence in support of the claim of Martin Fischer that
edema is due to imbibition of water by the colloids of the tissues. This
question has been fully discussed elsewhere (page 62).
(Monographs)
iHowell, W. H.: The Harvey Lectures, J. B. . Lippincott Co., xii, 272.
Starling, E. H.: Human Physiology, Lea & Febiger, 1915.
sKowe, A. H.: Arch. Int. Med., 1917, xix, 354,
^Williamson, C. S.: Arch. Int. Med., 1916, xviii, 505.
sTower and Herm: Proc. Soc. Biol. and Med., 1916, xviii, 505.
eRoua and Eobertson: Jour. Exp. Med., 1916, xxiii, 219, 239, 549.
TButler, G. G.: Quart. Jour. Med., 1912, vi, 145.
SHowell, W. H.: cf. Harvey Lecture; also Am. Jour. Physiol., 1913, xxxii, 264.
'^Drinker, C. K., and K. E.: Am. Jour. Physiol;, 1916, xli, 5.
loDenny and Minot: Arch, Int. Med., 1916, xvii, 101; Am. Jour. Physiol., 1915,
xxxviii, 233.
"Addis, T.: Quart. Jour. Med., 1910, iv, 14.
12Cannon and Mendenhall: Am. Jour. Physiol., 1914, xxxiv, 225.
"Howell, W. H.: Arch Int. Med., 1914, xiii, 80.
"Brodie, T. G.: Jour. Physiol., 1897, xxi, 403.
isWhipple, G. H.: Arch. Int. Med, 1912, ix, 365; Jour. Exp. Med., 1911, xiii, 136.
leWhipple, G. H.:' Arch. Int. Med., 1913, xii, 637. .
i?Duke, W. W.: Arch. Int. Med., 1912, ix, 258.
CHAPTER XV
BLOOD PRESSURE
The object of the circulation is to maintain through the tissues a sup-
ply of blood that is adequate to meet their demands for nutriment and
oxygen and to remove the effete products of their metabolism. The de-
mands vary according to the activities of the tissue, being particularly
variable in the case of such tissues as the muscular and the glandular.
In studying the physiology of the circulation we have therefore to bear
in mind two aspects of the problem: (1) the cause for the continuous
bloodflow, and (2) the mechanism by wrhich alterations in this bloodflow
are brought about.
If we open an artery we shall find that the blood escapes from it
under such a pressure that it is thrown to a height of about six feet,
that its outflow is proportional to the size of the artery, and that it pul-
sates. If, on the other hand, we open a vein, we shall find that the
blood wells out without any very evident pressure, and that it flows
in a continuous stream, its outflow being the same in a unit of time as
that of the artery, provided the two vessels are the only ones supplying
the particular area. The general conditions governing the bloodflow
are the same as those governing the flow of fluid through any system of
tubes. For example, in the city water mains it is known to every one
that the rate of outflow from any part of the system depends finally on
two factors: (1) the difference in pressure at the beginning and end of
the system, and (2) the caliber of the tube at the outlet. We may in-
crease the outflow by raising the pressure at the beginning of the system,
the caliber of the outlet meanwhile remaining constant, or we may
maintain the pressure constant but increase the caliber of the outlet.
In the circulation of the bjood, the difference in pressure at the be-
ginning and end of the circulation is furnished by the pumping action
of the heart, and the alteration of the caliber of the outlet is provided
for by the constriction or dilatation of the blood vessels. These simple
physical principles indicate the direction which a study of the circulation
122
BLOOD PRESSURE 123
should take. They indicate that our first consideration should be of the
mean blood pressure, how it is maintained, and how it can be made to
vary. After we have learned this, we may then proceed to a more
particular examination of the mechanism of the pump — that is, of the
heartbeat; then finally we may proceed to examine the nature of the
processes by which the caliber of the arteries is controlled.
THE MEAN ARTERIAL BLOOD PRESSURE
The first prerequisite to the investigation of the blood pressure, as of
any other physical problem, is that we should possess some means by
which it can be quantitatively measured. The earliest attempt to accom-
plish this was made by the English scientist, the Rev. Stephen Hales, a
little over a century after Harvey published his account of the circula-
tion of the blood. Hales connected a glass tube nine feet in length with
a severed artery of a horse, the connection between the two being made
by means of a piece of brass pipe joined to the windpipe of a goose as a
substitute for rubber tubing. He found on untying the ligature on the
artery that the blood rose in the tube to a height of eight feet and three
inches above the level of the left ventricle of the heart, 'and that when
at full height it rose and fell with each pulse through a distance of two,
three or four inches.
Mercury Manometer Tracings
The somewhat crude but very significant experiment of Hales clearly
established the existence of the enormous pressure at which the blood is
made to circulate through the arteries. To render possible a further
investigation of the factors on which this pressure depends, it became
necessary to invent some more convenient means for its measurement,
but this was not accomplished until a century later, when Poiseuille ap-
plied the mercury manometer, which Ludwig subsequently adapted so
that tracings might be taken (Fig. 21).
Having before us such a tracing as shown in Fig. 22, let us consider
how it may be used in the study of blood pressure. The first thing we
must do is to measure the average height of the tracing above the line of
zero pressure; the mean arterial blood pressure is then equal to this
distance multiplied by two, because the distance through which the mer-
cury has moved up in the limb of the manometer carrying the writ-
ing point is only one-half of its total displacement. Since mercury
is about 13.5 times heavier than an equal volume of blood, the above
measurement must be multiplied by this figure if we desire to express
124
THE CIRCULATION OP THE BLOOD
our result in terms of the height to which the blood pressure could raise
a column of blood.
In arteries of approximately the same size, the mean arterial blood
pressure does not markedly vary in different mammals. Thus, in the
carotid artery of the dog it averages about 110 to 120 mm. Hg, in that
of the cat about 105 to 115 mm., in the rabbit from 90 to 105 mm., in
the sheep about 150 mm., in the horse about 200 mm., and in man some-
Fig. 21. — Mercury manometer and signal magnet, arranged for recording the mean arterial
blood pressure in a laboratory experiment. The pressure bottle (R) is filled with anticoagulating
fluid and is connected by tubing with the manometer (M), the cannula for the artery (U) being
connected with the T-piece (J). By this arrangement it is possible to flush out the tubing
when clotting interferes with the experiment. (From Jackson— Experimental Pharmacology.)
where between 120 and 140 mm. The pressure varies in different parts
of the vascular system, being greatest in the aorta and least in the small-
est arterioles but the fall in pressure — the pressure gradient — does not
become very pronounced until the arterioles have become so small that
it is no longer possible to insert, a cannula into them; thus, the mean
125
blood pressure in the renal or femoral artery is very little less than that
in the aorta.
If Ave examine the contour of the tracing which the pressure draws, we
shall find that it exhibits two types of wave, small and large ; and if we
observe the animal while the tracing is being taken, we shall find that
Fig. 22. — The arterial blood pressure recorded with a mercury manometer (lower tracing),
along with a tracing of the respiratory movement of the thorax. Note that the beginning of
respiration occurs distinctly before the rise in blood pressure.
the former are caused by the heartbeats and the latter by the respira-
tions— an observation which immediately raises the question as to the
trustworthiness of the method, for it will be asked, How can it be that
the heartbeat produces an effect on blood pressure which is less than
that of the respirations? Obviously the tracing must be faulty in re-
gard to the relative significance of the waves.
126
Spring Manometer Tracings
The cause of this inaccuracy depends on the inertia of the mercury,
an inertia which is so great that the sudden changes of pressure produced
by each heartbeat are not able to overcome it, whereas the much less
significant but more prolonged pressure changes produced by each respi-
ration develop their full effect on the mercury. These facts led investi-
gators to seek for instruments in which the inertia error is eliminated,
with the result that they invented what are known as spring manometers.
Fig. 23. — Hurthle's spring manometer.
Many varieties of this instrument have been produced, but for our pur-
pose it is necessary to describe the principle of only the simplest and
most efficient — the Hiirthle manometer. As shown in Fig. 23, it consists
of a variety of tambour, which differs from the ordinary tambour in two
important particulars: (1) its chamber is made as small as possible, and
(2) it is covered not with an elastic membrane but with one of leather or of
thin fluted metal. These two precautions are taken in order to avoid spuri-
ous waves set up on account o€ elastic recoil. Such errors are further
reduced by filling the tubing and chamber of the tambour with an anti-
coagulating fluid.
Fig. 24. — Arterial pressure recorded by a spring manometer. The effect of weak excitation of
the vagus is seen during the period marked by the signal m. (From Dubois.)
Before the tracing taken with the spring manometer can be em-
ployed for quantitative measurements, it must obviously be graduated
according to some scale. This is accomplished immediately before or
after the experiment by connecting the manometer through a T-piece
with a pressure bottle, which can be raised or lowered to a specified
height, and with a mercury manometer. The displacement of the writing
point of the spring manometer corresponding to each 10 mm. Hg of
pressure is then written on the tracing.
BLOOD PRESSURE
127
The tracings taken with such a manometer, as shown in Fig. 24, are
quite different from those with the mercury manometer. It will be seen
that now the cardiac waves are decidedly the more pronounced, the respira-
tory, being comparatively inconspicuous. Instead of there being a fairly
steady pressure in the arteries, this undergoes very considerable altera-
tion during each heartbeat.*
Examination of this tracing gives us accurate information regarding
the blood pressure both between the heartbeats — diastolic, as it is called —
and during them — systolic. It gives us a means of telling what must be
the dead load of the circulation — that is, the pressure that is constantly
present — as well as the live load that is superadded to this by each heart-
160
120
L ine of
SYSTOLIC PRESS U BE
L/r>e of
MEAN PRESSURE
Lt'nt
D/ASTOLIC
Pressure
D/ASTOL/C PRESSURE
Fig. 25. — Diagram based on experiments on dogs to show the magnitude of the systolic,
diastolic and mean blood pressures at different parts of the circulatory system. O is the line
of zero pressure, and the letters below it indicate the parts of the system to which the curves
refer. (From Brubaker.)
beat. This difference is often called the pressure pulse, and in man it
amounts to somewhere about 35 mm. Hg. If we take tracings with a
spring manometer from different parts of the arterial tree, we shall find
that, as we travel towards the periphery, the pressure pulse becomes less
and less marked, until finally by the time the capillaries are reached it
has almost entirely disappeared. This decline in the pressure pulse can
moreover be seen to be dependent more largely on a fall in systolic than
in diastolic pressure. In other words, the dead load of the circulation —
the diastolic pressure — remains practically constant all along the arte-
rial tree, whereas the systolic pressure falls relatively quickly (Fig. 25).
*The tracings shown in Figs. 22 and 24 are not typical, (he pulse pressure being too small in
the latter and too large in the former.
128 THE CIRCULATION OP THE BLOOD
Clinical Measurements
The methods of blood-pressure measurement in man have recently
become so perfected that the results are almost as accurate as those ob-
tained in laboratory animals by direct measurement through the use of
cannulae inserted into the vessels. Both the systolic and the diastolic'
pressure can be measured with equal facility and 'accuracy! Since the
technic for making the systolic measurements was described at a much
earlier date than that for the diastolic, it has until recently been the
habit with a great part of the medical profession to be satisfied with
systolic readings alone. This is most unfortunate, because the knoAvle'dge*
which such information gives us is incomparably inferior to that which
can be obtained by gauging the diastolic pressure. Until we have learned
more about the dynamics of circulation, it would be profitless to go
into any details as to the reasons for this statement, but it will soon
become self-evident. Suffice it for the present to- state that the diastolic
pressure is the more important because it gives us the load which the ves-
sels and aortic valves must constantly bear, and the resistance which must
be overcome prior to the opening of these valves at the beginning of
systole. Moreover, it helps us to gauge the peripheral resistance.
The first step in the technic of blood-pressure measurements in man
is the placing of an armlet or cuff around the arm or leg. This armlet
consists . of a rubber bag at least 12 cm. broad and covered on its outer
surface by cloth or leather. The bag is connected by tubing with! a pres-
sure gauge and a pump. The pressure gauge may be either an ordinary
mercury manometer or one of the numerous gauges built on the aneroid
principle that are now on the market (Fig. 26). For measuring the
blood pressure in the vessels of the upper extremities, the armlet should
be applied around the fleshy part of the upper arm and for the lower
limbs around the thigh. For accurate reading of both pressures the
following procedure should be followed. Having applied the armlet, the
pulse should be palpated at the radial artery, and the pressure in the
armlet then raised until the pulse can no longer be felt, at which moment
the pressure in the manometer should be noted. The cuff should then be
slowly decompressed and the pressure noted at which the pulse reappears.
These two readings of systolic pressure should be close together, but
they will not usually agree exactly for reasons which will be explained
immediately. 'They give us the palpatofy systolic index, as it is called.
The pressure is now lowered about 15 mm. Hg, and a stethoscope is
placed in front of the bend of the elbow over the artery and as close up
to :the cuff as possible. With each heartbeat a distinct sound like a pistol
shot will be heard. The decompression is now continued slowly, and as
the pressure falls the sounds will be heard to become louder and prob-
BLOOD PRESSURE
129
ably somewhat murnmrish in quality. At a certain pressure this loud
character of the sound will suddenly become much less marked, and the
murmurish quality if present will suddenly disappear. This point cor-
responds to the diastolic pressure, which is now read off from the
manometer.
It must be remembered that below this point, as the pressure in the
cuff is further lowered, a sound is still heard in the artery; indeed it
does not entirely disappear until the pressure has become quite low. This
point of final disappearance is, however, of no significance. The cuff is
the
Fig. 26. — Apparatus for measuring the arterial blood pressure in man. The pressure in thi
:uff is raised by means of the syringe until the pulse can no longer be felt at the wrist. Thi
pressure is read off on the mercury manometer (systolic pressure).
now entirely decompressed, and should be left so for a moment or more,
so that the circulation in the part of the arm below it may return to the
normal.
The above readings should then be controlled by a second observa-
tion, in which the methods employed are slightly modified. With the
stethoscope at the bend of the elbow the pressure in the cuff is run up to
a little above the previously determined diastolic pressure, so that the
sound is clearly heard. The pressure is then further raised till the
sound disappears. This point indicates the systolic pressure; it is called
130 THE CIRCULATION OF THE BLOOD
the auditory systolic index. It will be found to give a systolic pressure
a little higher than that obtained by palpation of the artery at the wrist.
The sound being now absent, the pressure in the cuff is lowered until
the sound reappears, and the point at which this occurs should almost
exactly correspond to that at which the sound was found to disappear.
If the palpatory systolic index is not below the auditory, it indicates
that some error has been made in the application of the apparatus, and
that the reading of the diastolic pressure will be unreliable. The usual
source of error is in the position of the stethoscope. If readjustment of
this does not bring the two indices into proper relationship, the auscul-
tatory method can not be relied upon for either systolic or diastolic
readings.
In case of the failure of the auscultatory method, we have to fall back
upon the palpatory method for systolic pressure; and for the measure-
ment of diastolic, we have to use the method known as the oscillator]/,
which until recent years was the only one used for gauging the dias-
tolic pressure. This consists in observing the oscillation of the indicator
of the pressure gauge; as the pressure in the cuff falls gradually from
below the systolic pressure, these oscillations will be observed to increase
in amplitude, until they reach a maximum beyond which with lower
pressure they rapidly decline. The pressure in the cuff at the moment
when the oscillations are at the maximum represents the diastolic pres-
sure. With a mercury instrument it is obviously difficult to employ this
method, but with a modern spring instrument it can with a little practice
be used with great accuracy and will serve as a valuable check on the
diastolic reading as taken by the auscultatory method.
The procedure may be altered in various ways, there being only one pre-
caution to bear in mind ; namely, that the pressure in the cuff should not be
applied continuously for more than a few moments of time, for if this
is done for long periods, not only will it interfere with the accuracy
of the reading, but it may cause considerable discomfort to the patient.
There are several conditions affecting the accuracy of the readings by
each method Avhich it is Avell to bear in mind. These have been investi-
gated by MacWilliam,1 Leonard Hill,2 and Erlanger.3 With regard to
the systolic pressure the most important of these are as follows: (1) The
compression cuff should be a wide one (12 cm.), and it should never be
applied so that there is any chance of its compressing the artery against
a bony surface. This precaution is necessary, since it has been found that
much less pressure is required to obliterate any perceptible pulse below
the armlet when the artery is flattened against some hard structure than
when it is uniformly compressed in the tissues in which it lies. (2) Dis-
crepancies are often noted between the systolic readings on compres-
BLOOD PRESSURE 131
sion and decompression of the artery ; that is, the pulse may reappear on
decompression at a lower pressure than that at which it disappeared on
compression, the difference being most marked when the decompression
is done quickly. This difference is owing to the fact that the full force
of the pulse does not reach the forearm until all the vessels have become
distended with blood. (3) There are often discrepancies in the systolic
readings taken from different limbs; thus, it is not uncommon to find
that the systolic pressure in the leg is higher than that in the arm even
when the observed person is in the horizontal position. These differences
are most commonly observed in patients suffering from aortic regurgi-
tation or thickened arteries. In aortic regurgitation the pulse is of the
water-hammer variety, and the greater systolic pressure observed in the
leg vessels in such cases seems to depend on differences in the phys-
ical conditions concerned in the transmission of this exaggerated pulse
wave to the vessels of the two extremities.
The. reason for the discrepancies in cases of hardened arteries is no
doubt that the hardening is likely to be more pronounced in the ves-
sels of the thigh than in those of the arms. When a hardened vessel is
compressed it does not collapse uniformly- — that is, it does not become
completely closed — but its walls come together at the middle part while
chinks still -remain at the sides. The blood continues to pass through
these chinks, and a very considerably higher pressure in the cuff is re-
quired to obliterate them. That this is probably the correct explanation
is supported by the observation that, although in such patients the pulse
does not disappear in the vessels of -the foot at the same pressure as it
does at the wrist, a distinct change is nevertheless perceptible in the
pulse of the foot at a cuff pressure equal to that producing obliteration
in the wrist. In a patient showing a systolic pressure of 115 mm. for the
upper arm and 198 mm. for the leg, at 116 mm. the pulse in the leg,
although not obliterated, became notably cut down in volume. It there-
after persisted at a small volume with little alteration until the pressure
became sufficient to obliterate it. It is said that repeated compression
and decompression of the hardened arteries greatly reduces the dis-
crepancy in the systolic readings. Differences in systolic readings are
also sometimes observed in normal individuals, particularly after mus-
cular exercise, but for these no satisfactory explanation can be given.
While palpating the radial artery, it will often be noticed, as the
pressure in the cuff is gradually raised from zero, that the force of the
pulse increases perceptibly until a pressure of about 50 mm. is reached.
This paradoxical behavior of the pulse can also be demonstrated by the
sphygmograph (sec page 201). Its cause is not understood, but it is
of significance that the greatest augmentations occur at a cuff pressure
132 THE CIRCULATION OF THE BLOOD
at which a sound first conies to be heard by listening over the artery
at the elbow.
"With regard to the diastolic pressure, there has been some controversy
as to whether it is more accurately gauged by the oscillatory or the aus-
cultatory method. If both methods are employed it will usually be found
that the oscillatory gives a higher reading than the auscultatory. The
concensus of opinion seems to be that the latter method is the more accu-
rate, and certainly it is the easier to apply, for with the oscillatory
there is often great difficulty in deciding just exactly when the maximum
oscillation occurs.
The strongest evidence supporting the conclusion that the auscultatory
readings are more -reliable than the oscillatory has been gained by ex-
periments with an artificial schema, consisting of a wide glass tube rep-
resenting the armlet, filled with Ringer's solution and closed by rubber
stoppers pierced by tubes, which are connected with a fresh artery, which
therefore runs from end to end inside the tube. Through tubing connected
with the artery a pulsatile flow of oxygenated Ringer's solution is made
to flow at varying pressures, which are indicated by valved manometers
(see page 152) connected with the artery tubing just beyond the com-
pression tube. The pressure in the latter is also measured by a manom-
eter, and it is caused to vary by. a suitable compressor. By comparing
the behavior of the artery with the pulsating movement of a spring
manometer connected with the compression chamber, under different
degrees of pressure inside and outside the artery, it has been observed
that the maximal oscillation occurs when the artery is actually some-
what flattened between the pulse beats; that is, it occurs at an outside
pressure above the diastolic pressure, at which of course the vessel should
retain its circular shape. When a stethoscope is applied to the tube leading
from the artery just beyond the compression chamber, in the above de-
scribed model sounds similar to those in the arm are heard with each pul-
sation. While the pressure is being gradually lowered from above the ob-
literation point, these sounds will be found to become first audible as soon
as a certain amount of fluid is forced through the compressed area at each
pulse (the systolic index), and to become louder and often murmurish in
quality as the decompression is proceeded with, until a pressure is reached
at which they suddenly become less intense and change in character. At
this moment it will be observed by watching the artery that the external
pressure is no longer capable of producing any flattening of the vessel
between pulses. Evidently, therefore, the change of sound corresponds
exactly to the diastolic pressure.
With regard to the cause of the sound, it should be clearly understood
that it is the systolic wave that produces it, but its occurrence and its
BLOOD PRESSURE 133
character when present are dependent upon the intra-arterial pressure
existing during the diastolic phase. The cause of the sound has been
shown to depend on the production of a water-hammer in the blood ves-
sels below the compression cuff (Erlanger3). By a water-hammer is
meant the pressure changes which are caused by suddenly stopping the
flow of water in a pipe. When a sudden pressure occurs in vessels with
elastic walls, these walls are thrown into vibration and so produce a
sound. In the taking of blood-pressure measurements, as above de-
scribed, when the pressure in the cuff is between systolic and diastolic,
the volume of the compressed artery will increase abruptly with each
heartbeat and thus permit a considerable volume of swift-flowing blood
to enter the rest of the artery underneath the cuff. /When this quickly
moving column of blood comes into contact with the stationary blood
filling the uncompressed artery ~below the cuff, it will become immedi-
ately checked, and thus distend the arterial wall with unusual violence
and set it into vibration.
CHAPTER XVI
THE FACTORS CONCERNED IN MAINTAINING THE
BLOOD PRESSURE
Having become familiar with the principles of the methods by which
blood-pressure measurements are made, the next problem is to examine
into the causes which operate to maintain the pressure. Two of these
causes may be considered as fundamental, since without them no such
pressure could exist. These are: (1) the pumping action of the heart,
and (2) the peripheral resistance — that is, the resistance to outflow of
blood from the ends of the arterial system. Less essential though im-
portant factors are: (3) the volume of blood in the blood vessels, (4)
the viscidity or viscosity of the blood, and (5) the elasticity of the
walls of the vessels. We shall now proceed to examine the experimental
evidence which indicates the relative importance of each of these factors.
1. The Pumping' Action of the Heart
Changes produced in the mean arterial blood pressure by alteration
in the pumping action of the heart are most strikingly demonstrated by
observing this pressure after cutting or during stimulation of the vagus
nerves. As will be explained later (page 217), impulses- conveyed
through these nerves to the heart make the beats slower and weaker.
These impulses are constantly acting in the heart, so that when both
vagus nerves are cut, the beats become more frequent and stronger,
with the result that the mean arterial pressure rises considerably. A
lesser degree of this effect can usually be obtained by cutting the vagus
nerve on one side (Fig. 27). If now the peripheral end of a cut vagus
nerve is stimulated, as by applying an electric current to it, the heart will
either stop beating altogether or become very much slowed, with the result
that the mean arterial blood pressure will fall, in the former case almost to
zero and in the latter, to a level Corresponding to the degree of slowing
of the heart (Fig. 28).
2. The Peripheral Resistance
To demonstrate the influence of peripheral resistance on mean arte-
rial blood pressure, the most striking experiment is performed by cut-
ting or stimulating the great splanchnic nerve. In this nerve impulses,
134
BLOOD PRESSURE 135
which are called vasoconstrictor because they constrict the lumen of the
blood vessels, are transmitted to the blood vessels in the abdomen.
The vessels are under the constant influence of these impulses so that,
when the nerves that transmit them are sevei-ed, the vessels dilate and
thus offer less resistance to the movement of blood along them. The
result produced on the mean arterial blood pressure by cutting the two
splanchnic nerves is therefore a marked and sudden fall, which is im-
mediately recovered from if the peripheral end of one of the cut nerves is
stimulated artificially (Fig. 29). In choosing this experiment to prove the
relationship between peripheral resistance .and the. mean arterial blood
Fig. 27. — Effect of cutting the vagus nerve on the arterial blood pressure.
pressure, it must be remembered that it is not entirely conclusive, since
the results observed on the mean arterial blood pressure from cutting
or stimulating the nerve may be in part explained as due to variation
in the total capacity of the circulation ; more room is created by cutting
the nerves, less room by stimulating them.
3. The Amount of Blood in the Body
This can be altered by hemorrhage or transfusion, and the results
of such procedures are of interest not only on account of their physi-
ologic bearing, but also because of their great practical importance.
136
THE CIRCULATION OF THE BLOOD
To appreciate the significance of the results, it is important to bear in
mind that the total volume of the blood constitutes from 5 to 7 per cent
of the weight of the animal. This fact has been determined partly by
postmortem, and partly by antemortem measurements. In the post-
mortem method, the total amount of blood is determined by collecting the
blood while bleeding the animal to death and then washing out the
vessels with saline solution until the escaping fluid is no longer tinged
with red. The blood contained in the saline solution is estimated by
colorimetric methods (see page 92), and is added to that directly col-
TVmt C. OttS.
Fig. 28. — Effect of stimulating the peripheral end of the right vagus on the arterial blood
pressure.
lected. In the antemortem method some substance that does not dif-
fuse through vessel walls or become quickly destroyed is added to the
blood. By determining the concentration of this substance in a speci-
men of blood, the volume with which it has become mixed can readily be
calculated. Acacia has recently been found suitable for this purpose
(Meek), but the best known work (of Haldane) was done by causing the
animal to inspire a known amount of carbon monoxide. This combines
with the hemoglobin of the blood (see page 401) to displace an equal
quantity of oxygen. By determining the difference between the volume
BLOOD PRESSURE
137
of carbon monoxide in the blood before and following its administration
we can calculate with how much blood the known inspired quantity of
carbon monoxide must have combined. The results vary somewhat in
different animals; in the dog, the blood constitutes about 7.7 per cent
of the body weight, and in man, about 5 per cent.
The immediate effect of hemorrhage on the blood pressure depends on
the rate of bleeding. If a large artery, such as the femoral, is cut across,
Fig. 29.— -Effect of stimulation of the left splanchnic nerve on the arterial blood pressure.
Note the primary and secondary rises.
the pressure will show an immediate but moderate fall, due largely to the
fact that we have suddenly decreased the" peripheral resistance. If on
the other hand only a small artery or a vein is opened, the bleeding will
at first produce no effect on the blood pressure, and it is only after some
considerable amount of blood has been removed that it begins to fall (Fig.
30). To be more exact, we may state that the removal of 5 c.c. of blood per
kilogram of body weight does not influence the blood pressure. The re-
moval of a second portion of 5 c.c. per kilogram causes the blood pres-
sure to begin to fall, the fall of pressure for each subsequent 5 c.c. of
138
THE CIRCULATION OF THE BLOOD
blood per kilogram removed averaging about 6 mm. Hg, until after 20
to 25 e.c. of blood per kilogram have been removed, when a more rapid
fall in pressure sets in (Downs'4). "When the pressure reaches the level
of from 20 to 30 mm. Hg, the danger limit is reached, for there now
supervenes a train of symptoms known as ' ' shock, ' ' and the chances for the
animal's recovery become uncertain. That the removal of the first por-
tion of blood, if this removal is slow enough, does not influence the blood
pressure, indicates that some adjustment has occurred in the vascular
system to hold up the pressure in spite of the loss of blood. This adjust-
ment is believed to consist in vasoconstriction.
Starr* |n Endof
ifowRerrwBKemov/al
Time in Sees 8t
Abscissa
Fig. 30. — The effect of rapid and slow hemorrhage on the arterial blood pressure. Between
the second and third pieces of tracing an interval of two minutes elapsed.
Recovery from hemorrhage is remarkably rapid, the original volume of
blood being restored within a few hours. The chances of recovery de-
pend upon the amount of blood lost. A loss equal to 2 or 3 per cent of
the body weight can almost always be recovered from in laboratory ani-
mals, and in the case of man there is reason to believe that recovery
may occur after as much as 3 per cent of the body weight has been lost.
The recovery of blood pressure is brought about partly by a transfer
of fluid from the tissues to the blood. This abstraction causes a drying
out of the tissues, which soon excites an extreme degree of thirst. The
dilution of blood by fluid derived from the tissues occurs very rapidly,
as can be shown by comparison of the hemoglobin content, or the number
of blood corpuscles, in samples of blood removed immediately before
BLOOD PRESSURE 139
and immediately after a hemorrhage. The specific gravity of the post-
hemorrhagic blood is also decidedly below normal, indicating that the
diluting fluid contains a lower concentration of dissolved substances than
the blood plasma. The dilution of the blood is indeed often so great that
hemolysis occurs, the plasma being distinctly tinted red.
Hemorrhage also slightly raises the hydrogen-ion concentration of the
blood plasma, and diminishes the store of reserve alkali, so that the ad-
dition of a certain amount of acid to the blood (e.g., carbon dioxide)
causes a greater rise in the hydrogen-ion concentration.
The deficiency in the blood elements produced by the dilution is recti-
fied by the manufacture of new corpuscles in the bone marrow, etc., but
this process in a liberally fed animal takes several days for accomplish-
ment, and while it is going on microscopic examination o'f the blood will
reveal the presence of immature corpuscles.
Careful studies of blood regeneration following the removal on two
successive days, of 25 per cent of the blood, by Whipple and Hooper,
have shown that even in starving animals the total amount of hemo-
globin (percentage of hemoglobin multiplied by the volume of blood)
slowly recovers. Eecovery is greatly hastened by feeding with flesh or
even with gelatin. Removal of the spleen or the establishment of a bili-
ary fistula does not interfere with the recovery.
Incidentally it will be advantageous to consider here the effects of
transfusion! These are very different according to the nature of the fluid
used for transfusion. Three transfusion fluids have been investigated:
(1) blood itself, (2) physiologic saline solution (see page 95), and (3)
physiologic saline solution containing viscid substances such as gelatin.
The effects are also very different according to whether the solutions are
injected into animals with normal blood pressure or into those whose
blood pressure has been lowered by preceding hemorrhage.
When blood is injected into animals with normal blood pressure, it
will very soon cause the pressure to rise, and as the injection is main-
tained the rise may continue until the pressure is perhaps 50 per cent
or more above its normal level. If the injection is long continued, how-
ever, a sudden fall of pressure occurs, on account of engorgement of the
right side of the heart. If the injection is not pushed so far, the increased
blood pressure after being maintained for a short time returns to its old
level.
Injection of saline into a normal animal, if made slowly, has no effect
at all on the blood pressure; if more rapidly injected, the pressure will
rise slightly, but to a much less extent than that observed when blood
itself is injected. Much larger quantities of the saline than of the blood
can be tolerated before cardiac embarrassment ensues. After the dis-
140 THE CIRCULATION OF THE BLOOD
continuance of the saline injection, the blood pressure returns very
rapidly to its old level. The most striking result of such experiments is
the enormous volume of saline solution which can be slowly injected
without perceptibly affecting the pressure. The question is, Where does
the fluid go ? If the urinary outflow is examined, a certain increase will
usually be observed, but never by any means sufficient to account for
the disappearance of the injected saline. If we open the abdominal cav-
ity, we shall find that a considerable transudation of the saline into the
peritoneal cavity has occurred, and that the liver is conspicuously edem-
atous. A certain degree of edema is also usually evident in the tissues
of the extremities.
Still more interesting and important, from a practical standpoint, are
the results obtained by injecting the above solutions into the animals
whose blood pressure has been lowered by a previous hemorrhage. If
the blood removed during the hemorrhage is defibrinated (see page 101),
and then reinjected into the animal, it will bring the blood pressure al-
most but not quite back to its original level, which will then be 'fairly
well maintained. If, on the other hand, saline solution instead of blood
is injected, the restoration of blood pressure (with an amount of saline
equal to that of the removed blood) will amount to only about three-
quarters of the extent to which it had fallen. This partial recovery is,
moreover, maintained for only a short time, after which the pressure
rapidly falls nearly to the level to which it was reduced by the hem-
orrhage.
These observations raise two important practical questions: (1) Why
is saline relatively ineffective in the restoration of pressure? and (2)
Why does. the pressure thus restored so quickly fall again?
The answers to these questions brings us to a consideration of the next
of the factors concerned in the maintenance of the blood pressure,
namely, the viscosity of the blood.
4. The Viscosity, of the Blood
The importance of this factor arises from the fact that facility of flow
in a tube is inversely proportional to the viscosity of the fluid and
directly proportional to the driving pressure to which it is subjected —
that is, to the difference in pressure between two points in the tube.
If therefore the output of the heart should remain constant, but the
viscosity of the blood be decreased by a saline injection, the facility of
flow would be increased and the pressure decreased. This fact can easily
be shown experimentally in a model by causing gum solutions of various
concentrations to be driven through a glass tube by means of a small
piston pump delivering a constant amount of fluid into the tube with
BLOOD PRESSURE 141
each movement* Although the outflow from the narrow end of the tube
must remain constant, the pressure in the tubing will vary in proportion
to the viscosity of the gum solution (Bayliss5.)
Transferring these results to an animal whose blood pressure has been
lowered by hemorrhage, it has been found that if saline solutions con-
taining a sufficient amount of gum acacia or gelatin to make the viscos-
ity about equal to that of blood, are injected, the original level of blood
pressure is recovered as well as it would be had blood itself been in-
jected. A 7 per cent solution of gum acacia almost fulfills these require-
ments, but unfortunately this solution contains a slightly greater amount
of calcium than it is safe to inject into an animal. The excess of calcium
may, however, be removed by exactly neutralizing the gum solution with
sodium hydroxide, neutral red being used as an indicator. Most of the
calcium becomes precipitated as phosphate. The mucilage of the British
Pharmacopeia, diluted five times with water, makes a 7 per cent solu-
tion of gum acacia. A 6 per cent solution of gelatin, after being heated
to 100° C., gives a viscosity similar to that of blood, but on account of
the possible presence of tetanus spores such solutions must be very care-
fully sterilized before injection, and the process of sterilization causes
a decrease in viscosity. The injection of a quantity of one of the above
solutions equal to that of blood lost by a hemorrhage will usually bring
the blood pressure back to its original height and hold it there for an
hour or so.
Viscosity is, however, not. the only property of such solutions upon
which their desirable effect depends. The osmotic pressure of the colloids
also comes into play. By injecting saline solution containing a sufficient
amount of a colloid such as soluble starch, which gives it the correct
viscosity but has no osmotic pressure, the blood pressure, although it
temporarily recovers after transfusion, does not maintain its recovery in
the same way as with solutions containing gum or gelatin. The difference
between a starch solution and one of gum or gelatin is that the former
has no osmotic pressure, the effect of which is developed mainly on the
excretion of urine, as can be shown by observing the outflow from the
ureters during the injection into animals of equal quantities of saline
alone or of saline containing starch or gelatin (Knowlton6.) With the
first two fluids diuresis is produced, but not with gelatinous solutions.
The reason that the osmotic pressure of certain colloids prevents passage
of water from the blood into the urinif erous tubules is that the develop-
ment of -this pressure on the blood side of the renal epithejium tends to
counteract the filtration pressure by which the urine is formed (see
page 514.)
Although the urinary factor will not in itself explain the efficiency of
142 THE CIRCULATION OP THE BLOOD
the colloids in recovering the blood pressure, the conditions controlling
it reveal the mechanism by which the passage of fluid from the blood
vessels into the tissues is prevented when solutions of correct composi-
tion are injected. Normally the protein content of the blood plasma is
higher than that of the tissue lymph, so that there is a continual attrac-
tion of water from the tissues to the blood — an attraction Avhich is nor-
mally balanced by filtration going in the opposite direction. When the
filtration pressure in the blood vessels exceeds the difference existing
between the osmotic pressure of their contents and that of the tissue
fluids, water will pass into the tissue spaces. When the blood is diluted,
as by the injection of saline solution, the osmotic pressure of the colloids
in a given volume becomes lowered and, the filtration pressure remaining
constant, fluid passes into the tissue spaces. Of course these explanations
rest on the assumption that the wall of the blood vessels consists of a
membrane which is permeable to crystalloids but impermeable or nearly
so to colloids.
Another important property of the transfused saline solution to con-
sider is its hydrogen-ion concentration. This value increases in the blood
left in the body after hemorrhage, and injection of sodium chloride solu-
tion aggravates the acidosis; addition of NaHC03 so as to make a 0.2
M solution restores the correct PH, and at the same time restores the
lost buffer influence (Milroy7.) These observations are of interest in the
light of the recent discovery of Cannon that a condition of acidosis, as
judged by the C02-combining power of the blood, is present in shock,
and that the development of this condition can often be guarded against
by bicarbonate injections.
5. Elasticity of Vessel Walls
The elasticity of the vessel walls is essential to the maintenance of the
diastolic pressure. If the walls presented no elasticity but were rigid,
blood pressure would fall to zero between the heartbeats. This fact can
very readily be shown by a simple physical model consisting of a pump
to represent the heart, connected through a T-piece with two tubes, one
of \vhich is elastic, the other rigid. The free end of each tube is con-
tracted to a narrow aperture representing the peripheral resistance, and
either tube may be shut off from the pump by means of a stopcock (see
Fig. 30). Each tube should also be connected with a mercury manom-
eter. If now the stopcocks are arranged so that the fluid passes into
the rigid tube while the pump is in action, it will be found that with
each stroke of the pump the pressure in the tube rises considerably, but
that it falls to zero between the strokes. If now the stopcocks are turned
so that the flow is through the elastic tube, the action of the pump being
BLOOD PRESSURE 143
meanwhile kept up, it will be found that the pressure between the strokes
is maintained at a height which is dependent on : ( 1 ) the rate at which
the pump is operating, and (2) the resistance to outflow from the tube.
The quicker the action of the pump and the higher the resistance, the
lower the fall of pressure between the beats.
The physical explanation of this result is clearly that the fluid within
the elastic tube when the wave of pressure travels into it from the pump
distends the Avails of the tube, so that when the pressure from the pump
ceases to act, the stretched elastic Avails recoil on the column of fluid
and maintain the pressure. We may say that the elastic fibers in the
vessel Avails store up some of the systolic pressure and then transmit it to
the blood during diastole.
Fig. 31. — Diagram of experiment to show that the diastolic pressure depends on the elasticity
of the vessel wall. The pulse (produced by compressing the bulb B) disappears when fluid
flows through an elastic tube (F) when there is resistance (g) to the outflow. A, basin of
water; B, bulb syringe; C and E, stopcocks; D, rigid tube; F, elastic tube; G, bulb filled with
sponge.
These considerations would lead us to expect that patients with hard-
ened arteries should exhibit a lower diastolic pressure than normal per-
sons, which, however, is not usually the case, since such patients also
suffer from an increase in the resistance to the floAV of blood in the periph-
ery. The pressure pulse in these patients is, hoAvever, very marked.
On the other hand, Avhen the Aressel Avails become more extensible and
elastic, as in certain cases of aneurism, the pressure pulse in the vessels
beloAV the aneurism is distinctly less than that observed in normal ves-
sels of the same patient.
CHAPTER XVII
THE ACTION OF THE HEART
Having studied the methods for measurement and the main factors con-
cerned in the maintenance of the arterial blood pressure, we may now pro-
ceed to study in greater detail the two most important of these ; namely, the
action of the heart, and the peripheral resistance.
The heart action has to be studied from two viewpoints, the physical
and the physiological. From the physical viewpoint we have to study
the heart as the pump of the circulation. We must see how it acts so as
to raise the pressure of the blood within it, and how the valves operate
so as to direct the bloodflow always in one direction. We must also ex-
plain the causes of certain secondary physical phenomena, such as the
heart sounds which accompany the heart action, and of certain secondary
changes in pressure produced in the other thoracic viscera by each heart-
beat. From the physiologic Viewpoint we must investigate the conditions
responsible for the constant rhythmic activity of the heart and the con-
trol to which this is subjected through the nervous system.
THE PUMPING ACTION OF THE HEART
When the heart is viewed in the opened thorax of an animal kept alive
by artificial respiration and lying in the prone position, it can be noted
that with each contraction the ventricles become smaller and harder, that
the apex tends to rise up a little, so 'that if the thorax were intact it
would press more firmly against the walls, and that it rotates slightly
from left to right, but does not move nearer the base of the heart. If
the auriculoventricular groove is carefully observed, it will often be
noted that it moves slightly toward the apex with each systole, whereas
th'e base of the heart itself, where it is attached to the large vessels, re-
mains fixed. The auricles can often be seen to contract and relax before
the ventricles.
The most noteworthy results of this inspection are that during sys-
tole the apex of the heart does not move toward the base, but that
the auriculoventricular groove moves slightly toward the apex. That
these same movements occur in the intact animal can be shown by the
very simple experiment of pushing two long steel knitting needles
144
THE ACTION OF THE HEART 145
through the thoracic walls into the heart walls, one of them so placed
that it pierces the apex of the ventricle, the other so that it pierces the
base. The needles then act as levers with their fulcra at the. chest wall,
and if the movements of their outer free ends, produced by the movements
of the heart, are observed, they will be found to confirm the observations
made on the exposed heart.
More particular investigations of the changes occurring in the shape
of the heart cavity during systole and diastole have been undertaken by
making measurements of sections across the heart in one or other of
these conditions. For such purposes the heart in diastole is easily ob-
tained, but for the heart in systole it is necessary to use the somewhat
artificial means of injecting the heart with hot chromic acid solution
just before the death of the animal. The chromic acid causes the cardiac
muscle to contract and maintains it in this condition. The outcome of
these investigations is, however, not of much practical importance.
Although it is now common knowledge that the direction of the flow
of the blood is from the veins to the arteries, yet it may be of interest
to consider for a moment the general principle of the methods by which
William Harvey succeeded in making this discovery. His evidence was
partly anatomic, partly experimental. He pointed out that the walls of
the veins, arid of the auricles to which they lead, are very thin, whereas
those of the arteries and ventricles are very thick, and he concluded that
in the veins the blood must flow gently from the tissues toward the
heart, to which the valves in the veins direct it, and that in the arteries
it must be propelled by pulses with each systole through the arteries
towards the tissues by the contraction of the walls of the ventricles. The
experimental support for this hypothesis he furnished partly by clamping
the large vessels, veins and arteries leading to or from the heart, and
observing the resulting distension or collapse of the vessel ; and partly by
calculation of the amount of blood which must be expelled from the
ventricles in a given period of time.
Harvey's discoveries concerning the events of the cardiac cycle were
not much added to until experimental methods were devised by which
the pressure changes occurring in the various cavities could be measured
and compared. Until such measurements were elaborated, it was impos-
sible to investigate the mechanism by which the various valves between
the heart cavities and the vessels connected with them perform their
function, or to describe with any degree of accuracy the events occurring
in the heart chambers during the various phases of the cardiac cycle.
It is for the purpose of ascertaining the exact time relationship of these
changes that intracardiac pressure curves are studied.
146
THE CIRCULATION OF THE BLOOD
Intracardiac Pressure Curves
The earliest method for taking such curves consisted in introducing
into the cardiac chambers and the blood vessels of the horse, so-called
cardiac sounds. These consisted of a more or less rigid tube furnished at
one end with a little elastic bag or ampulla and connected at the other
with a tambour, by means of rubber tubing. One of these little bags
was placed in one of the ventricles, another in the auricle or aorta, the
tube being inserted in the former case through one of the large veins at
the root of the neck ; in the latter case through the carotid artery. The
intracardiac pressure curves obtained in this way marked a great ad-
vance over the methods that had previously been used to study the events
of the cardiac cycle, but they were so faulty in comparison with tracings
C--
Fig. 32. — Diagram of Wiggers' optical manometer. The wide glass tube (A) (connected
with the ventricle, etc.) is connected with a brass cylinder (B) provided with a stopcock (C),
the lumen of which comes in apposition with a plate (a) having a small opening in it. The
freedom of communication between B and a is regulated by the position of the tap. Above a is
a segment capsule (fc) 3 mm. in diameter and covered by rubber dam. This carries a small
mirror (C) fastened so that it pivots on the chord side of the capsule. Above the capsule is
arranged an inclined mirror, from which a strong beam of light is reflected on to the mirror
(c) on the capsule. This beam then travels back and the mirror (£) is adjusted so that it
impinges on a moving photographic plate. The slightest movements of the small mirror (C)
are thus greatly magnified.
taken by more modern methods that it is not worth while considering
them any further here.
The physical errors involved in the use of the older instruments were
due mainly to the elastic recoil of the membranes, etc., used in their
construction. A great improvement in technic was afforded by the use
of the spring manometer of Hiirthle (see page 126), which was connected
with one of the heart cavities by a cannula filled before insertion with
some anticoagulant fluid. The cavity of the tambour was made as small
as possible, and either left empty or filled with the anticoagulating fluid.
THE ACTION OF THE HEART 147
A searching investigation into the physical principles involved in tak-
ing records of sudden changes in pressure by such instruments has, how-
ever, shown that considerable errors are incurred, the inertia of the
moving mass of fluid in the tubing and the necessity of using levers in
order to secure records being responsible for most of them (cf. Wig-
gers). Their elimination has recently been achieved by using a so-called
optical manometer, one of which (Wiggers') is shown in the accom-
panying figure. It consists of a wide glass tube A, connected above with
a hollow brass cylinder B, provided with a stopcock C, the lumen of which
tapers from below upward till it assumes the same diameter as an aper-
ture in the segment capsule 6, above it — that is, a capsule cut away at one
end — which is 3 mm. in diameter and covered with rubber dam. By ad-
justment of this stopcock the pulsations of the fluid in A and B can be
damped to a greater or less extent before they are transmitted into the
Fig. 33. — Optical records of intraventricuiar pressure; a-l, auricular systole; b-d, presphygmic
period; d-f, sphygmic period; after /, diastole. Instruments of varying degrees of sensitiveness
were employed in taking the curves. ('From Wiggers.)
segment capsule. A small piece of celluloid carrying a tiny mirror rests
on the rubber dam, being pivoted on the chord side of the capsule. A
mirror is attached to the capsule with its plane so adjusted that the
image of a strong light placed at some distance from it is focused on the
little mirror carried by the celluloid. The ray reflected from the little
mirror and again reflected from the larger mirror is adjusted so as to
impinge upon a moving photographic plane travelling at a uniform rate
in a suitably constructed photographic apparatus. By the use of such
an apparatus the chief errors encountered by the use of the older in-
struments are eliminated, because there is no moving mass of fluid and
there are no levers to set up spurious vibrations. Curves secured by
the use of this instrument are shown in Fig. 33.
Two objects must be kept in view in analyzing the curves: (1) Curves
obtained from the different cavities may be compared in order to de-
termine the exact moment during the cardiac cycle at which such pres-
•148 THE CIRCULATION OF THE BLOOD
sure changes occur as must serve to produce opening or closing- of the
various valves; and (2) the contour of the curves obtained from each
cavity may be examined in order to find out exactly how the pressure
in that particular cavity is behaving.
Comparison of the Curves
Before using the curves for ascertaining the relative pressure in the
different cavities, they must be graduated according to some scale, for
it is clear that by the use of instruments like those we have been describ-
ing, the absolute pressure value of each curve will vary according to the
construction of the instrument (thickness of membrane, etc.), and in-
deed instruments of varying degrees of resistance must be employed in
taking curves from places having such different pressures as exist in
the auricles and ventricles. The graduation is, however, a very easy
matter, and consists, as already explained (page 126), in connecting the
instrument by means of a T-piece with a mercury manometer and a pres-
sure bottle and then marking on the tracing, the points corresponding to
each 10, 20 or 50 millimeters of increase of pressure, as the case may be.
To ascertain the time relationship between the opening and the closing
of the auriculoventricular valve, the tracings should be taken from the
right auricle and the right ventricle, and to ascertain the same with re-
gard to the semilunar valve, from the left ventricle and the aorta.*
By comparing the curves it is now an easy matter to ascertain the
exact moment at which the pressure in the one cavity comes to equal
that in the other. This moment, read on the accompanying time tracing,
will obviously indicate that at which the particular valve is just about to
open or close. From the results of such experiments, the curves may be
superimposed as in Fig. 34. ,
In the first place let us compare the curves from the right auricle and
ventricle. The curves begin at the very end of diastole, and they show
that a distinct increase in pressure is occurring in both auricle and ven-
tricle and lasting about 0.05 second. This is of course caused by auric-
ular systole, and since it occurs in both cavities, it indicates that the
passage between them, the auriculoventricular orifice, must be open.
The ventricular curve then suddenly shoots away beyond the auricular
because of the onset of systole in the ventricle, and the point at which
the two curves begin to separate indicates the moment at which the
auriculoventricular valves close. From this time on until ventricular
systole has given place to diastole, (about 0.2 second), the auricle is
*The connections with the heart may be made by pushing long cannulae down the large veins or
arteries, or in the case of the ventricles by inserting a cannula with a sharp point directly through
the wall of the ventricle.
THE ACTION OF THE HEART
149
therefore shut off from the ventricle. The exact moment in diastole at
which the two cavities are again brought into communication — i.e., the
ventricular valves open — is indicated by the curves coming together.
Having thus determined the exact moments of opening and closing
of the auriculoventricular valve, we may now proceed to compare the
intraventricular pressure curve with that taken from the aorta. After the
necessary calibration corrections, this curve has been placed in Fig. 34
in its true relationship to the ventricular curve. Beginning again at the
end of diastole, we find that the aortic pressure is very considerably
above that of the ventricles, indicating that the semilunar valves must
be closed; and it will be observed that the intraventricular pressure at
Fig. 34. — Pressure curves after being graduated have been superimposed. The presphygmic,
sphygmic and postsphygmic periods of ventricular systole are shown by the vertical lines. The
A-V valves close at the first line. The aortic valves open at the second line and close again at
the third line. The A-V valves open at the fourth line. The position of the two main heart
sounds is also indicated.
the beginning of systole does not rise sufficiently to open them until an
appreciable interval (0.02 to 0.04 second) after the closure of the auric-
uloventricular valves; that is to say, there is a period at the beginning
of ventricular systole during which the ventricle is a closed cavity. It
is a period during which the ventricle by its contraction is getting up a
sufficient amount of pressure in the fluid contained in it to force open
the semilunar valves against the resistance of the pressure in the aorta,
and it has been popularly called ''the period of getting up steam," or,
in physiologic language, the isometric, or the presphygmic, period. We
shall use the last-mentioned term in our further discussion here.
150 . THE CIRCULATION OF THE BLOOD
After the aortic valves have been opened, it will be observed that the
pressure in the ventricles is just a little above that in the aorta, and that
it continues so during the whole of ventricular systole. When diastole
sets in, the pressure in the ventricles quickly falls, and a point is soon
reached at which equality of pressure in ventricle and aorta is again
attained. This corresponds to the moment of the closure of the semi-
lunar valves. The pressure in the ventricle, although now rapidly fall-
ing, takes a little time before it has fallen low enough to permit the
auricular valves to open. Here again, then, the ventricle is a closed cavity,
and we have what is known as the postspliygmic period.
CHAPTER XVIII
THE PUMPING ACTION OF THE HEART (Cont'd)
THE CONTOUR OF THE INTRACARDIAC CURVES
The Ventricular Curve
From an analysis of the contour of each curve, further interesting
points are brought to light. The ventricular curve in the diagram alluded
.to above (Fig. 34) is shown as having a flat top or plateau. By the use
of the more modern, optically recording, instruments it has been shown
that this plateau becomes displaced by a peak if every precaution is
taken to prevent dulling down of the pressure changes in the instrument,
as by opening wide the stopcock in the instrument (Fig. 33). The peak
is, however, by no means a sharp one, so that we may fitly describe the
contour of the ventricular curve during the sphygmic period as consist-
ing of a rising portion, almost continuous with the curve during the pre-
sphygmic period, a summit and then a declining portion, which is usually
slower than the ascending. The practical value arising from a study of the
curves lies in the insight which they give us into the nature of the stroke
of the cardiac pump. They show us that the impulse which the ventricle
gives to the moving mass of blood in the aorta is rather a sudden than a
sustained one. The column of blood in the aorta is a mighty thing to
move, and it would appear as if a sustained pressure brought to bear on
it during the sphygmic period would be far more efficient in bringing
about an adequate movement of the blood than a sudden jerk. In closing
a heavy gate a slow sustained pressure is far more effective than a short
push.
It is further of interest to note on the intraventricular pressure curye
that there is very little indication of any secondary waves or vibrations
at the moment during which the semilunar valves are opened or closed.
Nevertheless, by close scrutiny it can usually be, seen that a slight
change in the direction of the ascending curve is evident when the valves
open (see Fig. 33), and similarly that the moment of closing is indicated
by a sharper bend in the curve. As a matter of fact, Wiggers has shown
that the exact contour of the curve during the sphygmic period depends
partly on the degree of sensitiveness of the optical manometer used and
partly on the tension existing in the ventricle just before contraction.
151
152 THE CIRCULATION OP THE BLOOD
In the case of the right ventricle the contour of the curve also depends
on the degree* of resistance to the bloodflow through the pulmonary
circuit. The top of the curve becomes broader when the initial tension
is high, and more rounded when there is a high pulmonary resistance.
Another point of interest in connection with the ventricular curve is
that early in diastole it descends below the line of zero pressure, indicating
that a negative or suction pressure must exist in the ventricle at this
time. It will be further observed, however, that this subatmospheric
pressure exists for only a very short time. The auriculoventricular
valves being opened, a similar negative pressure is also present in the
auricular tracing. Were we to depend on such records alone for evidence
of the actual existence of this negative pressure in the heart, objection
might be taken to the conclusion on the ground that it was due to the
to manometer
max valve
to heart
Fig. 35. — Von Frank's maximal and minimal valve, which is placed in the course of the
tube between heart and mercury manometer. By turning the stopcocks, it may be used as a
maximum, minimum, or ordinary manometer (central tubes open). (From Starling.)
sudden recoil to which the instrument is subjected at the beginning of
diastole. It is necessary therefore to control these observations by the
use of an entirely different method. This consists in connecting the
heart with a valved mercury manometer (see Fig. 35). This instru-
ment does not of course record any sudden changes of pressure in the
cardiac cavity, but in obedience to changes in pressure the mercury slowly
moves in the direction in which the valve permits it to move. Such an
instrument, with the valve opening towards the heart, is called a minimal
manometer, and after it has been connected with the ventricle, it will te
found that a negative pressure of perhaps 40 or 60 mm. Hg is recorded.
Evidently, then, the negative pressure does actually exist in the ventricle
during some phase of the cycle, and the question arises as to whether it
is of importance in connection with the pumping action of the heart. At
first sight, considering the heart as an elastic structure, we might con-
THE PUMPING ACTION OF THE HEART 153
ceive that the negative pressure would serve to suck blood into the heart,
just as it sucks water in an ordinary ball syringe. Closer consideration
will, however, show that this conclusion is untenable, partly because the
negative pressure exists in the ventricle for so short a period of time, and
partly because it would have to operate on the slowly moving column of
blood in the thin-walled veins, with the result that it would cause the walls
of these vessels to come together rather than produce a movement of the
blood contained in them. The negative pressure of the heart can not
therefore be of much consequence in attracting the venous blood into the
ventricle.
Several factors may cooperate to produce this negative pressure,
among them being the sudden opening out of the base of the ventricles
at the beginning of diastole, the elastic recoil of the tissue which becomes
compressed in the heart walls during systole, the turgescence of the walls
of the ventricles produced by the sudden inrush of blood into the coro-
nary vessels at the beginning of diastole, all of which factors tend to1
cause an opening out of the walls of the ventricles with a consequent
increase in the capacity of their cavities.
The Auricular Curve
Examination of the intraauricular pressure curve is of particular in-
terest because of the relationship which it has to a tracing taken of the
movements in the jugular vein at the root of the neck (see page 274).
This jugular pulse curve, as it is called, is produced mainly by the
changes of pressure occurring in the auricle, from which it differs only in
the relative height of the various waves. By graduating the. intra-
auricular pressure curve by the method described above, we can tell
exactly the magnitude in the changes of pressure occurring during each
cardiac cycle. This obviously can not be done with a tracing taken from
the jugular vein, although qualitatively the tracings reflect exactly the
changes that are occurring in the auricle.
On examining the auricular pressure curve (consult Figs. 34 and 97), we
find that after the wave of presystole, which of course coincides exactly
with that on the intraventricular curve, a second wave occurs culminating
in a peak almost exactly at the beginning of the sphygmic period. The
curve then rapidly descends, usually indeed below the line of zero pres-
sure, and slowly rises throughout the rest of ventricular systole, until
the moment of opening of the auriculoventricular valve, when it descends
again and thereafter runs parallel with the ventricular curve. The let-
ters used to designate the waves are the same as those employed for
similar waves shown on the jugular pulse tracing, and although the
154 THE CIRCULATION OF THE BLOOD
lettering is more or less arbitrary, we must accept it because of its gen-
eral usage in all Avork of this kind.
As to the causes of the waves A is of course caused by auricular systole
or presystole; C, occurring as it does at the beginning of the period of
ventricular systole, is caused by the bulging into the auricle of the closed
auriculoventricular valve. The floor of the auricle, in other words, at
this moment becomes somewhat elevated and imparts to the blood which
is resting upon it a slight wave of pressure, which is transmitted along
the veins for a considerable distance. The succeeding depression is
marked x, and the negative pressure which it indicates is- probably due
to the co-operation of three forces, all tending to increase the auricular
capacity: (1) the diastole of the walls of the auricle; (2) the descent
of the auriculoventricular groove, thus tending to open out somewhat
the folds in the walls of the auricle; and (3), probably most important of
all, the tendency of the thin-walled auricles to become dilated as a result
of the sudden diminution in intrathoracic pressure produced at each heart-
beat by the discharge of blood from the heart and intrathoracic blood
vessels into those of the rest of the body. All thin-walled structures
in the thoracic cavity, the auricles included, will expand to take up the
extra room created in the thoracic cavity. Similar negative heart pulses,
as they are called, can be observed with each systole in the lungs and
in the esophagus.
THE MECHANISM OF OPENING AND CLOSING OF THE VALVES
When physical valves open and close as a result of the changes in pres-
sure on their two surfaces, a certain amount of fluid must succeed in
passing the valve flaps before these become perfectly closed. But there
is every reason to believe that such is not the case in the heart, the flaps
of both the auriculoventricular and the s'emilunar valves being already
completely closed before pressure conditions entailing a possible regur-
gitation of blood through them become established.
Auriculoventricular Valves
During diastole the flaps of the auriculoventricular valves are hanging
down into the ventricle and floating in a half-open position in the blood,
which is meanwhile accumulating in the chamber. This position is de-
pendent upon the operation of tAVo opposing forces on the valve flaps:
the pressure of the blood floAving from the auricle on their upper aspects,
and reflected waves of pressure from the Avails of the ventricle on their
under aspects (centripetal reflux). When presystole occurs, the pres-
sure of the auricular stream momentarily increases, thus slightly dis-
tending the Avail of the meanAvhile relaxed ventricle and after a moment 's
THE PUMPING ACTION OF THE HEART
155
delay causing the reflected wave to become more pronounced. At the
same time the muscular fibers in the valve flaps (Kurschner's fibers)
contract and make the flaps shorter, the total effect of the two factors
being that the valve takes up a position nearer that of closure. When
presystole suddenly stops, the reflexed waves will persist for an instant
of time longer than the auricular wave which causes them, because of
the elastic nature of the ventricular wall, so that the valve flaps close
with perfect opposition not merely at their edges but also for a con-
siderable distance along their upper surfaces.
When ventricular systole starts, the only effect of the high pressure
which is brought suddenly to bear on the under surfaces of the already
closed valves is to cause them to vibrate and to bulge into the auricles,
being meanwhile anchored down and prevented from flapping into the
auricle by the chordae tendineae. Although there is reason to believe that
the musculi papillares to which these are attached begin to contract at the
1 23
Fig. 36. — Diagram to show the positions of the cardiac valves: 1, during diastole; 2, during
the presphygmic period; 3, during the sphygmic period.
very outset of ventricular systole — indeed slightly to precede it (see
page 263), and thus keep the chordae taut, yet as systole continues the
contraction of these muscles becomes more and more pronounced, and the
resulting tightening of the chordae serves to draw down the valve flaps,
so that progressively larger proportions of their upper aspects tend to
become opposed. Meanwhile the auriculoventricular orifice is also be-
coming narrowed down on account of the contraction of the musculature
of the auriculoventricular groove.
Semilunar Valves
The mechanism involved in the operation of the semilunar valves is
somewhat different. It has been shown that, when fluid is flowing in a
tube, the pressure and velocity are not equal in the axial and peripheral
parts of the stream. In the axis the velocity is greater than in the layers
of fluid next to the walls, but the pressure is less. These facts can be
156
THE CIRCULATION OF THE BLOOD
demonstrated by observing the flow through a wide tube of water in
which are suspended lycopodium spores. By placing in the tube small
bent tubes so arranged that one open end lies near the periphery and
the other near the center, it can be seen that the differences in pressure
are such as to cause the fluid to flow from periphery to axis (centripetal
eddies).
If the bent tubes are used to study the conditions of flow in a tube which
suddenly becomes wider, it will be found that Avhere the wide portion
starts centripetal eddies are set up, which tend to carry the seeds into
the axis of the' stream, where their velocity is greatly increased. Now
these are the conditions obtaining at the beginning of the large arteries
S.a.-D.v.
D.a.-S.v.
Fig. 37. — Diagram showing the position of the cardiac chambers and valves during presystole
(S.a. — D.v.) and during the sphygmic period. (From Landois.)
of the heart, the orifice into the ventricles being constricted, while at
the sinus valsalvae the vessels are dilated. A centripetal vortex must be
set up in the sinus, tending to throw the valve flaps into a closed posi-
tion, which, however is prevented by the blood rushing between them
from the ventricles. They thus take up a mid-position and vibrate in
the stream. When the efflux from the ventricle stops at the end of sys-
tole, the reflux, lasting for a moment longer and being now unopposed,
immediately closes the valves, in which position they are then maintained
by the greater pressure on their upper surfaces.
The position of the valves relative to the events of the cardiac cycle is
shown in Figs. 36 and 37.
THE PUMPING ACTION OF THE HEART 157
THE HEAET SOUNDS
During certain phases of the cycle distinct sounds, the heart sounds,
can be heard by applying a stethoscope to the thoracic wall. The first
occurs at the beginning of ventricular systole and is best heard over the
apex beat; the second occurs at the beginning of diastole and is heard
best at the second right costal cartilage or in the second left intercostal
space. A third sound, much less distinct, is sometimes heard in diastole
a short time after the second. To study the exact time relationship of
the sounds the vibrations which they set up can be recorded graphically
alongside cardiac tracings by means of a microphone attachment to the
electrocardiograph (see page 259).
Causes of Sounds
It has been found that the first sound consists of two distinct elements,
one high pitched and the other of a dull character. The former element
is believed to be the result of vibrations set up in the flaps of the auric-
uloventricular valves, and therefore in the blood in the heart, by the
sudden rise in systolic pressure. The dull element on the other hand
is undoubtedly of muscular origin. The evidence for these conclusions is
as follows: (1) When the auriculoventricular valves are prevented from
closing properly either by disease or by pushing a loop of wire down the
large veins, the high pitched quality disappears, and nothing but a rush-
ing sound accompanies the dull bruit produced by the contracting muscle.
(2) In a heart that has been rendered bloodless by an incision near the
apex, or even in an excised but still beating heart, the dull element of
the first sound still continues to be heard for a short time. That con-
tracting muscle produces a sound is a well-established fact.
There are, however, many obscure phenomena connected with the
causation of the first sound, but we can not go into such controversial
matters here. A close inspection of the electrophonographic tracing
shows that the sound starts at the beginning of the presphygmic period,
and that it lasts with gradually declining variation in intensity until
well into the sphygmic period (Fig. 38).
The second sound occurs accurately at the beginning of diastole and
can readily be shown to be caused by the sudden shutting and stretching
of the semilunar valves, which throws them, the blood in contact with
them, and the neighboring walls of the aorta into vibration. Proof of
this conclusion is furnished by the following facts: The second sound
immediately disappears if the blood is let out of the heart by opening
the apex, and it is replaced by a rushing "bruit" if the flaps are pre-
vented from closing as a result of disease or of hooking them back by
158 THE CIRCULATION OF THE BLOOD
passing a wire down the carotid artery. The third sound, although audi-
ble only in some individuals, can nevertheless be shown to exist by the
electrophonograph, and since it occurs at the time when the auriculo-
ventricular valves open, it is believed to depend upon the sudden inrush
of blood from auricles to ventricles.
The greatest importance of the sounds is in the clinical diagnosis of val-
vular and other lesions of the heart. When a valve leaks, for example,
the blood escapes past it under great pressure, and is ejected into a mass
of blood at low pressure, these being conditions which are well known
to create sounds or 'bruits. By examining the exact relationship of such
bruits to the normal heart sounds, deductions can be drawn concerning
the condition of the various valves.
Record of Heart Sounds
The heart sounds have been graphically recorded by transmitting them
through a stethoscope to a microphone placed in circuit Avith a string
galvanometer (electrophonograms,). Through this circuit passes a cur-
rent the strength of which depends on the resistance offered by the
microphone, this resistance being proportional to the number and ampli-
tude of the vibrations of the sounds transmitted to it through the stetho-
scope. There are several objections to this method. One of these is de-
pendent on the varying distance of the heart from the chest wall, which
causes many of the sound vibrations to be lost before they reach the
stethoscope ;_ another, on adventitious sounds arising from contracting
muscles, the impact of the heart against the chest wall, etc., and still
another on unequal resonation by the air in the neighboring portions of
lungs. To investigate the problem more thoroughly, Wiggers,37 using
anesthetized animals, has recorded the sounds by carefully stitching to
the heart (exposed through a small opening in the pericardium) a lever,
the end of which was attached to a "transmitter" consisting of a
small capsule covered with rubber dam. The transmitter was connected
by rubber tubing to a ' ' recorder ' ' consisting of another small capsule carry-
ing on its membrane (made of rubber cement) an eccentrically placed small
mirror, on to which a beam of light Avas throAvn. The movements of the
beam of light reflected from the mirror, and caused by the sound vibra-
tions, were photographed. Mechanical vibrations set up in the apparatus
itself Avere largely eliminated by a side opening on the recorder, and the
effect of outside sounds minimized by surrounding the recorder by a
ventilated glass housing.
Although this apparatus is not free from faults due to inherent vibra-
tion frequency and resonance, the records secured by it are valuable in
shoAving the exact relationship of the sounds to the events of the cardiac
THE PUMPING ACTION OP THE HEART
159
cycle. The vibrations from the two ventricles are alike, but differ from
those taken from the aorta. The first ventricular sound consists of from
five to thirteen irregular vibrations, usually in three groups, the first
composed of two small vibrations, the middle one of several large vibra-
tions, and the third of a varying number of small vibrations. The
3 r
It II
m pi •-.•>»<•. ;
A.
^^ff ^| ' , ^^^K
B. C.
Fig. 38.— Electrophonograms along with intraventricular pressure curves from three dif-
ferent experiments. In A the uppermost curve shows the pressure, the middle one the sounds
o_f the right ventricle, and the lowermost one those of the aorta. P indicates the relative posi-
tion of the curves. M is due to mechanical oscillations. Sz indicates the second sound, and
/, 2, 3, and 4 the corrected time relations of the first sounds. In B, the pressure and sound
curves are both from the left ventricle (letters same as in A). In C, the aortic and pulmonary
arterial sounds are shown (letters same as in A). (From Wiggers and Dean.)
duration of the sound is from 0.05 to 0.152 seconds, and the periodicity
from 0.004 to 0.054 per second. When compared with an intraventricu-
160 THE CIRCULATION OF THE BLOOD
lar pressure curve, the initial vibrations occur 0.01 second prior to the rise
in pressure, the main vibrations reaching their greatest amplitude before
the sphygmic period begins, and the final vibrations occurring during
the early part of the sphygmic period and therefore just before the aortic
pressure has reached its height. The main vibrations therefore occur
during the descending limb of the R wave of the electrocardiogram (be-
ginning 0.01 second before its completion), the small preliminary vibra-
tions occurring during the ascending limb. When taken from the aorta,
the record of the first sound is someAvhat different, there being no initial
vibrations and the main ones being of greater frequency and reaching
their maximum earlier than those taken from the ventricle. The sub-
sequent vibrations are also larger, especially when the aortic pressure
is high (Fig. 38).
The record of the second sound at the ventricle is much simpler and
usually of less amplitude than the first, consisting of two to six vibrations
lasting 0.015 to 0.056 second. They begin a short time after the ventricu-
lar pressure begins to fall, approximately at the dicrotic notch of the aortic
curve, being completed in from 0.015 to 0.025 second after the bottom
of the notch. Their relationship to the T wave is variable. Taken from
the aorta, the record of the second sound shows vibrations of greater
amplitude and of a greater frequency than that from the ventricle.
CHAPTER XIX
THE NUTRITION OF THE HEART
THE BLOOD SUPPLY
In cold-blooded animals, such as the frog, the heart muscle is nourished
by blood soaking into it from the heart chambers, which indeed do not
form definite cavities as in the mammalian heart, but exist as an inter-
lacement of muscular tissue. In the hearts of higher animals, the muscu-
lature is supplied by special arteries (the coronary), although a certain
amount of blood may still pass directly from the cardiac cavities into
the musculature through the veins of Thebesius.
The relative importance of the various branches of the coronary artery
in maintaining an adequate nutrition of the heart has been studied by
observing the effect of occlusion of one or more of them (W. T. Porter9.)
Occlusion of the circumflex branch of the left coronary artery caused
arrest of the heartbeat in about 80 per cent of cases, the arrest being
usually accompanied by fibrillary contraction. Occlusion of the right
coronary arrested the ventricular contraction in about 20 per cent of
the cases. Smaller branches may be occluded without any evident
change in the heartbeat.
These results indicate that the capillary areas supplied by the branches
of the coronary artery do not freely anastomose with one another. They
are more or less terminal arteries ; that is, each branch supplies a distinct
region of the cardiac muscle. If one of the smaller branches of the coro-
nary is occluded, although there is 110 immediate stoppage of the heart-
beat, yet after some time the area supplied by that branch usually under-
goes necrosis, again indicating that collateral circulation can not have
become established. It is interesting, however, to note in this connection
that anatomic studies have shown that a certain amount of anastomosis
does occur between capillaries of different branches, although it is evi-
dent, from the above observations, that no adequate collateral circulation
becomes established through this anastomosis.
PERFUSION OF HEART OUTSIDE THE BODY
In order that the blood supply through the coronary arteries may
adequately maintain the normal nutrition of the cardiac muscle, certain
161
162 THE. CIRCULATION OF THE BLOOD
conditions must be fulfilled. The recognition of these conditions has
been accomplished by observations on the excised heart, for it has been
found that if they are fulfilled the mammalian heart can be made to beat
in perfectly normal fashion for several hours after its removal from
the animal's body. Indeed certain mammalian hearts, such as that of the
rabbit, may be made to beat for several days outside the body. We may
consider the essential conditions of the blood supply under four headings:
(1) the temperature; (2) the oxygen supply; (3) the pressure; and (4)
the chemical composition. Successful perfusion may be performed with
artificial saline solutions (e. g., Locke's), but it is simplest in investigating
the relative importance of the above conditions to start the heart per-
fusion with defibrinated blood.
After bleeding an anesthetized animal, such as a dog or a cat, until
no more blood can be removed, the blood is defibrinated and filtered
through gauze to remove the fibrin. The thorax of the dead animal is
then quickly opened, ligatures placed around the main arteries springing
from the arch of the aorta, a cannula with its end pointing toward the
heart inserted into the descending thoracic aorta, and the latter cut
across below the point of insertion of the cannula. The heart is then
quickly removed from the thorax and an artificial saline solution
(Locke's) allowed to run into the aortic cannula through a side tube,
until all the blood has been washed out -from the coronary vessels. Dur-
ing this operation the heart may develop a few beats even though the
solution is quite cool. The aortic cannula is now connected with a bottle
containing the defibrinated blood diluted with Locke's solution and
brought to body temperature by immersion in a water-bath. By means
of a suitably regulated air pressure exerted on the surface of the diluted
blood in the bottle, this is forced through an outlet at the foot of the
bottle into tubing which runs to the aortic cannula. The fluid thus finds
its way into the coronary vessels; for in passing toward the heart in the
aorta it will close the semilunar valves and force its way under pressure
into the coronary vessels, subsequently escaping by the coronary sinus into
the right auricle. Very soon after the perfusion is started the heart
begins to beat vigorously and regularly, thus offering a suitable prepara-
tion upon which to test the first three mentioned conditions necessary
for the nutrition of the cardiac musculature (Fig. 39).
If the temperature of the solution is allowed to fall considerably, the
beat becomes much slower, and if the cooling is proceeded with, the heart
will after a while cease beating altogether. If the pressure is lowered,
the beat will not necessarily become slower but very much feebler, and
will soon cease. In general it may be said that the temperature of the
solution affects the rate of the beat, and the pressure affects its strength.
THE NUTRITION OF THE HEART
163
Funnel (refilling
& air vent)
Stock solution
(Diluted blood +
a salt solution)
• Metal pan
Hot water bath
"fiver
Tube soldered
between palls
Fig. 39. — One form of apparatus for recording tracings from an excised heart (Langendorff
method). The heart is kept warm by a water bath (heart warmer), and the perfusion fluid is
also warmed. The driving pressure in this apparatus is supplied by gravity. (From Jackson.)
164 THE CIRCULATION OF THE BLOOD
It is, however, obvious that in perfused preparations changes in pres-
sure are likely to cause alterations in rate as well as in force, unless
great care is taken to keep the heart itself as warm as the perfusion
fluid.
The importance of an adequate pressure in the coronary vessels has
been clearly brought out in certain experiments in which the beat has
been maintained for a short time by establishing a pressure in the cor-
onary vessels by means of indifferent fluids or gases. Thus, if oxygen
gas is alloAved to pass through the vessels under pressure, the heart will
beat for a short time, and the same result has been observed even when
mineral oil or mercury has been perfused under pressure (Sollmann).
The necessity for an adequate oxygen supply is very readily demon-
strated. If the darker blood ejected from the right auricle with each
heartbeat is transferred immediately to the perfusion bottle, the heart-
beat will soon become feeble and irregular, to be readily restored to
normal when this dark blood is shaken up with air or oxygen.
By artificial perfusion in the manner above described, the automatism
of the heart may be restored many hours after death. Partial restora-
tion, confined to the auricles or to that part of the ventricles lying im-
mediately adjacent to the large blood vessels, can also be accomplished
in the heart of man several days after death, provided death has not
been caused by some acute toxic infection such as diphtheria or septice-
mia. The Russian physiologist Kuliabko, has succeeded in restoring for
over an hour the normal beat of the heart of a three-months-old boy
twenty hours after death from double pneumonia, but here again the
pulsation returns only in certain parts of the heart. As will be pointed
out, the remarkable resistance of the heart muscle displayed in these
experiments has been taken as an argument in favor of the myogenic
hypothesis for automatic rhythmic power of cardiac muscle, the argu-
ment being that nervous structures could not live so long a time after
death. The fallacies in this argument are discussed elsewhere.
RESUSCITATION OF THE HEART IN SITU
A suitable intracoronary pressure is a sine qua non for the mainte-
nance of the heartbeat, and this is a fact of great clinical significance,
for it indicates that any attempts to resuscitate a dead animal are cer-
tain of failure unless the method is such as will bring a nutrient fluid
under a certain pressure to bear on the coronary arteries. Injection of
fluid, even of defibrinated blood, into a vein will obviously fail to ful-
fill this condition, for the perfusion must be made into an artery so that
the fluid is carried down the aorta and thence into the coronary arteries.
THE NUTRITION OF THE HEART 165
The practical question, in so far as resuscitation of the heartbeat is
concerned, is therefore, How can we get the necessary fluid under pres-
sure into the beginning of the aorta f Even if we were to transfuse fluid
under considerable pressure into the aorta through the carotid artery,
it would mainly follow the large vessels leading away from the heart,
only a fraction of it reaching the beginning of the aorta. To compel the
fluid to pass towards the heart AVC must introduce some obstruction to
its passage peripherally. This can be done by the injection of a consid-
erable dose of epinephrine (adrenaline) in normal saline solution through
the needle of a hypodermic syringe inserted into the tubing leading
from the burette or pressure bottle to the cannula in the carotid artery.
As the perfusion fluid is running in, the epinephrine injection is quickly
made, artificial respiration and cardiac massage being meanwhile prac-
ticed. In the majority of animals it will be found that complete res-
toration of the normal blood pressure can be effected by this method.
Indeed by performing the resuscitation under aseptic conditions, some
animals may be permanently resuscitated so far as the circulation is
concerned, although the nervous structures, even after a few minutes
of "death," never reacquire their normal condition.
The epinephrine acts mainly by constricting the small arterioles and
thus directing the bloodflow towards the heart, but partly also by a direct
stimulating action on the cardiac muscle. It does not, however, con-
tract the coronary vessels; on the contrary, it is said to cause these
slightly to dilate.
THE RELATIVE IMPORTANCE OF THE VARIOUS CONSTITUENTS
OF THE PERFUSION FLUID
We can study the chemical conditions necessary for resuscitation
of the heartbeat by observing the beat of an artificially perfused heart
while solutions of different chemical composition are being perfused
through the coronary vessels. At the outset we are impressed with the
fact that for successful resuscitation the organic constituents of the
nutrient fluid are of trivial importance compared with the inorganic
constituents. With a solution containing the proper proportion of in-
organic salts, and of course an adequate supply of oxygen, the heart
of a rabbit, for example, may be made to continue beating for several
days.. It is true that it will beat longer if some of the organic con-
stituents of the blood plasma, particularly carbohydrate, are present,
but on the inorganic constituents alone its ability to beat is truly
remarkable.
166 THE CIRCULATION OF THE BLOOD
Observations on Cold-Blooded Heart
The earlier experiments for the investigation of the chemical condi-
tions necessary for the maintenance of the heartbeat were performed
on the heart of the frog or turtle. By perfusing either of these hearts
with physiologic sodium-chloride solution, it was observed that though
the beat might continue for some time, yet it gradually grew feebler
and feebler, until at last it ceased altogether with the heart muscle
in a condition of extreme relaxation or diastole. If small proportions
of potassium and calcium salts (as chloride) were added to the sodium-
chloride solution, the beat was much better maintained. Doctor Sidney
Ringer proved that the optimum concentration to produce efficient and
prolonged contraction for the heart of the frog or terrapin is as follows:
potassium chloride, 0.03 per cent; calcium chloride, 0.025 per cent.
The effectiveness of the solution was also found to be increased by the
addition of 0.003 per cent of sodium bicarbonate. This acts as a buf-
fer substance (page 36), holding the hydrogen-ion concentration at a
constant level. More recent work has shown that the hydrogen-ion con-
centration of the perfusion solutions is of considerable importance in
determining the efficiency of the beat, but the optimum is not the same
for the hearts of different kinds of animal, and indeed it may differ
for different parts of the same heart.
The question naturally arises as to the relative importance of each
of the above salts; or rather, we should say, cations, since the anion,
chlorine, is the same for all of them. The function of the sodium chlo-
ride in the solutions is twofold: (1) to endow the solution with the
proper osmotic pressure (see page 4) ; and (2) to perform the special
role of the sodium ion in the origination and maintenance of the auto-
matic beat. The latter function of Na can be shown by observing the behav-
ior of strips cut out from the ventricle of the turtle heart and placed
in solutions of correct osmotic pressure but containing no sodium chlo-
ride— isotonic solutions of cane sugar, for example. They soon cease
to beat, but if a small amount of sodium chloride is added to the cane
sugar solution, rhythmic contractions return. The role of the calcium
ions is almost entirely a pharmacologic one. If a strip of turtle ven-
tricle which has been made to cease beating by immersion in isotonic
sugar solution is placed in a weak solution of calcium chloride before
it is transferred to sodium chloride solution, the spontaneous contrac-
tions will return earlier and continue for a longer time. On the other
hand, if more than the correct amount of calcium salt is present in the
solution, the beats will soon be found to become smaller and smaller
in amplitude, because relaxation does not properly occur between them,
and ultimately they will cease altogether with the ventricle in a condition
THE NUTRITION OF THE HEART 167
of extreme contraction, called calcium rigor. The importance of calcium
may also be shown by attempting to perfuse a turtle heart with blood
serum from which the calcium has been removed by the addition of
sodium oxalate (which precipitates it as insoluble calcium oxalate). The
heart soon ceases to beat, but can readily be made to do so again by
adding a slight excess of calcium chloride.
The potassium ions do not appear, like those of calcium and sodium, to
be absolutely essential for the maintenance of the heartbeat; at least the
heart of the turtle will beat for a long time when perfused with a solu-
tion containing only sodium and calcium salts. The explanation of this
result need not, however, necessarily be that potassium is an unessential
constituent of the perfusion fluid, for it may well depend on the fact that
there is a sufficient store of potassium locked away in the muscle fiber
to supply the requirements of the heart muscle for this ion for at least
as long as the beat would continue under any circumstances. In any
case, we know that potassium has a profound influence on the heart-
beat, for when the proportion of it in the perfusion fluid is increased, the
beat becomes very slow and the tone of the heart is greatly diminished —
that is, it becomes extremely relaxed between the beats; and if the
amount is further increased, will very soon come to a standstill in a
greatly dilated or diastolic position.
The striking antagonism displayed by these inorganic cations upon
the heartbeat has led some investigators to suggest that the stimulus re-
sponsible for the rhythmic activity of the heart depends on some sort
of chemical union occurring between the inorganic cations and the con-
tractile substance of the heart. Union of calcium with the contractile
substance will lead to systole or contraction, whereas union of sodium
or potassium will lead to relaxation or diastole.
Observations on Mammalian Heart
Investigation of the efficiency of various saline solutions on the iso-
lated mammalian heart has shown that the proportion of the above salts
must be somewhat different from that used for the cold-blooded heart.
As might be expected, the most efficient proportions are those present
in the blood serum of the particular animal whose heart is being per-
fused. Basing his proportions upon the results of analyses of the inor-
ganic constituents of mammalian blood serum, Locke found that an
inorganic solution of the following composition is most efficient: so-
dium chloride, 0.9 per cent; calcium chloride, 0.024 per cent; potassium
chloride, 0.042 per cent; and sodium bicarbonate, 0.01 to 0.03 per cent.
When "Locke's solution," as it is called, is perfused, with oxygen in it,
under pressure through the isolated mammalian heart at body tempera-
168 THE CIRCULATION OP THE BLOOD
ture, efficient beating can be maintained for many hours. More recently
a solution known as Tyrode 's is commonly used. It contains a small amount
of magnesium and of phosphates. Although undoubtedly superior for
some perfused preparations, such as the intestine, it does not seem to be
in any way superior to Locke's for the perfusion of the heart. The bicar-
bonates and phosphates in these solutions endow them with a hydrogen-ion
concentration near that of the blood (slightly on the alkaline side of
neutrality), and at the same time they act as buffer substances.
As already pointed out, the organic constituents of such perfusion
fluids do not appear to be relatively of nearly so much importance as
the inorganic. Nevertheless it appears that a small percentage (0.01
per cent) of glucose does materially improve the nutritive qualities of
the solution, and it has moreover been shown that after a while the con-
centration of glucose in the perfusion fluid distinctly decreases. This
does not of itself necessarily mean that the glucose is actually utilized
by the heart muscle: it might be stored away in it as glycogen. That
some consumption of carbohydrate does however occur in the heart has
been demonstrated by measuring the intake of oxygen and the output
of carbon dioxide through the lungs of an isolated heart-lung prepara-
tion perfused outside the body with defibrinated blood. By experiments of
this type the attempt has been made to show that the heart of diabetic
animals loses the power of burning glucose as compared Avith the hearts
of normal animals. While the experiments are very suggestive, the
results do not as yet justify us in claiming that in the latter disease the
power of burning glucose in the tissues has been materially depressed.
The concentration of hydrogen ions in the perfusion fluid has an im-
portant influence on cardiac efficiency. We also know that the most
convenient method for changing the hydrogen-ion concentration of such
fluids is by altering their tension of carbon dioxide (see page 354). In
a heart-lung preparation,* such alteration in carbon-dioxide tension can
very readily be brought about by altering the percentage of this gas in
the air with which the lungs are ventilated. To measure the efficiency
of the heartbeat in such an experiment, it is convenient to enclose the
organ in a cardioplethysmograph, the tracing of which will tell us the
degree to which the heart is contracted or relaxed, as well as the output
of blood per minute. By increasing the tension of carbon dioxide, it
has been found in such experiments that the dilatation of the ventricle
is encouraged, so that the heart with each beat discharges a larger quan-
tity of blood (Fig. 40). When defibrinated blood is used the optimum
*A heart-lung preparation is one in which both heart and lungs are perfused outside the body,
the vessels being suitably connected to maintain a continuous circulation.
THE NUTRITION OF THE HEART
169
pressure or tension of carbon dioxide has been found to lie between 5
and 10 per cent of an atmosphere.
That the effect of carbon dioxide in encouraging the relaxation of the
heart between beats is dependent upon the change in hydrogen-ion con-
centration of the perfusion fluid has been shown by securing the same
results in experiments with perfusion fluids to which different quanti-
tities of weak nonvolatile acids have been added. These observations are
Fig. 40. — Volume curve of ventricles of cat (lower curve) in a heart-lung perfusion prepara-
of practical importance because of the light which they throw on the
cause of cardiac failure following upon conditions in which there has
been excessive removal of carbon dioxide from the blood, as in forced
ventilation of the lungs. Yandell Henderson has suggested that sur-
gical shock may be, partly at least, due to cardiac failure following the
"washing out" of carbon dioxide from the blood by the dyspnea so
often incident to the administration of anesthetics in surgical operations.
CHAPTER XX
THE PHYSIOLOGY OF THE HEARTBEAT
THE ORIGIN AND PROPAGATION OF THE BEAT— THE PHYSIO-
LOGIC CHARACTERISTICS OF CARDIAC MUSCLE
The origin and propagation of the heartbeat are studied on the excised
heart of a frog or turtle, or on the mammalian heart by perfusing it
under suitable conditions, which have already been described. The results
obtained on the cold-blooded heart apply more or less directly to the
warm-blooded. In the first place it is clear that the rhythmic contrac-
tility of the heart is not at all dependent upon the central nervous sys-
tem, for if it were so, 'the excised heart could not continue beating. This
fact does not, however, necessarily imply that the beating power is in-
dependent of nervous structures, for in the heart itself an extended net-
work of nerve cells and connecting nerve fibers can readily be demon-
strated. It might quite well be the case that the rhythmic beat is de-
pendent upon the transmission to the muscle fibers of the heart of
impulses generated in the nerve cells and transmitted along the nerve
fibers of this local nervous system. Such is the neurogenic hypothesis of
the heartbeat.
On the other hand, it may be that these nervous structures are not at
all responsible for the origination of the beat, but serve merely as sta-
tions on the pathway of the nerve impulses, transmitted to the heart
from the central nervous system along the vagus and sympathetic nerves,
for the purpose of altering the rate of the heartbeat so as to adjust it
to the requirements of blood supply in the various parts of the body. In
such a case the rhythmic power would reside in the muscular tissues of
the heart — that is, each cardiac muscular cell would have the power,
not merely like skeletal muscle of contracting in response to a stimulus
transmitted to it, but also of originating that stimulus within itself.
This is the myogenic hypothesis. Much controversy has raged around
these two hypotheses and although space will not permit a detailed study
of the question, it will be necessary, on account of the great importance of
the subject from the physiologic standpoint, briefly to review the main
arguments of each school of thought.
There is no piece of evidence offered by the advocates of either the
neurogenic or the myogenic hypothesis that can, taken singly, be con-
170
THE PHYSIOLOGY OF THE HEARTBEAT 171
sidered as absolutely conclusive. Although some of "the proofs" may
at first sight appear to be conclusive, yet each of them breaks down when
subjected to a closer scrutiny. It is only after we have collected all the
evidence for and against each view that we shall be in a position to come
to any conclusion, and even then it will be plain that our conclusion can
be only tentative.
Myogenic Hypothesis
Taking first of all the evidence in support of the myogenic hypothesis,
the following stands out most prominently:
1. The heart beats in the embryo chick before any nerve cells have
grown into it, and not only this, but if portions of heart muscle are re-
moved from the embryo and placed in blood plasma, they will continue
beating for many days It has also been observed that cells may wander
off from this mass of cardiac muscle and undergo multiplication and
differentiation, so as to produce isolated muscle cells which exhibit
rhythmic contractility. The rebuttal on the part of the neurogenists of
this apparently unassailable evidence is to the effect that, although em-
bryonic muscle cells may exhibit the power of rhythmic contraction, this
does not mean that the fully developed muscle cells will necessarily have
such power. In the eary stages of embryonic development, it is of course
evident that the functions which in the fully developed animal are del-
egated to various special organs and tissues should be performed by cells
having several such functions in common. The muscle cells of the heart,
for example, may to start with be possessed of a power not only of con-
tracting but also of initiating the contraction. It may be that they are
partly nervous in character and that only later, when the differentiation
is consummated, does the power of rhythmic contraction become dele-
gated to the nervous element and that of contraction retained by the
muscle itself.
2. The nervous structure in the heart may be damaged either by me-
chanical means or by drugs without apparently interfering with the
power of rhythmic contraction; for example, in the heart of large tur-
tles it is possible to dissect out a considerable amount of nervous tissue
without any disturbance of the beat, and in all animals the administration
of atropine, which paralyzes the postganglionic fibers of the autonomic
nervous system (see page 226) found in the heart, does not affect it.
3. The apex of the ventricle in such hearts as that of the turtle can
be shown, by careful histologic examination, to contain no nerve cells,
and although a few nerve fibers may be found, these are functionless
without nerve cells. This virtually nerveless piece of heart muscle can
be made to contract rhythmically by perfusing it with suitable saline
172 THE CIRCULATION OF THE BLOOD
solution under pressure and starting the beating by application of elec-
trical stimuli. Isolated strips of ventricular muscle, in which also no
nervous element can be demonstrated, may under favorable conditions
be caused to beat quite regularly if supplied with proper nutrient fluid.
The rebuttal of this evidence is twofold: In the first place, skeletal mus-
cle itself under certain conditions, such as exposure to solutions con-
taining an excess of phosphate (Biedermann's), may exhibit rhythmic
contractility, especially on cooling, which indicates that exhibition of rhyth-
mic power in isolated portions of cardiac muscle need not mean that under
ordinary conditions such power is responsible for the normal heartbeat.
In the second place, it is pointed out that although we can not reveal
their presence by present-day histologic methods, this is not conclusive
evidence that the heart-muscle fiber may not possess some nervous struc-
tures capable of functioning as nerve cells.
The heart even of mammals can be made to continue beating for sev-
eral days after excision from the body. The nerve cells, as we know them
in the central nervous system at least, can not, on the other hand, be
made to functionate for more than a few hours after death. Therefore,
it is argued, the heartbeat in surviving mammalian hearts can not de-
pend on the nervous structures. The argument is hoAvever easily refuted:
on the one hand, we do not know that the nerve structures situated
peripherally in the heart muscles are of the same viable nature as those
composing the central nervous system ; and, on the other, the survival
of the heart may in itself be sufficient to maintain around the nerve cells
embedded in it a nutrient environment which is much more physiologic
than that which AVC can supply in artificial perfusions of surviving
nervous tissues.
4. Circumstantial but nevertheless strong evidence is furnished by
the fact that many other varieties of involuntary muscle are endowed
with rhythmic contractility; thus, the muscle of the intestines, of the
ureters, of the bladder, of the uterus, of the blood vessels of certain
animals, and of the lymph vessels in the so-called lymph hearts, main-
tain rhythmic contractility after isolation from the animal body. The
rhythmic power seems in certain of these cases to be independent of
nervous control.
Neurogenic Hypothesis
In favor of this hypothesis the following evidence is offered:
1. The heart of certain animals — of Limulus, the king-crab, for exam-
ple, is definitely dependent for its rhythmic contractility upon neigh-
boring nervous structures. The heart of this animal is a tubular sac-
culated organ, and along its dorsal surface there runs longitudinally a
THE PHYSIOLOGY OF THE HEARTBEAT 173
nerve cord containing ganglion cells and giving off fibers which proceed
in part directly to the heart and in part to lateral cords (Fig. 41). Re-
moval of this median nerve cord is followed by total abolition of the
heartbeat; the heart becomes perfectly quiescent like an unstimulated
skeletal muscle. In appraising the evidence at its true value, it must be
noted that although by stimulation of the nerve fibers contraction of the
heart can be produced, the contraction is like that of a skeletal mus-
cle— it is not rhythmic ; and moreover — and this is most important — if
the various physiologic properties of muscle as described below be stud-
ied (page 176), it will be found that in all of them the quiescent heart
muscle behaves, not like the heart muscle of other animals, but like that
of skeletal muscle. This evidence, therefore, while indisputably showing
that the heart of Limulus depends for its rhythmic .power upon neigh-
boring nerve structures, does not justify the assumption that this will
be the case in the heart of animals having different physiologic properties.
2. The disposition of the nervous structures in the heart, especially of
the frog and turtle, exactly corresponds to the degree of development of
Fig. 41. — Heart and cardiac nerves of Limulus polyphetmis. (Carlson.) aa, anterior ar-
teries; la, lateral arteries; In, lateral nerves, mnc, median ganglionic chain; os, ostii or afferent
stomata, each pair of which corresponds to one of the segments into which the Limulus heart
is divided.
the rhythmic power of the different parts of the heart ; thus, the greatest
rhythmic power is manifested by the sinus and the least by the tip of the
ventricle at the bulbus arteriosus. In the former position the nerve
structures are very prominent; in the latter, no nerve cells and but few
nerve fibers can be detected. This proof is, however, easily assailed.
In the first place, it may merely be a coincidence that the disposition of
the nerve structures and the development of rhythmic power correspond.
The unequal rhythmic powers may depend primarily on a difference
in structure of the muscle fibers themselves, such differences having
been shown to exist between the muscle cells of the sinus and those
of, say, the ventricle. The former cells, for example, have much less'
developed crossed striation and their protoplasm is much more gran-
ular; in short, they are much more embryonic in type than the cells from
the tip of the ventricle.
If a jury had to return a verdict from evidence of so conflicting a char,
acter, it would no doubt be equivalent to that of the Scottish court — "not
174 THE CIRCULATION OF THE BLOOD
proven." But it is likely that the majority of the jury would vote
in favor of the myogenic hypothesis. Probably the safest viewpoint to
take at the present time is that the power of rhythmic contraction is
inherent in the cardiac muscle fibers, being most highly developed in
those of the venous end of the heart, and least developed in those of
the arterial end. Such a conclusion does not deny to the nervous struc-
tures of the heart the power under certain conditions of also assuming
rhythmic activity. In one case at least — namely, the heart of Limulus —
we know that this is so. For some reason in this animal the cardiac
muscle fiber has lost its inherent rhythmic power, and is now dependent
for its activities upon rhythmic nervous discharges transmitted to it
from the neighboring nerve cords, a condition which is paralleled in
the higher animals, in the innervation of the respiratory muscles. The
respiratory center rhythmically discharges impulses to the muscles, which
are quiescent in the absence of these impulses.
In a volume of this nature, devoted primarily to the practical appli-
cation of physiology, the discussion of these problems may seem a little
out of place, but that this is not the case is seen when we consider that
the experiments upon which the various points of evidence depend
bring to light facts of the very greatest importance in the study of the
physiology of the heartbeat. One fact which stands out prominently
is that the greatest rhythmic power resides in the basal portion of the
heart — that is, in what corresponds, in the more primitive hearts, to the
sinus venosus.
Although the muscle of the entire heart possesses rhythmic power, it
does not do so to an equal degree; in the sinus the rhythmic power is
extraordinarily developed, while in the bulbus arteriosus it is scarcely
recognizable. This observation suggests the possibility that the sinus
may dominate the heartbeat — that it may be the "pacemaker" for the
heart as a whole. The most natural method for demonstrating such a
possibility would be to observe the effect on the heartbeat of some Hock
between the sinus and the rest of the heart. Such a block can be intro-
duced in the heart of cold-blooded animals by local compression around
the various junctions. If a thread is tied around the sinoauricular
junction, the sinus will go on beating uninterruptedly, but the auricles
and ventricles — that is, the greater part of the heart below the ligatures
— will cease beating, sometimes entirely (Stannius' ligature). After a
while, however, the heart below the ligature will usually begin to beat,
but at a rhythm which is slower than, and independent of, that of the
sinus.
THE PHYSIOLOGY OF THE HEARTBEAT
175
The experiment can be still better performed by using a wedge-
shaped clamp. (GaskelPs clamp.) If this is applied so that the heart
can be pinched either at the sinoauricular junction or at the auriculo-
ventricular, it will be found that, as the cardiac tissue is gradually
Fig. 42. — Heart-block produced by applying clamp at a-v junction. The clamp was tightened
at a. (From Brubaker.)
pinched, the portion of the heart below fails to beat as quickly as that
above the clamp (Fig. 42). This is known as partial heart-block, and
the degree of the block is indicated by the numerical expression 2 to 1,
3 to 1, 4 to 1, etc., meaning that the sinus is beating either twice as
quickly as the ventricle, or three times, or four times as the case may
Fig. 43. — Tracing of contraction of ventricle, showing the effect of the local application
of heat to the auricle at z, and to the apex of the ventricle at z. Note that the rate in-
creased in the former case.
be. Similar conditions of heart-block may also be produced by cutting
the cardiac tissue partly across at various places in the heart.
Further evidence that the sinus dominates the beat in the heart of
176 THE CIRCULATION OF THE BLOOD
cold-blooded animals is furnished by observing the effects of local heat-
ing or cooling of the various parts of the heart. In all rhythmically
acting structures it is well-known that heat increases the rate of the
rhythm and cold depresses it. If we locally warm the region of the
sinus, as by holding a heated wire near it the whole heart will immedi-
ately beat quicker; but if we locally heat the tip of the ventricle, no
alteration of rhythm will be observed to occur (Fig. 43).
The establishment of the fact that the sinus dominates the heartbea*
—that it is the pacemaker of the beat — raises the question as to how the
impulse originated at this place is transmitted over the rest of the
heart, and here again a neurogenic and a myogenic hypothesis have to
be considered. Before going into this question, however, it will be well
for us to consider briefly the manner of response of cardiac muscle
fiber to a stimulus, because the behavior of cardiac muscle under such
conditions is considerably different in many regards from that of skel-
etal muscle, and it is to these differences that many of the peculiar
alterations in the beat observed after interfering with the conducting
structures between the sinus and the rest of the heart, are to be ex-
plained.
The Physiologic Characteristics of Cardiac Muscle
It is necessary to bring the heart into a quiescent state in order to
investigate the properties of its musculature. This is accomplished by
the application of the Stannius ligature betAveen the sinus and the auri-
1....L-
Fig. 44. — Frog heart showing the position of the first and second ligatures of Stannius-
(Iledon): /, auricles; 2, sinus; j, ventricle. It is the first ligature which brings the heart
to standstill.
cles (Fig. 44). After tightening the ligature the auricles and ventricles
become quiescent, and by observing the effects produced by the appli-
cation of electric or other stimuli we can compare the behavior of the
cardiac muscle with that of skeletal muscle similarly stimulated. This
comparison is made because of the assistance which it offers in compre-
hending the properties of cardiac muscle. As a matter of fact, recent
investigations have shown that the differences between the two types of
muscle are not fundamental, since under certain conditions the one may
THE PHYSIOLOGY OF THE HEARTBEAT
177
be made to behave like the other. They are dependent upon the pres-
ence or absence of anastomosis between the muscle fibers.
1. When electric stimuli of varying strengths are applied to skeletal
muscle, the contraction produced by each stimulus is proportional to
the strength of the latter until this has become of such a strength that
the maximal response is elicited. In cardiac muscle, on the other hand,
an entirely different result is obtained, for the weakest stimulus, if it
produces any response at all, produces one that is maximal; that is, the
height of contraction is the same as it would have been had a much
stronger stimulus been applied. Expressing this result in general terms,
we may say that in cardiac muscle a minimal stimulus produces a maxi-
A. — Skeletal Muscle
B. — Cardiac Muscle
Fig. 45. — Effects of stimuli of increasing strength on skeletal and cardiac muscle to illustrate
the "all or nothing" principle in the latter. (From Practical Physiology.)
mal effect, whereas in skeletal, the effect, as measured by the height of
contraction, is proportional to the intensity of stimulation. This is some-
times known as the "all or nothing phenomenon" (Fig. 45).
2. If maximal stimuli are applied successively and at short intervals
of time to skeletal muscle, a slightly higher response results from each
succeeding stimulus, until about ten stimuli have been applied, after
which for some considerable time the same height of contraction follows
each stimulus. If each contraction is recorded, it will be seen that the
first few contractions give a staircase effect; that is, if a horizontal line is
drawn from the top of each contraction to the next one, the effect of a
178
THE CIRCULATION OP THE BLOOD
staircase with gradually diminishing steps will be produced. If we repeat
this observation with cardiac muscle, we shall find that the staircase
phenomenon or treppe, as it is called, is very pronounced ; and moreover,
in obedience to the all or nothing principle, the treppe is obtained in
cardiac muscle whatever may be the relative strengths of the stimuli
applied to the heart, provided always that all of them are effective;
whereas in the case of skeletal muscle it can be demonstrated only pro-
vided the stimuli are of equal strength (Fig. 46).
3. If an effective stimulus is applied to a skeletal muscle while in process
Skeletal muscle
Cardiac muscle
Fig. 46. — -The effects of successive stimuli on skeletal and cardiac muscle to show the prominence
of the staircase phenomenon, or treppe, in the latter. (From T. G. Brodie.)
of contraction, as in response to a preceding stimulus, the second stimulus
prolongs the contraction produced by the first one. If, however, the second
stimulus is applied during the latent period* of the first one, it will have no
effect — that is, the muscle during this period is refractory.! From these
results it follows that stimuli succeeding each other during the contraction
period will, in the case of skeletal muscle, cause a continuous contraction, or
tetanus, as it is called, because the contraction produced by each stimu-
lus will add itself to that of its predecessor before any trace of relax-
ation has set in. If, however, the second stimulus is applied so late in
the contraction period of the first that time is not available for the latent
*By "latent period" is meant the period after the moment of application of a stimulus during
which no effect of that stimulus is observed.
fBy "refractory period" is meant the time following the application of a stimulus during which a
second stimulus develops less than its full effect or no effect at all.
THE PHYSIOLOGY OP THE HEARTBEAT
179
period to expend itself, then obviously a slight relaxation will have oc-
curred before the effect of the second stimulus develops itself, and tet-
anus will be incomplete. These facts will be evident from the accom-
panying tracings (Fig. 47).
Skeletal muscle
Stannius' heart
Fig. 47. — The effects of successive stimuli and of tetanizing stimuli on skeletal muscle and
cardiac muscle. The small vertical marks show when the stimuli were introduced. (Compiled
from tracings published by T. G. Brodie and Leonard Hill.)
In the case of cardiac muscle the above described properties are quite
different, for the refractory phase extends throughout the whole period of
contraction; that is, a second stimulus applied during the contraction
produced by a previous stimulus has no effect whatsoever; it does not
180
THE CIRCULATION OF THE BLOOD
have one until the muscle has reached the full extent of its contraction
and is about to relax. Since a latent period must supervene upon the
application of this second stimulus, it follows that no complete fusion of
the contractions is possible. Complete tetanus therefore, does not occur
in cardiac muscle, however frequently the stimuli may be applied (Fig.
47).
The refractory phase is a property of extreme importance in under-
standing many of the peculiar irregularities observed in cardiac action.
If we observe the effect of stimuli applied at varying periods after the
Fig. 48. — Myograms of frog's ventricle, showing effect of excitation by break induction
shocks at various moments of the cardiac cycle. The line O indicates the commencement of
all the beats during which the shock is sent in. It will be noted that in /, 2 and J, the heart
is refractory to the stimulus. The signals indicate the moments at which the stimuli were ap-
plied. From 4 to 8 the heart reacts by an extrasystole, after a delay, which is progressively less
the later in diastole the stimulus enters, as shown by the sections shaded obliquely to make them
more conspicuous. The extrasystolcs increase in . height from 4 to 5, each being followed by
a compensatory pause. (From r.uciani's Human Physiology.)
termination of the refractory phase of a previous stimulus, we shall find
that the height of the extra contraction is directly proportional to the
time after the end of the refractory period at which it is applied. . If a
stimulus is applied at the very beginning of diastole, the extra contrac-
tion will be small, whereas if it is applied at the end of diastole, the
extra contraction will be at least as high as that of the preceding. It
may be higher because of the treppe.
THE PHYSIOLOGY OF THE HEARTBEAT 181
These observations enable us to interpret the results obtained by ap-
plying electric shocks (extra stimuli) to the beating heart during different
phases of systole and diastole. During systole, the muscle being refrac-
tory, no effect is produced by the extra stimulus, but during diastole
•extra systoles which are progressively more pronounced the later in
diastole they occur, follow the application of each stimulus. These re-
sults are so far exactly like those obtained with a quiescent heart. But
another phenomenon now becomes evident; namely, that following each
extra systole there is a compensatory pause in the action of the heart,
of such duration that, when the next natural beat occurs, it does so
practically at the same time as it would have occurred had no artificial
stimulus been applied. This Avill be apparent from the following dia-
gram (Pig. 48).
It should be noted that the refractory period is greatly diminished by
raising the temperature of the heart. Indeed, under these conditions
and with strong stimulation it may be possible to produce an almost
complete tetanus.
The importance of knowing the above facts is that we are thereby
enabled to explain the peculiar manner in which the ventricle responds
to stimuli transmitted to it from the sinus and the auricle. The muscu-
lature of the auricle and ventricle of the mammalian heart is not one
continuous sheet, but is separated by a space at the auriculoventricular
junction, across which, in specially organized structures, the beat of the
auricle is transmitted to the ventricle. Sometimes the stimuli are so
frequent that the ventricular muscle is unable to respond to each stimu-
lus transmitted to it, with the result that marked irregularities in con-
traction occur (see page 280). In this way certain of the cardiac irregu-
larities observed in man can be explained. Thus, the so-called pulsus
bigeminus is due to every second beat being an extra systole. This second
beat is therefore generally weaker than the preceding and succeeding nor-
mal beats, and it is almost always followed by a compensatory pause. When
the intervals separating the beats are of uniform length, although every
second beat is diminished in size, the pulse is termed pulsus alternans.
CHAPTER XXI
THE PHYSIOLOGY OF THE HEARTBEAT (Cont'd)
THE ORIGIN AND PROPAGATION OF THE BEAT IN THE
MAMMALIAN HEART
As has been shown in the preceding chapter, there is no doubt that
in the cold-blooded heart the beat originates at the sinus venosus, whence
it spreads to the rest of the heart. Very strong evidence has also been
presented to indicate that the beating power is inherent in the muscle
fiber itself and independent of nervous structure. This would suggest the
further possibility that the structures through which the beat is propa-
gated are the muscle fibers and not the nerve fibers — in other words,
that the propagation of the heartbeat, like its origination, is myogenic
rather than neurogenic. Direct proof of this hypothesis is readily fur-
nished by numerous experiments, among which may be mentioned mak-
ing interdigitating cuts across the heart, or excising a ribbon of ven-
tricular muscle by an incision simulating the walls of Troy. In both
these cases the beat will be found to travel from one end of the muscular
band to the other, although it is evident that all the nerves proceeding
from base to apex of the heart must have been severed. Of course this
evidence is not irrefutable, for it might be argued that there are nerv-
ous structures disposed in the form of a plexus continuously all over the
heart, and that some branches of the plexus remain uncut in the above
experiments. It is only in the heart of Limulus that undoubted evidence
exists that the beat is transmitted by nerves, but as we have seen, this
heart in all its properties is probably the proverbial exception which
proves the rule. The balance of evidence stands in favor of the view
that the propagation of the beat over the cold-blooded heart is myogenic
and not neurogenic.
CONDUCTING TISSUE IN MAMMALIAN HEART
When we attempt to investigate the problems of the origin and propa-
gation of the beat in the warm-blooded heart, many experimental diffi-
culties of course face us. In overcoming these, the first thing we must
do is to establish the structural relationship between cold-blooded and
warm-blooded hearts. In the embryo of both classes of animals the
132
THE PHYSIOLOGY OF THE HEARTBEAT
183
heart arises as the so-called cardiac tube. As development, proceeds,
diverticula grow out from the- walls of this tube to form the auricles and
ventricles. In the comparatively simple heart of the turtle these dispo-
sitions of the auricles and ventricles in relationship to the cardiac tube
are more or less evident even in the fully developed heart, particularly
in the case of the auricles (Fig. 49); but in the heart of the higher
mammalia it is impossible by superficial examination alone to show any
remains of the primitive cardiac tube. More careful anatomic investiga-
tions during recent years have, however, shown that it exists in the form
of certain definite structures composed of tissue histologically quite dif-
ferent from that of the rest of the heart, and disposed in such a manner
TH
Fig. 49. — Heart of tortoise as suspended. B, body of tortoise; TH, threads to levers; CL, clamp
holding aorta; A, auricle; C, coronary nerve; S, sinus; V, ventricle. (From Gaskell.)
as would indicate not only that it is derived from the primitive cardiac
tube, but also that it is the main pathway along which the beat is
transmitted.
This primitive cardiac tissue is much better developed in certain re-
gions than in others, the first portion of it to be discovered being that
known as the auriculoventricular node, or the node of Stanley Kent* (Figs.
50 and 51). This structure is found at the base of the interauricular sep-
tum on the right side and near its posterior margin. It exists as a collection
of peculiar small primitive cells and fibers, and is continued downward as
a bundle of the same peculiar tissue to the interventricular septum,
where, near the union of the posterior and median flaps of the aortic
*The discovery of this node is often erroneously attributed to His, and called after his name.
184
THE CIRCULATION OF THE BLOOD
valve, it bifurcates so as to send a branch down each side of the septum
immediately below the endocardium. Each main branch, as it proceeds
downward on the septum, divides up into an intricate system of smaller
branches, which become reflected over the inner surface of the ventricles,
where their existence has been known for some time as the so-called
Fig. SO. — Dissection of heart to show auriculoventricular bundle (Keith); 3, the beginning of
the bundle, known as the A-V node; 2, the bundle dividing into two branches; 4, the branch run-
ning on the right side of the interventricular septum. (From Howcll's Physiology.)
Fig. 51. — Photograph of model of the auriculoventricular bundle and its ramifications, con-
structed from dissections of the heart (Miss De Witt). All of the branches in the left ventricle
are not included. (From Howell.)
Purkinje fibers. The fibers ultimately end in close association with the
papillary- muscles. The node and main bundle and the two branches
before they have begun to divide are surrounded by fibrous tissue, and
they seem to have a liberal blood supply. It is of interest that they con-
tain a high percentage of glycogen. In the human heart the auriculo-
THE PHYSIOLOGY OF THE HEARTBEAT 185
ventricular node and bundle measure about 15 mm. in length and about
2 mm. in width.
The rest of the tissue between the auricles and ventricles is fibrous
in nature, although other connections like those of the auriculoventricular
bundle have been described by Kent. One of these, called, the right lat-
eral connection, runs between the right auricle and the external wall of
the right ventricle.
Another, but much smaller, mass of similar embryonic cardiac tissue
has more recently been discovered by Keith and Flack in the parts of
the auricle which correspond anatomically to the sinus venosus of the
heart of cold-blooded animals — that is, in the area lying between the
openings of the venae cavse and around the coronary sinus. To be more
explicit, this tissue lies "in the sulcus terminalis just below the fork
formed by the junction of the upper surface of the auricular appendix
with the superior vena cava." This sinoauricular node, as it is called,
is more or less club-shaped, the blunt end of the club being above, as
shown in the accompanying figure (Fig. 52). It is important to note
that there is no direct connection visible between the sinoauricular and
auriculoventricular nodes (Fig. 53).
Another anatomic fact seen also in the accompanying figure, concerns
the disposition of the muscular fibers of the auricle. These radiate in
bundles in a peculiar fan-shaped manner from a point which lies im-
mediately below the sinoauricular node to all parts of the superficies of
the right auricle. This point has been called the concentration point.
At the termination of the vense cavae, the muscle fibers are arranged more
or less circularly.
Having become familiar with the disposition in the mammalian heart
of the primitive cardiac tissue, along Avhich in the heart of the lower
animals we know that the heartbeat spreads, we may now proceed to
examine the evidence shoAving that this tissue is also responsible for the
origination and propagation of the beat in the heart of mammals. With
regard to the origin of the beat in a normally beating mammalian heart,
it is of course impossible to tell where this takes place. If the heart is
excised, however, it will continue to beat 'for a few moments, and as it
dies it will be observed that the power of contraction remains in the au-
ricular region, and particularly at the bases of the venae cavae, for a con-
siderable time after the ventricles have ceased to beat. This part — the
ultimum moriens — is situated in most hearts somewhat lower than the
sinoauricular node. That it is the last part of the heart to cease con-
tracting does not necessarily mean that it is the part of the heart in
which the beat ordinarily originates; it means simply that this is the
part of the auricle in which the power of contraction remains for the
186
THE CIRCULATION OF THE BLOOD
longest time after death. Although the observation does not enable us
to determine exactly where the heartbeat originates, yet it makes it
very probable that this is somewhere in the auricles ; a conclusion which
is borne out by many other pieces of evidence, such as those obtained by
Fig. 52. — Diagram of an auricle showing the arrangement of the muscle bands; the concen-
tration point (C.P.); and the outline of the S.A. node (S.A.N.). The diagram is to scale, and
illustrates by the circles and connecting dotted lines the method of leading off by paired contacts
and the subsequent orientation. (From Thomas Lewis.)
i Auricular appendage
,--S/'noaurtcu/ar node
-Auriculoventrlcular node
-Aurluiloventricular bundle
HR-
• Right & left ventricular
bundles
-Musculi papillares
Fig. 53. — Diagram to show the general ramifications of the conducting tissue in the heart of
the mammal. It will be observed that there is none of this tissue between the sinoauriculo- and
auriculoventricular nodes.
the study of polysphygmograms (page 273), of electrocardiograms (page
266), and of observations on the heart during heart-block (page 270).
Our problem therefore narrows itself down to determining the exact
point of the right auricle at which the beat originates.
THE PHYSIOLOGY OF THE HEARTBEAT 187
SITE OF ORIGIN OF THE BEAT
The working hypothesis from which we may proceed to attack this
problem is that the beat originates in the sinoauricular node, and to
put this to the test, various methods have been employed: (1) Warming
or cooling or injuring the node and noting the effect on the heartbeat.
Such procedures greatly affect the rate of the heartbeat, whereas they
produce no change when applied to other parts of the heart. (2) De-
termination of the comparative rhythmic power of strips cut out from
different regions of the auricular walls. It is greatest in those taken
from the region of the node. (3) Determination by the use of galvan-
ometric curves of the relation of the node to the seat of origin of cardiac
impulse. By all these methods the results indicate clearly that the beat
originates in the sinoauricular node, but on account of the great im-
portance in connection with the interpretation of electrocardiograms in
man, it is particularly with the result of the third group of experiments
that we will concern ourselves here.
Evidence Furnished by Studying the Current of Action Which
Accompanies the Heartbeat
To start with, it is essential that we make ourselves familiar with
the principles of the methods employed. These principles are briefly as
follows: When a wave of contraction passes along a muscle, it is im-
mediately preceded by a change in electrical potential, which can be
detected by means of a galvanometer connected with the muscle through
so-called nonpolarizable electrodes. The galvanometer employed must
be extremely sensitive, and must not vibrate after the current has ceased
to pass. The form generally in use today is known as the string galva-
nometer of Einthoven. It differs from the galvanometer ordinarily em-
ployed in physical laboratories in that the current instead of passing
through a coil of wire surrounding a magnetic needle, passes through a
silverized quartz thread suspended in the strong magnetic field which
exists between the two opposing poles of a horseshoe electromagnet.
The string is thus surrounded on all sides by innumerable lines of force
extending between the two poles of the magnet. When a current, how-
ever small, passes along the string, it will generate lines of force of its
own, and these by reacting with the stationary lines of force of the field
will cause the string to move. The string is placed in the pathway of a
strong beam of light, and its shadow, after being magnified by lenses,
is projected on a moving photographic plate or paper arranged in a
suitable holder. The nonpolarizable electrodes referred to are employed
in place of ordinary electrodes in order to obviate the generation of elec-
188 THE CIRCULATION OF THE BLOOD
trie currents set up by the contact of metal with the saline constituents
of the muscle juices.
If Ave connect a galvanometer by means of nonpolarizable electrodes
with, two parts of a denervated muscle (the curarized sartorius-of the
frog), it will be found that a current is set up whenever a wave of con-
traction passes over the muscle from one end to the other. The part
which first contracts becomes electrically negative to the rest of the muscle,
but as the wave of contraction passes along it, the "negativity" de-
creases at the end at which the wave started until, when the wave has
reached the middle of the strip, neither end of the muscle shows any
difference in potential, so that the string comes back to a position of
rest. However, as the contraction wave reaches the farther end of the
muscle, this lead in turn becomes negative, and the string swings in the
Fig. 54. — Diagram to illustrate the development and spread of the wave of negativity in a
strip of muscle (curarized sartorius) when stimulated at the end (P). The shaded portions show
the position of the negativity. The portion of the curve drawn by the deflections of the galvanom-
eter at each stage are shown at the right (a, b,'c, and d) . (After Lewis.)
opposite direction (Fig. 54). From this comparatively simple experiment
it can be seen that a muscular contraction wave arises at the electrode which
is negative first, and that the movement of the string of the galvanometer is
most marked — that is, the deflection is greatest — when the two electrodes
are applied at the extreme ends of the muscle. When they are brought
closer together, the initial deflection becomes much l«ss marked ; in other
words, the amplitude of the negative wave is greatest when the time
interval between the receipt of the excitation at the two contacts is
greatest.
The application of these facts to the study of the initiation of the beat
in the auricle requires that we should consider another proposition:
namely, if a pair of contacts are arranged in the center of a circular
sheet of muscle and the edge of this sheet is stimulated at different
THE PHYSIOLOGY OP THE HEARTBEAT 189
points, the amplitude of deflection of a galvanometer connected with the
pair of contacts will be most pronounced when these are radial to the
points of stimulation, for under these conditions it is evident that the
greatest possible difference will exist betAveen the intervals required for
the wave to reach each contact.
Bearing these principles in mind, we may now proceed to examine the
evidence pointing to the origin of the heartbeat at the sinoauricular node:
(1) When two electrodes are applied at different points of the au-
ricle, the amplitude of movement of the string of the galvanometer
produced by each heartbeat is greatest when the line joining the elec-
trodes converges on the sinoauricular node. To make this clear the
movement of the string must be photographed in the manner above
described, the resulting tracing being called an electrocardiogram. From
the experiments with the circular sheet of muscle alluded to it is evident
that the stimulus to produce this result must have arisen in the neigh-
borhood of the node. (2) If one electrode is placed on the sinoauricular
node and the other electrode is moved about from place to place on the
auricle, the deflection being noted at each new position, the electrode
on the node will always be found to be negative to the other electrode;*
which, however, will not be the case if both electrodes are moved about
on other parts of the auricle.
(3) As we shall see immediately, the current of action of the beating
heart may be recorded by connecting a galvanometer with various parts
of the body; for example with the right fore limb and the left hind
limb. On the electrocardiogram thus obtained are several waves, one
of which, called the P-wave, can easily be shown to correspond to the
contraction of the auricle (see Fig. 261). If now we compare such elec-
trocardiograms with those obtained while contractions of the auricle
are produced by applying artificial stimulation to various parts of it.
it will be found that the artificial simulates the normal curve only when
the stimulated part is in the neighborhood of the sinoauricular node.
In other words, it is only when the stimulus is applied to the sinoauric-
ular node that a characteristic P-wave is obtained. When the appendix
or the superior vena cava is stimulated, the P-wave is distorted although
the other waves of the electrocardiogram may be normal.
(4) By taking simultaneous electrocardiograms from direct leads
placed on the auricle and comparing the record with that of a standard
limb lead taken simultaneously, we shall find by exact measurement that
the time of onset of the excitation Avave of the auricle, as measured in
•relationship to the'P-Avave on the standard electrocardiogram, is shortest
*The connections between the electrodes and galvanometer are always arranged so that any
upward movement of the shadow of the string above the line of equal potential at the two electrodes
indicates electric negativity.
190
THE CIRCULATION OF THE BLOOD
when one electrode is over the upper end of the sinoauricular node, and
that in other regions of the auricle it always appears at a later interval.
Further details 011 this subject will be found in the papers by Eyster and
Meek8 and in Lewis, monographs.
Frequently, in taking electrocardiograms from different parts of the auricle, it is
found that certain of the curves show small waves of positivity below the line of equal
potential preceding the main wave of negativity. These initial deflections are most
marked when both the electrodes are far removed from the sinoauricular node — for ex-
ample, when they arc placed on the auricular appendix; but they are never present when
Fig. 55. — Simultaneous electrocardiograms to show the cause for extrinsic deflections. The
upper curves are from the appendix and the lower ones from lead II. The chief or intrinsic
deflection (Tn) is seen to disappear in the right-hand appendix electrocardiogram, because the
base of the appendix has been crushed. The extrinsic deflection (Ex) remains, as do the ven-
tricular deflections (F1 Vz). (From Lewis.)
one of the electrodes is placed on the sinoauricular node itself. In other words, curves
taken from leads at a distance from the sinoauricular node are more or less composite
in form, being made up partly of the main deflection due to the arrival of the excitation
and partly of the secondary deflections dependent upon extrinsic influences acting on
the electrodes; that is, the electrode picks up electric discharges from distant areas of
muscle while these are in a condition of contraction (Fig. 55). From these considera-
tions it follows that the intervals between the intrinsic and extrinsic deflections
should be longest in leads that are farthest from the node, and gradually become
less as one of the contacts approaches the node, until over this structure the ex-
trinsic deflection is no longer recorded. Such has been found to be the case.
CHAPTER XXII
THE PHYSIOLOGY OF THE HEARTBEAT (Cont'd)
THE ORIGIN AND PROPAGATION OF THE BEAT (Cont'd)—
FIBRILLATION
Mode of Propagation in the Auricles
From the mass of evidence we have little doubt that the heartbeat
originates in the sinoauricular node, and the question now presents itself
as to how the beat is propagated over the remainder of the auricles and
into the ventricles. Regarding the propagation of the beat over the
auricles, two possibilities exist: (1) it may spread uniformly over the
muscular tissue of the auricular wall until it reaches the auriculoventric-
ular node, or (2) there may be laid down between the sinoauricular and
the auriculoventricular node a special strand of highly conducting tissue.
It is no argument against this second possibility that we should so far
have been unable by histologic methods to differentiate any such struc-
tures.
There is considerable practical importance attached to the solution of
these questions, particularly with regard to the cause of certain types
of cardiac arrhythmia, such, for example, as that known as nodal rhythm.
Thus, it is evident that if the beat is transmitted uniformly over the
muscular tissue of the auricle, then the whole auricle -will have con-
tracted before the beat has reached the auriculoventricular bundle, by
which it is then transmitted to the ventricles. On the other hand, if the
beat should travel between the two nodes by special conducting tissue,
then the impulse will have arrived at the auriculoventricular node be-
fore the auricle has contracted. As a matter of fact, it is not quite settled
yet as to which of these two views is the correct one, although the balance
of evidence seems to favor the former — that is, that the wave is transmitted
uniformly over the muscular tissue of the auricle. (Lewis.)
The methods employed in attacking the problem have been essentially
the same as those described above. One of them may be called the direct,
the other the indirect. In the former, a series of pairs of contacts is
placed on the auricle, each pair being in a radial direction to the sino-
auricular node. The time at which the excitatory process arrives at that
contact of each pair which is proximal to the sinoauricular node is accu-
191
192 THE CIRCULATION OF THE BLOOD
rately determined from the galvanometric record. The exact distance be-
tween the contact and the sinoauricular node is then measured and from
the data the average transmission time is estimated. From his results
Lewis3 concludes that the transmission rates are uniform from the node
to all parts of the auricle, with the exception of the superior vena cava,
in which the rate is considerably lower. One thousand millimeters per
second represents very fairly the average rate at which the excitation
wave travels. On the other hand, Eyster and Meek8 state that the wave
is propagated throughout the sinus node, and that it spreads to the
contiguous venae cavse and to the auriculoventricular node with con-
siderable rapidity, reaching the mouth of the superior vena cava in 0.01
second, whereas its passage to the auricle itself takes 0.02 second. There is
therefore a delay in the passage of the wave to the auricle, which indi-
cates that the excitation must spread to the auriculoventricular node be-
fore involving the right atrium. These authors conclude that "this leads
to the inevitable conclusion that the cardiac impulse spreads to the ven-
tricle and to the right auricle by different paths, and does not pass to
the ventricle through the auricle, as ordinarily stated."
In the second, or indirect, method, the onset of the negative wave from
different leads in the auricle is compared against a standard. For the
standard Eyster and Meek have used the record of the mechanical sys-
tole of the auricle, but the interpretation of the result is extremely dif-
ficult on account of the rate at Avhich the changes are occurring. Lewis,
on the other hand, has used the standard electrocardiogram for purposes
of comparison.
Mode of Propagation of the Beat to the Ventricles
After reaching the auriculoventricular node, the beat is transmitted to
the ventricles along the auriculoventricular bundle — a fact which has been
most clearly demonstrated by the experiments on heart-block. We have al-
ready seen (page 174) that although each chamber of the heart of a
turtle or frog has a rhythm of its own, this is much more pronounced at
the venous end of the heart, and when the transmission of the beat to the
ventricles from the auricles is obstructed or blocked, as by compression
or partial cutting at the auriculoventricular junction, the ventricles,
after coming to a standstill for a time, subsequently contract with a
rhythm which is entirely independent of that of the auricles.
In the mammalian heart the same results may be obtained by arrang-
ing a clamp so that it compresses practically nothing but the auriculo-
ventricular bundle (Erlanger.) If the compression is extreme, the
rhythm of the ventricles is quite independent of that of the auricles, but
if it is only partial, the ventricular systoles follow regularly every sec-
THE PHYSIOLOGY OF THE HEARTBEAT 193
ond, third, or fourth auricular contraction. If after such a complete or
partial heart-block has been instituted, the clamp is removed, it will
usually be found that the heart-block disappears and the auricular and
ventricular contractions fall back into their usual sequence. The im-
portance of this discovery, apart from its physiologic interest, rests in
the fact that it is exactly duplicated in clinical experience. If the pulse
tracing of the radial artery is compared with that of the jugular vein
in certain types of heart disease, it will be found that the auricle is beat-
ing two or three times more quickly than the ventricles. In many of
these cases it has been found on autopsy that definite lesions often syphi-
litic in nature involve the auriculoventricular bundle. In other cases,
however, such lesions have not been discovered. Sometimes the bundle
is so severely diseased that the block is complete, the ventricles con-
tracting quite independently of the auricle (Stokes-Adams syndrome.)
In such cases it is assumed that the beat originates in the uninjured part
of the bundle below the seat of the block. It should be pointed out here,
however, that all cases of slow pulse in the arteries are not necessarily
dependent upon heart-block, but may depend upon a slow beat of the
auricle itself. This is called bradycardia.
Sometimes after complete destruction of the auricuioventricular bun-
dle the beat continues to be transmitted to the ventricle, and conversely
this transmission has sometimes been observed to be upset by lesions not
affecting the bundle. The explanation of both of these exceptional re-
sults almost certainly is that the right lateral connection described above
(page 184) is serving as the main pathway of transmission for the beat.
The facility of conduction through the auriculoventricular bundle is
subject to alteration by the impulses passing to it along the vagus nerve,
particularly the left vagus. It can also be altered by certain drugs,
especially digitalis and strophanthin. The clear demonstration that it is
along this bundle that the beat is transmitted is strong evidence in favor
of the myogenic hypothesis (page 171) concerning the transmission of
the heartbeat, but it does not necessarily disprove the neurogenic hypoth-
esis, for histologic investigation has shown that the bundle is closely
surrounded by an intimate plexus of nerve fibers.
Spread of the Beat in the Ventricle
After the impulse has been transmitted by the bundle into the ven-
tricles, it spreads along the many branches into which, as we have seen,
the tAvo main divisions of this bundle separate. The first part of the
ventricular musculature to contract is therefore located near the ter-
mination of these branches, at the papillary muscles. That these should
contract before the rest of the muscle of the ventricles, has an obvious
194
THE CIRCULATION OF THE BLOOD
significance in connection Avith their function of tightening the chordse
tendineas so as to prevent any bulging of the flaps of the auriculoven-
tricular valve into the auricles when, at the beginning of the presphygmic
period, the high intraventricular pressure is brought to bear on their
under surfaces. After starting at this point in the ventricle, the con-
traction wave seems to spread farther through the ventricular muscle at
a fairly uniform rate.
Investigation of this problem by means of the galvanometer has been
technically a very difficult matter, and the details of the researches by
Lewis and his pupils have not as yet been published in full. According
to the preliminary communications at hand, however,3a it appears that,
Fig. 56. — Diagram of experiment by Lewis showing the times at which the excitation wave
appeared on the front of the heart relative to the upstroke of R in lead II. K.A., right appen-
dix; D.B.L., descending branch of left coronary artery. (From Thomas Lewis.)
when nonpolarizable electrodes are placed at various parts of the outer
aspect of the ventricle, and comparison made of the moments at which
the cardiac impulse arrives, as judged by the appearance of the excita-
tion wave relative to R in a standard electrocardiogram, it has been
found that the time of arrival bears no relationship to the anatomic ar-
rangement of the muscle bundles of the ventricle. It arrives early and
simultaneously over an area of the surface near the anterior attachment
of the wall of the right ventricle. It arrives late at the base of the right
ventricle and in the part near the posterior intraventricular groove.
Histologic examination has shown that the branches of the right division
of the auriculoventricular bundle are most closely connected with the
THE PHYSIOLOGY OF THE HEARTBEAT 195
place where the wave arrives earliest. Somewhat different results are
obtained from the left ventricle, but again they are dependent upon the
relationship of the part to the Purkinje fibers (Fig. 56).
FIBRILLATION OF THE HEART
Ventricles
The even spread of the wave of contraction over the heart depends on
the uniform excitability of the muscular fibers. If certain of the muscu-
lar fibers, or bundles of fibers, have a greater or less excitability than
others, then, when the stimulus to contract arrives, it will not produce
a uniform contraction of neighboring bundles, and coordinated action of
the cardiac musculature will give place to a confused movement in which
each part of the heart is contracting independently of the rest. This
fibrillation, or delirium cordis, as it is called, can be produced by a large
variety of experimental methods, such, for example, as by stimulating
the ventricles with induced electric shocks, or by ligation of a large
branch of the coronary artery, or by the injection of lycopodium spores
into the coronary circulation, or by mechanical stimulation of the heart
in the region of the auriculoventricular bundle.
Fibrillation of the ventricles is undoubtedly a common cause of death
in man, for of course the confused movements make the ventricles in-
capable of contracting on the contents of the heart. It is a condition
which can probably never be recovered from in the higher animals, but
it is of interest that the ease with which it is set up as the result of the
application of an electric stimulus varies to a marked degree in differ-
ent animals, and that in those hearts in which fibrillation can be elic-
ited only with difficulty, recovery can usually be effected either by stop-
ping the heart by means of cold and then allowing it to beat again, or
by the administration of epinephrine. Of the hearts investigated in
this way, that of the rat has been found to be most resistant to stimula-
tion; then in order come those of the rabbit, the cat, the dog, and the
horse. There is good reason to believe that the heart of man is readily
affected. Fibrillation of the ventricle is undoubtedly the main cause of
death in most cases of electrocution. Curiously enough, however, it has
been stated that, whereas, a current of ordinary intensity (2300 volts
alternating current) produces ventricular fibrillation in the heart of cer-
tain of the lower animals, at least in that of the horse, a very much
stronger current does not do so, and may indeed cause ventricular fibril-
lation produced by a more moderate voltage to disappear. Unfortu-
nately, however, these stronger currents produce irreparable damage in
196 THE CIRCULATION OF THE BLOOD
the central nervous system, so that the method of applying stronger cur-
rents, even were it feasible to do so quickly enough, would be of no
therapeutic value in removing fibrillation.
The disappointing results that have followed the repeated attempts
to resuscitate persons killed accidentally by electric shocks is undoubt-
edly dependent upon the fact that in the heart of man it is impossible
to bring back the normal beat after the ventricles have been thrown into
fibrillation. Fibrillation of the ventricle is also the cause of the sudden
cardiac failure occurring when blood clots or emboli cause a blockage
of the coronary circulation (it is sometimes the cause of angina pec-
toris, for example). It must also be remembered in clinical practice
that mechanical stimulation of the ventricles may produce fibrillation, so
that in attempted resuscitation by cardiac massage care should be taken
not to apply this too vigorously.
Auricles
Although ventricular fibrillation is seldom recovered from, it has been
clearly shown in recent years that fibrillation of the auricles is relatively
common and that it is by no means immediately fatal. Indeed it is one
of the most common of the chronic cardiac disorders in man. Auricular
fibrillation can be produced experimentally by the application of a
strong electric stimulus to the auricles. If, however, a weaker stimulus
is applied, the auricles do not go into typical fibrillation, but come to
beat at a very rapid and regular rate, perhaps three or four hundred a
minute. This condition is called "auricular flutter," and is quite fre-
quently observed in the clinic.
The influence of auricular fibrillation and flutter on tie beat of the ven-
tricle is an extremely important one in connection with the irregular-
ities of the heart observed in man, and this influence in most cases is
explained by considering (1) the narrowness of the path (in the auric-
uloventricular bundle) along which the impulses have to travel, and (2)
the varying conditions of excitability of the ventricular muscle, depend-
ing upon the existence of the refractory phase (page 180).
In auricular flutter, when three or four hundred impulses per minute
are passing along the bundle to the ventricle, the contraction produced
by the first one will scarcely have started before the second and imme-
diately succeeding ones arrive, so that the ventricle will beat at a rate
that is much less than that of the auricle, and a condition simulating
heart-block will become established. The characteristic feature which
distinguishes this from true heart-block, however, is the fact that the
ventricular rate is above normal, whereas in true heart-block the rate
is much below normal. By means of the electrocardiogram or by
THE PHYSIOLOGY OF THE HEARTBEAT 197
polysphygmographie tracings, it can also be shown that the auricle is
beating with perfect regularity although very rapidly.
In auricular fibrillation the ventricles obviously will respond at a very
irregular rate to the impulses transmitted to them, and the auricular
contractions, if examined by the methods above described, will show no
regular sequence. Further details of the .method of eliciting these signs
will be described later (page 266).
CHAPTER XXIII
THE BLOODFLOW IN THE ARTERIES
THE PULSES
Returning to the physical laws that govern the circulation of the blood,
we may now consider the temporary changes produced in the bloodflow
in the arteries by each systolic discharge. These changes go under the
general term of the pulses, of which three may be distinguished: (1)
the pressure pulse, or the pulsatile increase of pressure produced by
each heartbeat (see page 127) ; (2) the velocity pulse, or pulsatile accel-
eration of velocity; and (3) the palpable pulse, or the pulsatile expansion
of the walls of the blood vessels produced by the sudden change of blood
pressure in their interior. The general characteristics of the three
pulses are the same, certain features being however more pronounced
in one than in another.
General Characteristics
Rate of Transmission of Pulse Wave. — The rate of transmission of
the pulse wave can be determined by taking simultaneous tracings of
the pulses from two far distant parts of the arterial system along with
accurate time-tracings. From records (cf. Fig. 98) taken from the apex or
the carotid and radial arteries we can determine how long it takes for
the beginning of the pulse wave to travel to the radial artery from the
point in the aorta from which the carotid artery springs. We shall find
that it takes about one-tenth of a second, which, considering the length
of the artery involved, would work out as a transmission velocity of
about seven meters per second or about seventeen miles an hour. The
pulse therefore travels along the blood vessels at a much greater speed
than the blood itself is moving, this being, as we shall see immediately,
about 0.5 meters per second in the larger blood vessels.
The pulse is a wave of sudden increase in pressure and velocity pass-
ing along a stream which is flowing in the same direction with a cer-
tain more permanent pressure and velocity. A simple physical experi-
ment may serve to make this clear: If the first of a row of billiard balls
be tapped with the cue, the end balls will fly off while the others are
moving slowly along in the direction of the stroke. Each ball becomes
accelerated by the ball behind it, and imparts its influence to the ball
198
THE BLOODFLOW IN THE ARTERIES 199
in front. In other words, a pulsatile acceleration of velocity is produced
by a pulsatile change in pressure between each two balls. The existence
of a pulse wave going in the same direction but quicker than a moving
column of fluid can also be illustrated by observing the waves traveling
down a stream when a stone is thrown into it.
The length of the pulse wave is such that the beginning of it has ar-
rived at the periphery of the arterial system before the end has disap-
peared from the beginning of the aorta. This is important to remem-
ber, for it is a common mistake to think of the wave as being a local
one. The determination of the length of the pulse wave depends upon
the following equation: L = VT, where L equals the length of the pulse
wave, V its velocity of transmission, and T its duration at a given point
in the artery. Under ordinary circumstances L would usually work out
from 3.25 to 4.5 meters.
The rate of transmission of the pulse wave varies according to the
rigidity of the walls of the arteries. To understand why this should be
so, it will be well for a moment to consider the physical conditions
upon which the pulse wave depends. If we connect a piece of rigid
tube with the nozzle of a large syringe, with each movement of the pis-
ton a wave of pressure will be transmitted to the fluid in the tube, along
which it will travel at such a high velocity that it will arrive at the
free end of the tube almost instantaneously, and incidentally the out-
flow of fluid from the end of the tube with each compression of the
pump will be exactly equal to that represented by the movement of the
piston. If, on the other hand, an elastic tube is employed, it will be
found that the sudden increase of pressure produced by each stroke of
the pump causes a distention of the walls, which travels along the tube
as a wave at a readily measurable velocity, which is slower the more
extensible the tube. Moreover, the outflow of fluid from the free end
of the tube will continue for some time after the cessation of the move-
ment of the pump. What happens in the tube with each discharge of
the fluid is that the portion which is immediately adjacent to the pump
undergoes distention and, being elastic, tends immediately afterward to
recoil and thus exert a recoil pressure on the fluid contained in the tube.
As a result, pressure waves are set up in the fluid in all directions. Those
that travel back come to a stop because of the piston, while those that
travel distally act on the fluid in front of them so as to accelerate it
and by temporarily raising its pressure distend the next segment of the
vessel wall, until the end of the tube is reached. From this considera-
tion it is clear that the more extensible and elastic the wall of the tube
is, the longer will it take for the wave of pressure to travel from one
end to the other.
200
THE CIRCULATION OP THE BLOOD
Alteration in the rate of transmission of the pulse wave in the arter-
ies of man depends entirely upon an application of these principles.
When the arteries become hardened in old age, the rate of transmission
of the pulse wave is markedly increased. The pulse is also transmitted
more rapidly in the vessels of the lower extremities than in those of the
upper, since in the former the blood vessels are somewhat more rigid.
Delay in the transmission of the pulse wave is further observed as one
of the signs of aneurism in a vessel; as is well known, aneurism of the
subclavian artery on one side causes a delay of the pulse on that side
that is perceptible to the fingers.
The Contour of the Pulse Curves
For more particular study of the exact contour of the pulse wave, and
especially for determining the time relationships of the secondary waves,
Fig. 57. — Diagram of Chauveau's dromograph. a, tube for introduction into the lumen of the
artery, and containing a needle or vane, which passes through the elastic membrane in its side
and moves by the impulse of the blood current; c, graduated scale for measuring the extent of
the oscillations of the needle.
a large variety of methods of varying degrees of accuracy have been
elaborated for each kind of pulse.
Those devised for measuring the pressure pulse have already been de-
scribed (see page 127), and for the other pulse® they are as follows:
Velocity Pulse. — Much ingenuity has been displayed in the elabora-
tion of methods for recording the velocity pulse. In one of these the
artery is cut across and the ends attached to a tube, into the lumen
of which projects a paddle or vane articulated with a light lever, which
passes through its wall (see Fig. 57). The vane floats in the blood
stream, and the outer end of the lever to which it is attached is con-
nected with some device to record its movements, which vary with the
velocity of bloodflow (hemodromograph). Another method consists in
the application of the instrument known as Pitot's tube used by phys-
icists. This consists of a horizontal tube having two side tubes, each of
THE BLOODFLOW IN THE ARTERIES
201
which is connected at its outer end with a manometer and prolonged
inside the horizontal tube, where they are bent at opposite right angles,
so that the inner end of one of them — the proximal tube — points up
Fig. 58.
Fig. 59.
Fig. 58. — Diagram to show principle of Pilot's tubes for measuring velocity pulse. In both
tubes the fluid will rise because of lateral pressure, but in the proximal (left-hand) tube it will
rise higher than in the distal, because it will also be affected by the velocity of flow.
Fig. 59. — Diagram to illustrate the principle of Cybulski's Photo-hematotachometer. The fluid
in C stands higher than that in D in proportion to the velocity of flow of the blood along
AB.
Fig. 60. — Dudgeon's sphygmograph. (From Jackson.)
stream, and records not only the lateral pressure but also the pressure
produced by the sudden increase in velocity of the flow, while the
202 THE CIRCULATION OF THE BLOOD
other — the distal tube — being bent down stream, records merely lateral
pressure. A photographic record of the movement of the fluid in the
two tubes gives the velocity pulse (see Fig. 58). For physiologic pur-
poses the form of apparatus used is constructed as shown in Fig. 59.
Palpable Pulse. — To secure a record of the palpable pulse, the so-
called sphygmograph is employed, although a tambour having a button
in the center which is made to press on the artery may also be em-
ployed. The commonest form of sphygmograph is that known as
Dudgeon's (Fig. 60). It consists of a small button connected with a
spring, the movements of which are transmitted and magnified by means
of a system of levers connected with a writing point arranged so as
to inscribe its movements on a moving surface.
The Analysis of the Curve
The general contour of the pulse waves taken by any of the above
methods are in general very much the same. The pressure and velocity
Fig. 61. — Pulse tracing (sphygmogram) taken by sphygmograph. a d, the period of the pulse
curve; b, the primary; c, the dicrotic wave. Time marked in fifths of a second. (From Prac-
tical Physiology.)
pulse curves are, however, not usually taken for the purpose of observ-
ing the contour of the wave but rather for measuring the difference in
pressure or velocity actually produced during each pulse. It is a record
of the palpable pulse that is usually employed for studying the contour
of the wave and the presence of secondary waves. A record of the pal-
pable pulse wave (Fig. 61) shows two separate waves on the descending
limb of the main wave. If a large number of similar pulse curves are
taken from different individuals or from the same individual under
different conditions, it will be found that of these two waves the second
one is by far the more constant ; and if the waves are timed in relation-
ship to the heart sounds, this second wave always occurs immediately
after the second sound, allowance, of course, being made for the time
required for the pulse to be transmitted from the heart to the artery
from which the pulse tracing is being taken. If the observation is
made very carefully, it can indeed be shown that the second sound cor-
responds exactly to the notch which precedes this wave. The waves that
THE BLOODFLOW IN THE ARTERIES 203
precede this notch can not be related to definite changes occurring in
the heart. Evidently, then, the secondary pulse waves are not all of
equal significance, by far the most important being that which occurs
immediately after the second sound, called the dicrotic wave (c), the
notch in front of it being called the dicrotic notch. Any secondary
waves occurring before the dicrotic are called predicrotic, or if they
occur on the ascending limb of the main pulse wave, as they sometimes
do, they are called anacrotic. Waves occurring after the dicrotic are
called postdicrotic.
The relative importance of the dicrotic, in comparison with the pre-
dicrotic and postdicrotic waves, is further evidenced by the fact that
it alone is seen on a so-called hemataugram, which is the tracing ob-
tained by allowing a fine stream of blood, escaping from a pinhole made
in the wall of an artery, to impinge upon a moving sheet of white blot-
ting paper. That such a tracing shows a dicrotic but no secondary wave,
indicates that only the former is present in the blood stream itself, and
that the other secondary waves must be produced by some condition
arising either in the elastic tissue of the walls of the blood vessels, or
in the elastic properties of the instruments used for taking the pulse
tracing.
The Dicrotic Wave. — Because of its obviously greater significance, we
shall first of all consider the exact cause of the dicrotic wave and of the
notch preceding it. Theoretically, two possible causes might explain
the wave: either it is due to some secondary wave set up at the heart,
or it i$ dependent upon waves reflected from the periphery of the cir-
culation back along the blood stream, just as secondary waves are re-
flected from the walls of a tub of water when a stone is thrown in the
center. In considering this second possibility, we are of course making
the assumption that at the ends of the arterial system there is a sudden
resistance to the onward movement of blood. The frequent branching
which occurs when the arterioles open into the capillaries no doubt of-
fers many opportunities for the reflection of pulse waves back to the
heart, but these waves must be reflected at such varying distances along
the arterial system that there can be little opportunity for them to be-
come added together so as to form a wave of sufficient magnitude to
make itself perceptible in the blood flowing in the larger arteries. These
waves are relatively so small and they occur at such different times that
the net result of their addition, so far as the production of a larger
wave is concerned, must be practically nil. Notwithstanding these con-
siderations, it is possible that under some conditions, such as in cases
of high arterial tension, certain of the predicrotic or postdicrotic waves
may be due to the above causes.
204 THE CIRCULATION OF THE BLOOD
That the dicrotic is not a reflected wave is clearly established by the
fact that if the distance between the dicrotic wave and the main pulse
wave is measured at different points of the arterial stream, it will al-
ways be found to be the same, which obviously would not be the case
were the dicrotic wave reflected. If, for example, we were to examine
the contour of the wave produced by throwing a stone into a tub of
water, we should find that near the edge the secondary wave was very
close to the main wave, whereas near the center the secondary wave
would occur much later.
Our problem therefore narrows itself down to an investigation of
the cause for the dicrotic wave at the central end of the circulation. It
occurs, as we have seen, immediately after the beginning of diastole.
That it can not be due to anything taking place in the ventricle itself is
evidenced by the fact that such a wave is absent from an intracardiac
pressure curve (see page 151), although it is present in the very begin-
ning of the aorta. Now, the only structures existing between those two
points which could be held responsible for this wave are the semilunar
valves — a conclusion which is sustained by the fact that, if the aortic
valves are rendered incompetent by hooking them back, or if the pulse
beat is examined in patients suffering from an aortic insufficiency, it
will be found that the dicrotic wave is not nearly so evident as usual.
To understand how the valves are responsible for the production of the
wave, the mechanical changes occurring at the root of the aorta must
be clearly understood (see page 155). The stretching of the elastic walls
of the aorta which occurs with each systolic outrush of blood is fol-
lowed by a powerful and sudden contraction of the stretched walls,
and the pressure thus brought to bear on the column of blood in the aorta
tends to impel it both forward and backAvard. The forward movement
adds itself to the wave of increased pressure already produced by the
ventricular contraction. The backward component travels as far as the
semilunar valve, from which it is reflected, and now proceeds peripher-
ally along the blood stream during the time at which the original pres-
sure pulse is declining. It therefore imprints itself on the pulse trac-
ing as a separate wave, and does so all the more markedly when the
decline in the main pulse wave is rapid, as in cases in which the periph-
eral resistance is low, but fails to be prominent when, on account of
a high peripheral resistance, the decline in the main pulse wave is tardy.
This explanation coincides exactly with the well-known clinical fact
that the dicrotic wave is conspicuous in pulses of low tension, but ill
marked or absent in pulses of high tension.
One point remains to be considered, and that is the cause for the
sudden decline in the main wave at the cessation of the ventricular out-
THE . BLOODFLOW IN THE ARTERIES 205
put, for, it might be said, why should there be such a sudden fall in
pressure near the heart, whereas toward the periphery, as we have seen,
the pressure between the heartbeats tends to be maintained on account
of the elastic recoil of the stretched arterial walls. The explanation
usually given is that the sudden cessation of outflow of blood from the
ventricle at the end of the sphygmic period causes a negative pressure
to be produced in the blood at the beginning of the aorta, thus tending
to cause a reflux of blood towards the heart, the effect of which is (1) to
bulge the closed valves, and (2) to produce the reflected dicrotic wave.
If, while fluid is flowing under pressure along a tube, the flow is sud-
denly arrested by turning a stopcock, it is possible by the use of manom-
eters to show that a negative wave is set up immediately beyond the
stopcock, and that this negative wave travels along the tube at a rate
depending on the elasticity of its walls.
Causes for Disappearance of the Pulse in the Veins
The disappearance of the pulse in the capillaries and its consequent
absence in the veins we have already seen to be owing to the combined
influence of the elasticity of the vessel walls and the peripheral resist-
ance. On account of these two factors the pressure conveyed to the
blood during systole is stored up to be released during diastole by the
recoil of the stretched vessels. Sometimes, however, the pulse gets
through to the veins, either because the elasticity of the vessels is not so
marked, or because the peripheral resistance has been lowered (vaso-
dilatation). In patients with hardened arteries, or in normal individu-
als after taking nitrite, which dilates the peripheral arterioles, a pulse
may come through at the periphery and appear in the veins. This may
be called the peripheral venous pulse, and it is to be carefully distin-
guished from the central venous pulse observed in the large veins, as
at the root of the neck, before any valves have intervened to block the
transmission of the auricular pressure wave back into the column of
blood in the veins. If a pulse is seen in a large vein and there is
doubt as to whether it is peripheral or central in origin, this doubt can
be immediately removed by locally constricting the vein; if the pulse
is peripheral, it will disappear on the heart side of the constriction; if
it is central, on the side away from the heart.
CHAPTER XXIV
THE RATE OF MOVEMENT OF THE BLOOD IN THE
BLOOD VESSELS
Since the object of the circulation is to maintain an adequate move-
ment of blood in the tissues and capillaries, it is evident that besides
measuring the pressure of bloodflow, we should also measure the rate
of its movement, or, as it is often called, the mean velocity. This measure-
ment may be undertaken either for a given vessel or for a complete
vascular area, such, for example, as that of one of the viscera or one
of the extremities — the mass movement of the blood. Or instead of
measuring the mean velocity we may desire to know how long it takes
for a particle of blood to traverse a given vascular area. Such a meas-
urement is called the circulation time ; it does not at all tell us how long
it takes for all the blood to pass through the given area, but only, as
stated, the time required for the circulation of a fraction of the blood
through a particular field.
VELOCITY OP PLOW IN A VESSEL
Special methods have been devised for the measurement of each of
these three velocities. For the measurement of the velocity of flow
through a main artery or vein, methods similar to those employed by
hydraulic engineers are employed; that is to say, the volume of blood,
in cubic centimeters, which passes a given point is measured for a
given time, and the result divided by the cross section of the vessel at
the point of observation. The result gives us the mean lineal velocity.
To measure the outflow of blood in a given time, the simplest method
would be to cut across the vessel and collect the blood in a graduate,
but obviously in this method an error wouldx be introduced, because
cutting the vessel would lower the peripheral resistance and remove the
natural obstruction to the flow present in the intact animal. Moreover,
the hemorrhage would in itself introduce a disturbing factor on account
of the loss of circulating fluid.
To make such measurements of any value, it is obviously necessary to
retain the peripheral resistance. For smaller vessels this can be done
by introducing in the course of the artery a long glass tube bent in the
206
RATE OF MOVEMENT OF THE BLOOD
207
shape of the letter U (Fig. 62), or by merely allowing the vessel to
bleed into a graduated tube and seeing how long the blood column takes
to travel from one end to the other. This method is of considerable
value in measuring the velocity of flow from small vessels such as the
veins coming from glands and muscles. For larger vessels a so-called
stromuhr is employed. There are numerous forms of stromuhr; that
shown in the diagram (Ludwig's) (Fig. 62) consists of two glass bulbs
united above, and connected below with tubes that open flush with the
surface of a brass disc. This is pivoted at its center with another similar
platform also having flush with the surface the openings of two tubes con-
nected with the cut ends of the artery or vein. In a certain position of
the platform, the tubes from the artery or vein are exactly opposite
those of the bulbs, so that the blood can flow from one end of the vessel
Fig. 62. — Forms of apparatus for measurement of blood velocities.
i. Volkmann's hemodromometer. The blood vessel is attached to the two short side tubes,
and according to the position of the stopcock, the blood flows either directly between them or
through the U-shaped glass tube.
z. I/udwig's stromuhr. The tubes on the lower end of each of the two glass bulbs pierce
a circular brass platform and end flush with its surface. This platform pivots at its center on
a similar lower platform with two openings connected with the tubes that lead to the blood
vessel.
through the bulbs to the other end. To use the instrument the proxi-
mal bulb is filled with oil and the peripheral one with physiologic saline.
The clip is then removed from the central end of the artery, and the blood
flows in and displaces the oil, which in turn displaces the saline in the
peripheral end of the artery. When the blood has risen to a mark on
the tube joining the two bulbs, the instrument is rapidly rotated so that
the oil is brought back again into the proximal position, the rotation
being effected so quickly that there is no distinct interruption in blood-
flow. The operation is repeated in this way for a given period of time.
Counting accurately the number of revolutions, then multiplying the
number of revolutions by the capacity of the bulbs, we get in cubic
208 THE CIRCULATION OF THE BLOOD
centimeters the amount of blood that has flowed through the instrument
in a definite unit of time. This gives us the volume flow and, if the
result is divided by the cross section of the vessel in square centimeters,
we obtain what is known as the mean lineal velocity. Many modifica-
tions have been made of this instrument, but it is unnecessary to go into
them here.
/" The general result of such measurements has been to show that the
I lineal velocity is inversely proportional to the cross section of the vessel
I at the point of observation. It is obvious that the volume of blood
flowing out of the .heart to the aorta in a given time is exactly equal
to that flowing into it by the vena cava, and likewise that the volume
floAving into an organ is exactly equal to that which flows out. Conse-
quently the lineal velocity will be inversely proportional to the sec-
tional area of the vessel. The principle is the same as that which gov-
erns the velocity of flow of a stream: when the bed is narrow, the cur-
rent is swift, but it becomes sluggish when the bed is wide. If the
arteries were of the same caliber as the veins, the mean velocity of the
bloodflow through the two would be the same, but actually it is much
greater in the arteries because the lumen of these at a given point in the
circulation is only from one-third to, one-half , that of-the corresponding
vein.
It must be understood that we are dealing above with the mean
velocity in a unit of time, and that there must be considerable alteration
with each systole and diastole, constituting the velocity pulse (page 200).
The degree of this alteration with each velocity pulse is much less at
the periphery of the circulation than near the heart. As the periphery
is reached, the flow becomes more uniform. It must further be re-
membered that, although the mean velocity depends essentially upon
the area of the vascular bed, yet it is subject to considerable variations
as a result of changes either in the force or rate of the heartbeat or
in the facility of outflow from the ends of the arterial system — that is,
changes in peripheral resistance.
It is usually stated that the mean lineal velocity in the carotid artery
is about 300 millimeters per second; and in the jugular vein, about 150
mi-llimeters ; whereas in the capillaries, where the total area of the
vascular bed has become enormously increased, being perhaps some 800
times that of the aorta, the velocity of -flow is only about half a milli-
meter per second.
MASS MOVEMENT OF THE BLOOD IN A VASCULAR AREA
Methods. — In considering the bloodflow or mass movement of the blood
in the different regions of the body, it is usually more practical to
RATE OF MOVEMENT OF THE BLOOD 209
measure, not the mean lineal velocity of the inflowing and outflowing
blood, but rather how many cubic centimeters of blood are traversing
the part per 100 grams of organ or tissue per unit of time. Such meas-
urements may be made in a variety of Avays. If there are but one artery
and one vein to the part, the stromuhr may of course be employed, and
it may be inserted in either the arterial or the venous circuit. For
measuring the mass movement of blood through such large viscera as
the liver, this is indeed the only method that can be employed, the
stromuhr being inserted either in the course of the portal vein and he-
patic arteries, or, better still, in the vena cava just below the openings
of the hepatic vein, the vena cava being shut off for a moment between
the liver and the heart and the blood, as it flows from the hepatic vein,
allowed to collect in the stromuhr. For other organs and tissues, how-
ever, methods which do not involve any interference with the blood
vessels may be employed. One of these is the so-called plethysmograpkic
method of Brodie. An organ, such as the kidney, is enclosed in a plethys-
mograph (see page 230,) and while a record of its volume is being
inscribed on a quickly revolving drum, the vein is suddenly clamped,
with the result that the kidney volume expands in proportion to the
mass of blood flowing into it. When the expansion has reached a cer-
tain degree, the clamp is removed and the bloodflow allowed to pur-
sue its course. It is then an easy matter, by graduating the plethys-
mograph, to determine how many cubic centimeters of blood must have
flowed into the organ in the given time. To avoid serious local asphyxia
in the tissue, the clamp must be applied to the vein for only the briefest
period of time. This method may also be employed for measuring the
bloodflow through the extremities. Thus, if the arm is enclosed in the
plethysmograph (Fig. 63) and a band encircling the arm above the
plethysmograph is tightened so as to constrict the veins but not the ar-
teries, the rate at which the volume of the arm within the plethysmograph
expands will correspond to the rate at which blood is flowing into it
(Hewlett).
For the purpose of measuring blood flow through the upper or lower
extremities, a much more serviceable clinical method is that of G. N.
Stewart. This depends on the principle that, provided the blood passing
from the thorax to the hands or feet is of constant temperature, the
rate at which heat is dissipated from the hands or feet will be directly
proportional to the rate of movement of the blood through these parts.
Fortunately for the method, the hands particularly, but also the feet,
are more or less perfect radiators — at least they are to this extent, that
if the temperature in their environment is not much lower than the
temperature of the blood, then while this is traversing the part, it will
210
THE CIRCULATION OF THE BLOOD
lose heat to the environment until the outflowing or venous blood is at
exactly the same temperature as the environment; for example, if the
hand is placed in water that is a little cooler than that of the blood,
and the temperature of the blood in one of the large veins of the hand
is measured, it will be found to be the same as that of the water in the
water-bath.
To measure the rate of flow, therefore, we must ascertain: (1) how
much heat has been given out by the part to the water surrounding it
in a given time, and (2) the difference in temperature of the inflowing
(arterial) and outflowing (venous) blood. We measure the amount of
Fig. 63. — Plethysmograph for recording volume changes in the hand and forearm. By observ-
ing the rate with which the volume increases when the arm is compressed, the mass movement of
the blood can be determined. (From Jackson.)
heat given out to the water in calories, a calorie being the amount of
heat required to raise the temperature of 1 c.c. of water from 0° C.
.to 1° C. Suppose, for example, a hand were placed in 3,000 c.c. of
water at 33° C., and that after ten minutes the temperature had risen
to 33.5° C., then the amount of calories given out would be 3,000 x 0.5=
1500. Since calories equal cubic centimeters multiplied by change in
temperature, it follows that if we divide the figure representing them by
the actually observed difference in temperature between inflowing and
outflowing blood, the result must equal the number of cubic centimeters
of blood that has flowed through the part. The temperature of the in-
flowing blood has been found to be practically identical with that of the
RATE OF MOVEMENT OP THE BLOOD 211
mouth under the tongue; whereas of course the temperature of the venous
blood, as already explained, is equal to the mean temperature of the
water during the time that the hand was immersed in it. Further de-
tails of the teehnic of this method will be found elsewhere, but it may be
said here that it is extremely simple and accurate, and that it requires
nothing more than (1) an accurate thermometer ranging between abo^J
40° C. and 50° C., with a scale so drawn out that readings can be made
to Moo of a degree, and (2) a well-constructed vessel of about 3,000
c.c. capacity, with double walls, the space between them being packed
with some heat-insulating material such as ground cork.
Results. — Regarding the results obtained with these methods, it has
•been found that the blood supply for each 100 grams of tissue in the
viscera, as measured by the stromuhr method, is about as follows: stomach,
21 c.c. ; intestine, 71 c.c.; spleen, 58 c.c.; liver, arterial, 25 c.c.; liver,
venous, 59 c.c.; liver, total, 84 c.c.; brain, 136 c.c.; kidney, 150 c.c.; thy-
roid gland, 560 c.c. The large blood supplies of the thyroid gland and
of the kidney are the most striking results of these observations.
By the use of the calorimeter method the bloodflow through the hands
and feet of a healthy young man has been found to be about 13 grams
per 100 c.c. of hand per minute for the right hand, and about half a
gram less for the left. The footflow is only about one-third to one-half
that of the hand per 100 c.c. of tissue — a difference which is largely
owing to the greater proportion of skin and the smaller proportion of
bone m the hand. The average footflow or handflow for a given indi-
vidual under ordinary conditions is remarkably constant from time to time,
but it is extraordinarily sensitive to changes in the temperature of the
environment in which the subject has been living for some time previous
to the measurement. In one individual, when the room temperature was
20° C., the flow in the right hand, expressed in grams of blood per 100
c.c. of hand or foot, was 10.1; when it was 22.8° C., the flow was 12.8;
when it was 25° C., 12.1 ; when it was 30° C., 18.5. On account of the
influence of temperature on the flow, it is extremely important that the
measurements should be made in a small room the temperature of which
is kept constant, or if it must be made in the wards, the bed should be sur-
rounded by curtains. The measurements made on the hands of dispensary
patients shortly after coming in from outside air are very likely to be
fallacious. The importance of making such bloodflow measurements in
the clinic will be alluded to later.
Of course the measurements made by the above method in man tell us
only the rate of flow in the periphery of the body, and furnish us with no in-
formation regarding the flow of blood through the viscera. It is, how-
ever, a well-established fact that the bloodflow in the central part of the
212 THE CIRCULATION OF THE BLOOD
circulation is more or less reciprocal with that at the periphery, an
increase in the one place being accompanied by a corresponding de-
crease in the other.
The Visceral Bloodflow In Man
The visceral bloodflow in man can be measured indirectly in the case
of the lungs, either, ( 1 ) by finding the quantity of oxygen absorbed by the
blood during an interval of time that is less than that required for the
blood to travel once round the circulation (60 seconds) and comparing
this with the oxygen content of samples of arterial and venous blood, or (2)
by causing a person to breathe a known quantity of nitrous-oxide gas and
then finding the concentration of this gas in the blood after leaving the
lungs. In the former method the difference in oxygen percentage be-
tween arterial and venous blood will be less for a given absorption of
oxygen from the alveoli the more rapid the circulation of blood through
the lungs, and in the latter method for the absorption of a given amount
of nitrous oxide, the less will be the concentration of this gas in the
blood the more rapid the circulation. Obviously these estimations must
be made only over periods of time, less than that taken for any of
the blood to complete one circuit of the circulation.
The methods are admittedly only approximate, but the results are of
much interest, mainly because of the indication they give us as to the
amount of blood pumped out by the ventricle with each heartbeat, or
during a given period of time. The results have been found to. vary
considerably; thus, one author (Krogh) places the output of blood per
minute as between 2.8 and 8.7 liters, which would correspond, at a
pulse rate of 70, to an output per heartbreat of from 40 to 120 c.c. An
immediate and very marked increase has been found to occur during
muscular work. By comparing the bloodflow through the hand with
that through the lungs, an estimate can be formed in a given individual
as to the relative magnitude of the peripheral and visceral moieties of
blood. Interesting results, which will be referred to later, have been
obtained from such measurements.
The Work of the Heart
Meanwhile it is of interest to note that we may calculate from the
ventricular output of the blood the amount of work that the heart is doing
in maintaining the circulation. Of course the calculation is again only
approximate, since we have to assume certain figures. If we assume that
in a 70-kilogram man the quantity of blood is 4,200 c.c. (see page 85),
and that it takes about one minute for all the blood to complete a cir-
culation, then the work performed by the left ventricle .in one minute
BATE OF MOVEMENT OF THE BLOOD 213
will be equal to that done in raising the above quantity of blood to a
height corresponding to the mean pressure in the aorta. If we take this
pressure as 130 millimeters of mercury, which would correspond to -a
column of blood 1,755 meters high (13.5 x 130=1755 mm. blood, or 1.755
meter), the work done by the left ventricle would be 1.755x4.2=7.37
kilogram-meters in one minute, or in twenty-four hours roughly about
10600 kilogram-meters. The work done by the right ventricle is probably
about one-third that of the. left, this being about the ratio of the pres-
sures in the two chambers. The total work of the two ventricles is there-
fore about 14000 kilogram-meters. This represents an enormous amount
of work; indeed it has been computed that it is sufficient to raise a man
of 70 kilograms to about twice the height of the highest skyscraper in
New York. The work thus expended in forcing the blood through the
capillaries becomes converted by friction in the small blood vessels into
heat, the heat equivalent of the above amount of work being roughly
about 350 calories (see page 537).
THE CIRCULATION TIME
The circulation time, or the time taken by a drop of blood to travel
between two points in the circulation, can be determined in laboratory
animals by a variety of methods, all depending on the principle of seeing
how long it takes for a drop of some substance injected into an artery to
appear in the corresponding vein. For example, to determine the time
taken for a drop of blood to pass from the jugular vein into the carotid
artery in a rabbit, a solution of methylene blue in isotonic saline is in-
jected into the former vessel and the moment of its appearance through
the walls of the artery determined by a stop-watch. If the walls are too
thick to admit of the employment of this method, a strong solution of
sodium chloride may be substituted for the methylene blue, and the mo-
ment of its appearance at another point of the circulation determined by
observing the electrical conductivity of the vessel. Since the con-
ductivity of a blood vessel depends partly on the concentration of elec-
trolytes in the blood flowing through it, the moment at which the salt
solution appears will be indicated by a change in electrical resistance
(G. N.Stewart).
By such methods, it has been found that the time for the pulmonary
circulation is very short compared with that of the systemic circulation.
In a rabbit it is usually a little less than four seconds; in an average-
sized dog of about 12 kilograms, it is about eight seconds; and in man
it is computed to be about fifteen seconds. On the other hand, the cir-
culation time in such viscera as the spleen and kidney is relatively long,
214 THE CIRCULATION OF THE BLOOD
and more susceptible than that of the lungs to different conditions of
temperature. In a dog in which the pulmonary circulation time was
about 8.5 seconds, that of the spleen was about 11 seconds, and of the
kidney about 17.5 seconds. The shortest circulation time of all is of
course that in the coronary artery, but that through the retina can not fall
far behind it. %
To determine the total circulation time, we must know: (1) the average
amount of blood passing by each part in a given time, and (2) the average
circulation time of each part. From such computations, which however
are obviously subject to considerable error, it has been reckoned that the
total circulation time in man must lie somewhere between 1 and 1.25
minutes.
MOVEMENT OF BLOOD IN VEINS
Before leaving this part of our subject, a few words may be said con-
cerning the forces concerned in the movement of blood in the veins from
the capillaries to the heart. By the time that the venules are reached,
owing to friction in the capillaries the blood will have lost most of the
force imparted to it by the heart action. Nevertheless, this remaining
vis a tergo must be considered as the basic cause for the movement of
the venous blood near the periphery. As the venules get larger, two
other factors come into play: the massaging influence of the muscles,
and the valves of the veins. By the movements of the muscles the veins
which lie between will be rhythmically compressed, and this will tend to
cause the blood to be moved forward and backward in them, the back-
ward movement being however prevented by the operation of the valve's.
When the tonicity of the muscles is subnormal, as in conditions of ill
health, the absence of this massaging action permits the blood to stag-
nate in the veins, especially in those of the lower extremities in upright
animals, with the consequence that the veins become dilated, particularly
just above the valves, thus causing the condition known as varicose veins.
As the thorax is approached, two other factors become operative: the
aspirating influence of the thorax during inspiration, and the negative
intraventricular pressure (see page 152). There is no doubt that the
former of these is of considerable importance in maintaining the venous
return near the heart, for although the change of pressure induced by in-
spiration amounts to only 5 millimeters of mercury, yet it acts so
slowly that it produces a considerable influence. The aspirating effect
of the ventricle at the beginning of diastole is, however, of no sig-
nificance in attracting blood to the heart, for although, as we have seen,
it may be considerable, yet it lasts for so short a time that it could not
RATE OF MOVEMENT OF THE BLOOD 215
overcome the inertia of the column of blood in the vena cava. Even if
the negative pressure did last for a longer period, it could not attract
more than a small amount of blood, because it would cause the thin
collapsible walls of the veins to come together and thus block the pas-
sage towards the heart.
CHAPTER XXV
THE CONTEOL OF THE CIRCULATION
The available blood in the body is parceled out to the various organs
and tissues according to their relative activities, and, since these vary
from time to time, the question arises as to the nature of the mechanism
or mechanisms involved in bringing about this adjustment. Two possible
methods of increasing the supply are: an increase in the mass movement
of all the blood in circulation, and a reciprocal adjustment of the resistance
to the flow in different vascular areas brought about by vasodilatation
in one and vasoconstriction in others. Both of these methods might
operate together.
Two agencies can be thought of as responsible for bringing about
the above changes: (1) chemical substances or hormones, present in
the blood, and (2) the nervous system.
The influence of chemical substances, or hormones, (page 729) in the
control of the circulation is undoubtedly an important one, and of those
known at the present time two groups may be mentioned: (1) sub-
stances which alter the hydrogen-ion concentration of the blood, and
(2) so-called pressor and depressor substances, produced either by duct-
less glands, such as the adrenal, or by the activity of tissues. An in-
crease in hydrogen-ion concentration of the blood not only affects the
heartbeat (see page 168), but causes a marked dilatation of the Mood
vessels, so that both the central and the peripheral changes will be such
as to encourage an increased flow of blood through the active organs
or viscus. Thus, during muscular "activity of the leg muscles there will
be a tendency to an increase in the hydrogen-ion concentration of the
blood as a whole, resulting in a greater cardiac activity and a greater
outrush of blood through the aorta, and at the same time the vessels of
the acting muscle will have become especially dilated because of the
production by the active muscles either of lactic acid or of carbonic acid.
The active muscle also produces such substances as imidazole, which
have a powerful vasodilating action. Such substances are sometimes
called depressor.
Though the hormone control of the circulation is undoubtedly of great
importance, it is probably much less so than that exercised through the
nervous system, and here again the control is centered partly in the
216
THE CONTROL OF THE CIRCULATION 217
heart and partly in the peripheral resistance. The nerve control of the
heart is effected through the vagus and sympathetic nerves, and that
exercised on the blood vessels, /through the so-called vasoconstrictor and
vasodilator nerves.
The activity of the nerve centers from which the cardiac and vaso-
motor impulses are discharged is controlled by afferent impulses com-
ing from the various regions of the body. When a gland becomes more
active, we must suppose that stimulation of the sensory fibers has caused
afferent impulses to be transmitted to the cardiac and vasomotor centers,
upon which they act in such a way as to produce increased heart ac-
tion and a local dilatation of the blood vessels of the active gland, with
perhaps a constriction of the blood vessels of the rest of the body.
THE NERVE CONTROL OF THE HEARTBEAT
The Vagus Control
With regard to the control exercised through the vagus nerve, we have
already seen that the cutting of the two nerves in the neck causes the
heart to quicken and the arterial blood pressure to rise, whereas a
stimulation of the peripheral end of the nerve causes the heart to be-
come slowed, if not stopped altogether, and the blood pressure to fall.
For the more detailed investigation of the nature of the vagus control
of the heart, it is necessary to observe the exposed heart itself — an ex-
periment which, for obvious reasons, can be most simply performed in
a cold-blooded animal, such as the frog or turtle, but which can also
be performed in mammals provided artificial respiration is maintained.
The general effect of the vagus in both groups of animals is the same,
although apparent differences may exist on account of the relative im-
portance of the different parts of the heart in the origination and propa-
gation of the heartbeat.
The Cold-Blooded Heart. — If the vagus nerve on the right side in the
turtle (the left nerve is inactive in this animal) is stimulated with a
very feeble electric current, while simultaneous records are being taken
of the contractions of the auricles and ventricles in the manner shown
in the accompanying tracing (Fig. 64), it will often be found that there
is a weakening of the auricular beats without any change in those of
the ventricle. If the strength of stimulus is somewhat increased, the
auricular beat, besides becoming weaker, will also become slower, but
meanwhile the ventricular, although also slower, may become distinctly
stronger. At first sight this result may be a little confusing, because
it would seem to indicate that the vagus nerve weakens the auricular,
218
THE CIRCULATION OP THE BLOOD
but strengthens the ventricular beat. It is clear, however, that the
strengthening of the ventricular beat is merely due to the fact that the
cavity has become better filled with blood during diastole as a result of
the slowing of the auricle. These results indicate, then, that with weak
stimulation the vagus exerts its direct influence only on the auricle. If
Fig. 64. — Simultaneous tracings from auricle and ventricle of turtle's heart. Between the crosses
the vagus was stimulated, with the effect that the auricular beat diminished in force but not in
frequency, while the ventricular beats were practically unaffected. (From Howell's Physiology.)
the stimulation is strong enough both auricles and ventricles cease to
beat altogether, and if the stimulus is maintained, the inhibition may go
on for a very long time (Fig. 65).
Usually, even though the stimulus is maintained the heart begins to
Fig. 65. — Effect of vagus stimulation on heart of turtle. Note the after effect of augmentation.
beat again "after a time, at first only occasionally but gradually more
rapidly. This is known as escapement, and it indicates that the energy
pent up in the heart during the vagus inhibition has at last overcome
the inhibiting influence of the nerve, which is meanwhile becoming
fatigued. All of these results could be quite satisfactorily explained on
the assumption that the action of the vagus is confined to the sinus,
THE CONTROL OF THE CIRCULATION
219
which, it will be remembered, dominates the beat in the rest of the
heart. There is evidence, however, that the vagus also directly affects
the rhythm of the ventricle. It may be stated as a general conclusion
from these results that the influence of the vagus upon the heartbeat is
chiefly centered upon those parts of the organ in which the rhythmic power
is most highly developed.
Besides affecting the rate and strength of the heartbeat, the vagus also
exercises a control on the conductivity of the cardiac muscle. Thus, if
a partial block is instituted in the turtle heart by applying a clamp be-
tween the auricles and ventricles, stimulation of the vagus enfeebles the
auricular beat and may also cause a complete heart-block as shown in-
the tracing reproduced in Fig. 66. It is important to point out here,
however, that under certain conditions the vagus may appear to increase
rather than decrease the conductivity of the tissue in the auriculoven-
Fig. 66. — Tracing to show that vagus stimulation may diminish transmission from auricles to
ventricles. It shows the effect of stimulating the left vagus on partial (2/1) block produced on
heart of turtle by application of clamp at auriculoventricular junction. Stimulation at •!• depressed
the conductivity and weakened the auricular contractions (lower tracing) without slowing their
rate. The result was an increase in the degree of block with cessation of ventricular contractions
(upper tracing). Initial auricular rate =: 35 per minute. (From Carrey.)
tricular junction; for example, it has been observed in the turtle heart
that when a clamp is so tight as to produce complete block — that is to
say, to render the ventricle inactive while the auricle is still beating at
the usual rate — stimulation of the vagus, besides causing the auricles to
become distinctly slowed, may at the same time cause the ventricles to
respond to the auricular beats. This result is probably due to the better
chances of slow beats getting through the junction than those which are so
frequent as to crowd one another on the narrow bridge which the con-
stricted tissue offers to their passage (Fig. 67).
Very important work was contributed in this field by G. B. Mines13
shortly before his lamentable death. He found that the local applica-
tion of atropine to the sinus eliminates the effect of stimulation of the
(intracranial) vagus on the rate of the heartbeat, while the effect on the
220
auriculoventricular junction and on the ventricle remains. After the
atropinization, vagus stimulation delays the transmission of beat from
auricle to ventricle and shortens the time of each beat in the ventricle.
It was further found that by the local application of atropine various
parts of the ventricle can be rendered irresponsive to the influence of
the vagus and the effects of this nerve on the form of the cardiogram
modified at will. These results have an important bearing in the in-
terpretation of the cause of the T-wave of the electro-cardiogram
which will be referred to later. Mines' results show that the proba-
ble explanation is that the T-wave is due to the greater duration of the
excitatory state at the base than at the apex, for by altering the relative
duration of this state at base and apex by the above methods, he could
cause the T-wave to appear or disappear.
The direct excitability of the heart muscle to external stimuli is also
depressed during vagus stimulation. This effect is, however, not evi-
Fig. 67. — Tracing to show that vagus stimulation may facilitate transmission from auricles to
ventricles. It shows the effect of right vagus stimulation on heart-block produced in the turtle by
a clamp. Upper tracing records ventricle; lower tracing, auricles. Weak faradization of the right
vagus nerve beginning at A affected the degree of block only at f1 , when a lengthened period
between auricular contractions caused a single ventricular contraction. At B stronger faradiza-
tion of the same nerve produced marked slowing of the auricles, in consequence of which the block
disappeared and the ventricles contracted after each auricular contraction. Block reappeared when
the rate again became rapid. Initial auricular rate = 36 per minute. (From Carrey.)
dent in the case of all hearts, but is seen in those of certain fishes (e. g.,
the eel).
The Mammalian Heart. — The action of the vagus on the mammalian
heart may be investigated either by exposing the heart and connecting
the auricles and ventricles with specially designed recording levers
(myocardiograph), or if we desire to study the influence on the heart as
a whole, by taking a blood-pressure tracing from one of the large arteries
by means of a spring manometer. The results are in general similar to
those observed on the frog or turtle heart, the main effects being de-
veloped on the auricle. Considerable differences are found in the effect
on the heart as a whole in different animals, particularly with regard to
the facility with which escapement occurs. In the dog when the vagus
THE CONTROL OF THE CIRCULATION 221
is continuously stimulated, the heart is likely to remain inhibited for a
long time, whereas in the cat the inhibition is very quickly broken into
by escapement. If the tracing is taken directly from the heart, it will
frequently be observed in the dog that, when the escapement occurs dur-
ing vagus stimulation it is only the ventricle that is beating, the auricles
still remaining inhibited.
If the stimulation of the vagus is discontinued after some time in an
animal whose blood pressure is being recorded, the pressure will not
only quickly recover, but will usually overshoot the normal level, mainly
because of the asphyxia which has been produced during the period of
inhibition. The asphyxia raises the hydrogen-ion concentration of the
blood and this stimulates both the vasoconstrictor center and the heart
action (page 168). The increased heart action is, also partly owing to the
fact that during vagus inhibition the beating power of the heart becomes
improved (page 225).
As an outcome of recent work,14 it has been shown that the right vagus
nerve acts mainly on the sinoauricular node, and the left vagus on the
auriculoventricular bundle. This is in agreement with the observations
described above on the cold-blooded heart (page 217). Stimulation of the
right vagus ahvays causes slowing and weakening of both the auricular
and the ventricular beats, but stimulation of the left vagus is sometimes
observed to have but little influence on the auricular beat, although it
may produce a condition of partial heart-block; or, if a clamp is ap-
plied to the auriculoventricular bundle so as to produce a partial heart -
block, then during stimulation of the left vagus, the block may become
complete. There is, however, a considerable overlapping of these in-
fluences, at least in the case of the left vagus, for this nerve also acts
considerably on the ventricle, influencing perhaps not so much the rate
as the force of the contraction. It has been found experimentally that,
in order to demonstrate the specific action of the left vagus on the bun-
dle, it is most suitable to study the relationship between auricular and
ventricular beats when the auricle is beating rapidly as during the
application of artificial (electrical) stimuli to it. Ordinarily the con-
traction produced by each stimulus passes into the ventricle, but during
stimulation of the left vagus it is found that every contraction does not
pass. These experiments raise the question as to what the influence of
either nerve may be in blocking impulses from the auricles to the ven-
tricles when auricular fibrillation is present. It might be expected that
the left vagus would prove more effectual in this regard, but actually it
has been found that both vagi have the same effect.
Tonic Vagus Action. — Impulses are constantly passing along the vagi
to the heart. On account of this so-called tonic action, the heart rate
222 THE CIRCULATION OF THE BLOOD
increases when the continuity of the vagus nerve is broken either by
cutting or by freezing a portion of nerve (Fig. 26). The effect is usually
inconspicuous when one nerve only is cut, but in most mammals it be-
comes quite marked when both are cut. Change in the heart rate pro-
duced by muscular effort is much more gradual in animals with marked
vagus tone, such as hunting dogs, than in those with little vagus tone, as in
domestic rabbits. The degree of vagus tone therefore bears a relation-
ship to the staying power of the animal for prolonged muscular effort.
It is usually ill developed in cold-blooded animals. It is quite marked
in the case of man, as is evident on observing the heartbeat before and
after giving a sufficient dose of atropine to paralyze the termination of
the vagus in the heart.
The exaet location of the nerve cells that form the center of discharg-
ing impulses along the vagus fibers to the heart can not be made out
with certainty, but they are no doubt part of the great motor nucleus
(ambiguus), from which arise the fibers not only of the vagus but of
the glossopharyngeal nerve. The tone of this vagus center is almost
without doubt dependent upon the constant transmission to it along the
sensory or afferent fibers of impulses coming from various portions of
the body. According to the strength or number of these impulses, the
tone may be increased or diminished, thus altering the rate of the heart.
It is possible of course that the tone can be maintained, independently
of the afferent impulses, by the action on the center of chemical meta-
bolic products or hormones produced in the cells or carried to them in
the blood. We know at least that, like the respiratory center, that of
the vagus is excitable by such hormones as the hydrogen-ion concen-
tration of the blood. The tonicity of the vagus center is, however, mainly
dependent upon the passage to it of afferent impulses, and as evidence
for this conclusion may be cited the observation that, after section of
most of the afferent nerves to the medulla (as by cutting the spinal cord
high up in the cervical region), subsequent section of the two vagi does
not produce anything like the usual degree of change in the heart rate.
The Afferent Vagus Impulses. — The afferent vagus impulses may come
from practically any part of the body, having been first discovered by
the simple experiment of tapping the abdomen of the frog with the han-
dle of a scalpel, when slowing of the heart rate is observed. Cutting the
vagi abolishes the reflex. Similar cardiac inhibition is produced by me-
chanical stimulation of the tail or gills of an eel. In mammals stimula-
tion of the central end of any sensory nerve usually slows the heart,
though sometimes the opposite effect occurs. The pulmonary branches
of the vagus are particularly sensitive in producing reflex inhibition,
and distinct results are usually obtained: by stimulation of the termina-
THE CONTROL OF THE CIRCULATION 223
tions of the fifth nerve in the mucosa of the upper respiratory passages,
as by inhaling ammonia vapor; by stimulation of the sensory nerve end-
ings in the pharynx, as by swallowing ; and of the mucosa of the larynx,
as when a substance is "swallowed the wrong way." The sensory
nerves of the abdominal viscera seem to be particularly active on the
vagus center, as is seen in irritation of the sensory nerves of the stom-
ach such as occurs in gastritis. Profound, inhibition may also be caused
by violent stimulation of the mesentery, as from a blow on the abdo-
men, or by irritation of the sensory nerves of the intestine, either me-
chanical or because of disease. Another interesting illustration of affer-
ent vagus stimulation is obtained by pressure on the outer canthus of
the eye. This oculomotor vagus reflex, as it is called, is very marked
in some individuals.
Through which of these afferent paths it may be that the constant
stimuli are transmitted to the vagus center to enable it to maintain its
tone, can not be said, although it is very likely to be through the vis-
ceral nerves.
In considering the cause for an observed change in heart rate, we
must of course bear in mind the possibility that the action may have
occurred, not through the vagus center, but through the sympathetic
center. Thus, when the heart becomes quicker, it may be owing either
to diminution in the vagus tone or to an increase in the discharges
along the sympathetic nerve from the augmentor center. That such
reflex action through the augmentor center does occur under experi-
mental conditions has been clearly shown; for example, if both vagus
nerves are cut and the peripheral end of one of them stimulated mod-
erately, so as to hold the heart at about its normal rate, the stimulation
of certain sensory nerves may cause increase in the heart rate. Reflex
sympathetic control of the heartbeat is however no doubt much less
important than control through the vagus center. When it does exist
it means that the actual rate of the heartbeat at any given moment
must represent the algebraic sum of two opposing influences, with that
of the vagus preponderating. The advantage of such a double inner-
vation is that it insures prompter adjustment of the beat. If, for ex-
ample, for any reason quickening of the heart rate is necessary, it is
brought about most promptly if the vagus tone is diminished at the same
moment that the sympathetic tone is increased. Such reciprocal action
of antagonistic influences is the usual rule in the animal economy. Thus,
when the knee joint flexes, it does so not merely because stimulating
impulses are transmitted to the hamstring muscles, but also because at
the same moment inhibiting impulses are transmitted to the extensor
muscles (see page 814).
224 THE CIRCULATION OF THE BLOOD
Several possibilities have to be kept in mind when we attempt to
determine the exciting cause for an observed change in the heart rate in
man. Thus, a slowing of the rate may be due to mechanical stimulation
of the vagus trunk, as in pressure on the nerves by a tumor or aneurism
in the neck. That such mechanical irritation may stimulate the vagus
is easily demonstrated in many individuals by applying pressure to the
vagus where it lies in the neck in front of the sixth cervical vertebra.
Such pressure sometimes produces so profound an inhibition of the heart
that temporary loss of consciousness occurs. It is often an unsafe ex-
periment to perform.
Change in the heart rate in man may be caused by direct stimulation
of the vagus center, as by the presence of a tumor or a blood clot in the
medulla, or by the action on the center of some unusual hormone in the
blood. A general increase in intracranial pressure also stimulates the
vagus center. The slowing of the heart which occurs in asphyxia might
be due either to the action of hormones (hydrogen-ion concentration)
in the blood as the result of the asphyxia, or to the increased intra-
cranial pressure. That the latter is the more important cause is shown
by the fact that, if the rise in blood pressure is prevented by connecting
an artery with a mercury valve, — that is, with a tube dipping into a
cylinder of mercury to a depth corresponding to the normal blood
pressure, so that when the pressure tends to rise the blood escapes, —
the sloAving of the heart is not observed. The excitability of the afferent
vagus fibers in the lungs is greatly increased during the earlier stage
of chloroform administration.
Finally it should be pointed out that, although we have no voluntary
control of the activity of the vagus center, its activities are subject
to great variation as a result of impulses transmitted from centers higher
up in the cerebrospinal axis. It is by the operation on the vagus center of
such impulses that changes in heart rate occur during emotional ex-
citement, fright, etc. The increased heart rate in muscular exercise is
probably dependent upon a number of causes, such as the irradiation of
the motor impulses on to the cardiac centers (see page 412), the rise in
temperature and changes in the hydrogen-ion concentration of the blood,
etc.
Mechanism of Action of Vagus on the Heart. — Physiologists have nat-
urally been curious as to the exact manner in which the vagus nerve
brings about inhibition of heart action. Similar inhibition as a result
of stimulation of efferent nerves exists in the case of the dilator fibers
to the blood vessels (page 234) and the sympathetic nerve to the intes-
tine (page 4.67). Inhibition of voluntary muscles can be produced only
through the central nervous system by stimulation of afferent nerves
Left Ant Caval vein .Ttiyht Ant Cavsl vein
^m^^^^f
neuron
"euron
Auricular
,- ning from n
Sinus to auricle [.Position of
Hook from
'Heart lever
vonBezold's Oanqlion
in Auricular seprum /„,, , _
I 'Bidders Ganglion
in aurkulo-ventricular junction
JtimulaHng electrodes
in sino-auricular junction [Crescent]
Sympathetic fibres = dotted lines
Fig. 68. — Diagram to show the innervation cf the heart in the frog or turtle. The electrodes
are represented as applied to the white crescentic line where they will stimulate some postganglionic
fibers. (From Jackson.)
THE CONTROL OF THE CIRCULATION 225
(page 814). It is not the nerve fibers themselves that are responsible
for the inhibitory effect, for it has been found that if the peripheral
end of a cut vagus nerve is connected with the central end of one of
the anterior roots of the cervical portion of the spinal cord, the axons
of the latter when they grow down into the vagus trunk during the
regeneration which follows, stimulation of the regenerated fibers will
still produce inhibition of the heart. The nature of the fibers can not
therefore be the factor upon which the inhibiting influence of the vagus
is dependent. This leaves the terminal apparatus of the vagus fibers in
the heart as the structures in which the stimulus conveyed to them is
rendered inhibitory in nature.
There has been considerable speculation as to what kind of change
must be occurring in the heart in order to cause the inhibition, but
practically nothing that is definite is known. One significant fact, how-
ever, is that the electrical current led off through nonpolarizable elec-
trodes from two portions of the auricle one of which is injured, does not
take the same direction when the vagus nerve is stimulated as that which
it takes when the motor nerve of a similarly observed muscle is stimu-
lated. A positive instead of a negative variation is observed. Now,
since a negative variation is always accompanied by active chemical
breakdown changes occurring in the muscle to supply its energy of
contraction, it is assumed that the positive variation accompanying stim-
ulation of the vagus must indicate that, instead of a katabolic process,
a building up, or anabolic process, is being excited. This conclusion
would fit in perfectly with the well-known fact that, after the heart has
been held in standstill for some time by vagus stimulation, the beats are
stronger after the inhibition has passed off than they were before. The
vagus seems to have a conserving influence on the heart. During the
inhibition produced by it energy material is apparently stored up in the
heart, so that when the beat is reestablished it is stronger than before.
The Manner of Termination of the Vagus Fibers in the Heart. — This
subject is of considerable pharmacologic and therefore therapeutic in-
terest. In approaching the problem it must be remembered that the
vagus fibers belong to the so-called cerebral autonomic system of nerves
(see page 882). They are therefore fibers which have cell stations situ-
ated near their peripheral termination — cell stations, that is to say, in
which ganglionic medullated fibers, by forming synapses around nerve
cells, become connected with postganglionic nonmedullated fibers. The
existence of ganglia in the heart, particularly of the frog, has been
known for a long time. These ganglia are located at the sinoauricular
junction, at the interauricular septum, and in the ventricle near the
226
THE CIRCULATION OF THE BLOOD
auriculoventricular junction. The function of the ganglia is to serve as
cell stations on the course of the vagus nerves. (Fig. 68.)
Nicotine is a drug which in certain concentrations, if applied locally
to sympathetic ganglia, specifically paralyzes the synapses between the
ends of the preganglionic fibers and the cells from which the post-
ganglionic fibers arise. If this drug is applied in a 1 per, cent solution
to the heart, stimulation of the vagus trunk no longer produces inhibi-
tion, but if the stimulus is applied to a portion of the heart known as
Fig. 69. — Frog heart tracing showing the action of nicotine. The vagus trunk was stimulated
as indicated. In the normal (lower) tracing inhibition occurs but after nicotine (second tracing)
no inhibition follows. Stimulation of the crescent in the next two lines still is followed by inhibi-
tion. The final effects of the drug are' shown in the last two (upper) tracings. (From Jackson.)
the white crescentic line, inhibition still occurs, because at this point the
postganglionic nerve fibers come near to the surface and therefore are
stimulated (Fig. 69). On the other hand, atropine is a drug which
paralyzes the postganglionic fibers, so that after its application to the
heart inhibition can not be produced by stimulating either the vagus
trunk or the white crescentic line. Pilocarpine and muscarine are drugs
which have an action exactly opposite or antagonistic to that of atro-
Medulla
oblong a ta
N.XI
Cervical l
Accessory n.
to trapeziu
Spinal
medulla —
(cord)
Kami
communican-
tes going to
Symp. gang,
(preganglionic)
Ansa
subclavia-
(Annulus of
Vieussens)r
Thoracic— 3
nerves ^
N.I
Postganglionic fibers
are dotted thus
-Jugular ganglion (Gang, of the root)
-Depressor (Fall in pressure or slowing of heart.)
(Sensory) separate nerve in rabbit and opossum.
Vodosum ganglion (Gang, of the trunk) lHHaurnr^ton]
-Inhibitory cranial autonomic fibers
-Superior cervical ganglion
-Descending sympathetic fibers in cord
^Cervical vago- sympathetic trunk
^Electrodes (slowing or stoppage of
Subclavian heart. Augmentation in some
animals.)
-Aortic arch
First thoracic qanqlion
(Stellate)
5
Electrodes
(Acceleration, or
augmentation of heart.)
Fig. 70. — Schematic representation of the innervation of the heart of the mammal. The red
continuous lines represent the sympathetic (accelerator) preganglionic fibers, and the broken red
lines, their postganglionic fibers. The cell stations are in the inferior cervical and stellate ganglia,
some extending up to the superior cervical ganglion. The green continuous lines represent the
vagus preganglionic fibers, and the broken green lines, their postganglionic fibers. The cell stations
in this case are located in the heart itself. It will be observed that electrodes applied to the so-
called vagus low down in the neck may stimulate some sympathetic fibers. (From Jackson.)
THE CONTROL OF THE CIRCULATION 227
pine; that is, they stimulate the postganglionic fibers and produce a
slowing and possibly an enfeebling of the beat.
In the mammalian heart a large number of the fibers in the right
vagus nerve proceed directly to the sinoauricular node, where it can
be shown histologically that considerable masses of nervous tissue exist.
On the other hand, the great majority of the fibers in the left vagus
proceed to the auriculoventricular bundle, in which also nervous struc-
tures are abundant (page 184). As already indicated, the experimental
results which follow stimulation of either nerve can be explained by the
influence which 'the nerve exerts on the particular structure to which
the majority of its fibers proceed. In brief, stimulation of the right
vagus is likely to produce slowing and weakening of the beat, whereas
stimulation of the left vagus is more likely to institute a condition of
partial heart-block.
On account of the different results which may be obtained by stimu-
lating the vagus, some authorities have assumed that the heart must
contain four kinds of fiber, more strictly, of vagus nerve endings, one for
each kind of influence which the vagus can develop. These four influ-
ences are, it will be remembered, on the strength, the rate and the
propagation of the heartbeat, and the excitability of the cardiac muscle.
It is, however, almost certainly unnecessary to make such an assump-
tion, for the results can be explained as merely dependent upon dif-
ferent degrees of stimulation of the same kind of fiber and upon the
exact part of the heart to which the fiber runs. Sometimes, for ex-
ample, when the right vagus nerve is stimulated very feebly, there may
be only a diminution in the force of the beats -without any change in
their rate, indicating that the effect has been upon the musculature of
the auricular walls and not on the sinoauricular node. If the stimulus
is increased a little, then both an enfeebling and a slowing of beat occur,
indicating that the stimulus has now passed both to the auricular mus-
culature directly and to tlie sinoauricular node.
The Sympathetic Control
The effect of the sympathetic nerve on the heart may be described as
being exactly opposite to that of the vagus. The pathway along which
the fibers of this nerve travel to the heart is more or less a devious one.
They arise in the mammal from nerve cells in the gray matter in the
upper thoracic portion of the spinal cord. The fibers leave by the cor-
responding spinal roots and pass by the white rami communicantes into
the sympathetic chain, up wrhich they travel to the stellate and inferior
cervical ganglia. Around the nerve cells of the stellate ganglion the
fibers end by synapsis, and the axons of the cells are then continued on
228
THE CIRCULATION OF THE BLOOD
as postganglionic fibers, proceeding to the heart through branches com-
ing off from the stellate ganglion itself, or from the ansa subclavii or.
inferior cervical ganglion. (Fig. 70). In cold-blooded animals, such as
the frog, the sympathetic fibers run up to the upper end of the cervical
sympathetic and join the vagus immediately after it leaves the cranial
cavity. They then proceed along with this nerve — forming the vago-
sympathetic — to the heart. The effect of stimulation is shown in Fig. 71.
The sympathetic nerve differs from the vagus in that a much longer la-
tent period elapses before its influence becomes effective, and this persists
for a much longer period after the stimulus is withdrawn. If the vagus
B.
Fig. 71. — Tracings showing the effects on the heartbeat of the frog resulting from stimulation of
the sympathetic nerves prior to their union with the vagus nerve. (From Brodie.)
and sympathetic are stimulated at the same time, as by exciting the vago-
sympathetic in the frog, the first effect observed is that of the vagus
usually followed, after removal of the stimulus, by the sympathetic ef-
fect. If the stimulus is maintained for a long time, so that the vagus
becomes fatigued, escapement will occur earlier than with pure vagus
stimulation, and augmentation may become apparent. The sympathetic
influence is, however, never so strong as that of the vagus. The two
nerves are therefore not antagonistic in the sense that the one neutralizes
the effect of the other; but when both are stimulated, the heart responds
first to the vagus and later to the sympathetic.
CHAPTER XXVI
THE CONTROL OF THE CIRCULATION (Cont'd)
THE NERVE CONTROL OF THE PERIPHERAL RESISTANCE
As already explained, the nerve control of the peripheral resistance
takes place through the action of vasoconstrictor and vasodilator nerve
fibers on the musculature of the arteriole walls. The vasoconstrictor
impulses like those in the vagus of the heart are tonic, so that when a
nerve containing such fibers is cut, the corresponding blood vessels un-
dergo dilatation (see page 135), and when their peripheral ends are stim-
ulated artificially, constriction occurs. On the other hand, the vasodi-
lator impulses do. not appear, at least under ordinary circumstances, to
be tonic, so that the cutting of such fibers does not cause vasoconstriction ;
their stimulation, however, causes marked dilatation. Vasomotor fibers
are contained in most of the efferent (motor) nerve trunks, and to
detect their presence the nerve must be either cut or stimulated and the
condition of the blood vessels of the innervated area observed.
Methods for the Detection of Constriction or Dilatation
Several methods, varying with the exact area under observation, can
be used for the detection of vasoconstriction or dilatation. In many cases
visual inspection is sufficient, as in the well-known experiment of Claude
Bernard on the blood vessels.in the ear of the rabbit (see Fig. 106). When
this is held with a light behind it, and the cervical sympathetic of the
corresponding side is cut, marked dilatation will become evident and
vessels will spring into view where previously there were none visible.
Visual inspection is usually also a satisfactory method of demonstrat-
ing vasodilatation or constriction in exposed glands, in mucous pas-
sages and in the vessels of the skin.
Another comparatively simple method is the observation of the tem-
perature of the part, this being particularly useful when the vascular
area is one situated in the peripheral part of the body, such as the hand
or foot (see page 209). When dilatation occurs the temperature of the
part rises, because the warmer blood from the viscera flows with greater
freedom through the peripheral regions, where it is cooled off by radia-
tion. When a thermometer is placed between the toes of a dog or cat, a
229
230
THE CIRCULATION OF THE BLOOD
distinct rise in temperature will be observed when the sciatic nerve of the
corresponding limb is cut. The application of this principle in deter-
mining the mass movement of blood by the amount of heat given off from
the hands or feet has already been explained.
Other methods depend up.on observation of the outflow of blood from
the veins of the part. A simple application of this method can be used in
the case of the ear of the rabbit. If the tip of the ear is cut off, bleeding
under ordinary circumstances is only very slight, but if the cervical
sympathetic is cut, it becomes quite marked, slowing down again or
even stopping entirely when the peripheral end of the nerve is stimu-
lated. By making measurements of the volume of the outflow of blood
from a vein by this method, the extent of constriction or dilatation can
tube to recorder
oil enclosed
by membrane
Fig. 72. — Roy's kidney oncometer. (From Jackson.)
be followed quantitatively. Vasodilatation also causes changes in the
character of the venous flow, the usually continuous flow becoming pul-
satile and the color of the blood brightening. Comparison of the pressures
in the arteries and the veins of a part is also often of value in the detec-
tion of changes in the caliber of the blood vessels, for, of course, the
greater the . difference in pressure between the tAvo manometers, the
greater must be the resistance offered to the flow.
For experimental purposes, however, the standard method is that
known as the plethysmofjraphic. For this purpose the organ or tissue is
enclosed in a so-called plethysmograph or volume recorder, the prin-
ciple of which will be clearly seen by consultation of the accompanying
diagram of one adapted for the kidney (Fig. 72). Any increase de-
tected by this means in the volume of the part must be due either to
THE CONTROL OF THE CIRCULATION 231
an increase in blood flowing into the vessels because of increased heart
action or to a local vasodilatation; and vice versa, when shrinkage oc-
curs. We can not tell from the volume tracing itself which of these
changes is really responsible for the observed alteration, but we can do
so by simultaneously observing the mean arterial blood pressure. If this
falls when the volume decreases, it means that the volume of blood flow-
ing to the part must have become diminished. If, on the other hand, the
blood pressure remains constant or rises while the volume decreases, it
means that the blood vessels have locally constricted.
Methods for the Detection of Vasomotor Fibers in Nerve Trunks
If we wish to find out through which nerve trunks a given vascular
area is supplied with vasoconstrictor or vasodilator impulses, we should
proceed by the use of one of the above described methods to observe the
effect produced on the vessels by cutting the nerve and then by stimu-
lating the peripheral end of the cut nerve. As a result of such observa-
tions it has been found that the vasomotor fibers are frequently dis-
tributed so that those having a vasoconstricting action are collected
mainly in one nerve trunk and those having a dilating action in another;
in some nerve trunks, however, the relative numbers of the opposing
fibers are about equal. Nerves containing a great preponderance of vaso-
constrictor fibers are the great splanchnic and the cervical sympathetic ;
and those containing a great preponderance of vasodilator are the chorda
tympani nerve to the submaxillary gland and the nervi erigentes to the
external genitalia.
It must be clearly understood that, although one kind of vasomotor
fiber may preponderate in one of these nerves, yet the opposite kind is
also present. In the cervical sympathetic, for example, some vasodila-
tor fibers extending to the blood vessels of the mucous membrane of the
nose and cheeks can readily be demonstrated, as shown by the flushing
of these parts when the peripheral end of the nerve is stimulated; and
similarly, even in the great splanchnic nerve itself, vasodilator fibers
supplying the suprarenal capsule can quite readily be made out. When'
the vasoconstrictor fibers greatly preponderate over the vasodilator, the
effect of the latter may be demonstrated by taking advantage of the fact
that ergotoxine paralyzes the vasoconstrictor but not the vasodilator
fibers, so that after its administration stimulation of the great splanch-
nic nerve gives rise to a vasodilatation instead of a vasoconstriction.
The presence of vasoconstrictor fibers in the so-called vasodilator nerves
(chorda tympani and nervi erigentes) has not however, been demon-
strated.
A good example of a nerve trunk containing about an equal admix-
232 THE CIRCULATION OF THE BLOOD
ture of both kinds of vasomotor fibers is the sciatic. If the hind limb of
a dog is placed in a plethysmograph and simultaneously a record of the
mean arterial blood pressure taken, it will be found on cutting the sciatic
nerve that the volume of the limb increases, whereas the blood pressure
remains practically constant. Before placing the limb in the plethysmo-
graph, the muscles must of course be paralyzed by means of curare;
otherwise muscular contractions would confuse the result. If the
peripheral end of the cut nerve is now stimulated, vasoconstriction will
readily be observed. So far, then, the results demonstrate the presence
of vasoconstrictor nerve fibers alone.
To demonstrate the presence of vasodilators a different procedure is
necessary. This is based on the following facts: (1) The vasodilator
nerve fibers degenerate more slowly than the vasoconstrictor; (2) they
are less depressed in their excitability by cooling the nerve; and (3) they
are more sensitive to weak slow faradic stimulation than the vasocon-
strictor fibers. Accordingly, if we cut the sciatic nerve two or three
days before the actual experiment, and then, while observing the volume
of the limb, proceed to stimulate the half-degenerated nerve with feeble
electric stimuli of slow frequency we shall usually observe a dilatation
of the limb instead of constriction; and even if we cool a stretch of a
freshly cut nerve before applying the stimulus, the same result will
often be obtained.
The Origin of Vasomotor Nerve Fibers
Having seen how the presence of vasomotor fibers may be detected in
peripheral nerves, we must now proceed to trace them back to their
origin from the central nervous system. The method for doing this con-
sists, in general, in observing the effect on the blood vessels produced by
cutting or stimulating the various nerve roots through which the fibers
might pass on their way to the nerve trunks. As a result of such obser-
vations it has been found that all of the vasoconstrictor fibers emanate
from the spinal cord in the region between the level of the second thoracic
and that of the second or third lumbar spinal roots, but from nowhere
else in the cerebrospinal axis. Section of the spinal cord below the level
of the second lumbar spinal roots produces no change in the volume of
the hind limb, provided the muscles be thoroughly curarized, nor does
stimulation of the lower end of the cut spinal cord have any effect. If
the last two thoracic or the first two lumbar spinal roots are stimulated,
however, evidence of vasoconstriction will be obtained.
The restriction of the origin of vasoconstrictor fibers to the above-
mentioned regions of the spinal cord indicates that in proceeding to
the mixed nerve trunks they must travel along special nerve paths.
THE CONTROL OF THE CIRCULATION 233
These are provided by the sympathetic chain and its branches (Fig. 228).
The vasoconstrictor fibers in the anterior spinal roots leave the latter
by way of the corresponding white rami comnmnicantes, and pass into
the neighboring sympathetic chain, along which they either ascend or
descend, according to their ultimate destination. In their course they
come into contact with the sympathetic ganglia, through one or two of
which they may pass without any change, but ultimately each fiber ar-
rives at some ganglion, in which it terminates by forming a synapsis
around one of the ganglionic nerve cells. The axon of this nerve cell
then continues the course by the nearest gray ramus communicans back
to the spinal nerve beyond the union of its anterior and posterior roots.
Up to the point where the fiber forms a synapsis with a ganglionic nerve
cell, it is medullated and is known as the preganglionic fiber. Beyond
the nerve cell, it is nonmedullated and is known as postganglionic
(page 877).
The exact ganglion in which a given vasoconstrictor fiber becomes connected with a
nerve cell can be determined by the nicotine method of Langley. Local application to
the ganglion of a weak solution of this drug (1 per cent) paralyzes the synaptic con-
nection, so that a stimulus applied to the preganglionic fiber no longer produces its
effect. Suppose, for example, that a vasoconstrictor fiber has been found by the stimula-
tion method to travel through several ganglia, and we wish to determine in which of
these the synapsis occurs: we can do so by applying the stimulus at a point central to
the ganglia after painting each of them in turn with the nicotine solution. If the
application of the drug to a given ganglion is found to cause no alteration in the
effect produced by stimulation, then we know that there can not be any synaptic
connection in that ganglion, and we proceed in the same way till we have located ,
the ganglion in which synapsis occurs. It is important to remember that the post-
ganglionic vasoconstrictor fibers in a gray ramus communicans do not come from the
preganglionic fibers of the corresponding spinal rcot, but from fibers coming through
white rami at a higher or a lower level.
The above description applies to the vasoconstrictor fibers proceeding to the vessels of
the anterior and posterior" extremities, those for the former arising (in the dog) from
about the fourth thoracic to the tenth ; and those for the latter, from the lowest thoracic
and the first three lumbar nerve roots. The cell station for the fibers to the fore limbs
is in the stellate ganglion, and for the hind limbs in the last two lumbar and first two
sacral ganglia of the abdominal sympathetic chain.
The vasoconstrictor fibers to the vessels of the head and neck run a somewhat dif-
ferent course, there being no convenient cerebro spinal nerve along which the post-
ganglionic fibers may run. The fibers to the blood vessels of the head leave the cord
by the second to the fourth or fifth thoracic roots and pass by the corresponding white
rami communicantes into the sympathetic chain, up which they run, passing through the
stellate ganglion, the ansa subclavii, and the inferior cervical ganglion, then ascending
in the cervical sympathetic to the superior cervical ganglion, where their cell station
exists. The postganglionic fibers on leaving this ganglion travel to their destination
mainly along the outer walls of the blood vessels.
The vasoconstrictors to the abdominal viscera are carried by the splanchnic nerves,
the fibers of which come off from the lower seven thoracic and the uppermost lumbar
234 THE CIRCULATION OP THE BLOOD
roots. The thoracic fibers pass down the sympathetic chain, which they leave by the
great splanchnic nerves. The lumbar fibers form the lesser or abdominal splanchnic
nerves. As preganglionic fibers, therefore, these fibers are carried by the greater and
lesser splanchnic nerves into the abdomen, where the former comes into close relation-
ship with the suprarenal glands, giving off a branch fco the suprarenal ganglion. The
main course of the nerve is continued on to the solar plexus, in the various .ganglia of
which most of the preganglionic fibers end by synapsis, the postganglionic fibers then
proceeding along the blood vessels to the vessels of the abdominal viscera. (See also
page 879).
Vasodilator fibers have a more varied origin than vasoconstrictor, and
they run an entirely different course. Vasodilator impulses may be
transmitted by fibers arising from practically any level of the cerebro-
spinal axis, not only by the motor roots, but by the sensory as well.
Thus, they pass out of the spinal cord in the posterior sacral roots to
enter the nerves of the hind limbs, as has been demonstrated by observ-
ing an increase in the volume of the curarized limb during electrical
stimulation of the exposed rootlets. The apparent inconsistency of these
observations with the well-known law concerning the direction of the
impulses contained in the posterior spinal roots is explained by assum-
ing that the dilator impulses are transmitted along the ordinary sensory
fibers, since th'ere are no efferent fibers in these roots. They are impul-
ses which go against the ordinary stream (antidromic). In support of
this explanation it is of importance to note that at their termination
near the skin many sensory fibers split into several branches, some of
which run to blood vessels, and others to receptor organs' (page 797).
Stimulation of the latter branches may cause dilatation of the local blood
vessels nearby, indicating that impulses must be transmitted up to the
point at which the branching occurs and then down the vascular branch,
this result being obtained even after the main trunk of the nerve has
been cut above the division.
For the blood vessels of the anterior extremity, the vasodilator impulses are similarly
transmitted through the posterior spinal roots of the lower cervical region of the spinal
cord. The vasodilator fibers to the abdominal viscera are transmitted with the splanchnic
nerves, but they may also be derived from the posterior spinal roots, for it has been
found that stimulation of posterior rcots in the splanchnic area causes dilatation in the
intestine (Bayliss). Vasodilator fibers are also contained in the cranial nerves, par-
ticularly the seventh and the ninth, being distributed in .the former nerve to the an-
terior portion of the tongue and the salivary glands, and in the latter to the posterior
portion of the tongue and the mucous membrane of the floor of the mouth. The vaso-
dilator fibers for the mucous membrane of the inside of the cheeks and nares have their
course in the cervical sympathetic, being distributed to the buccofacial region in the
branches of the fifth cranial nerve.
There is evidence to show that the vasodilator fibers, like the vasoconstrictor, become
connected by synapsis with nerve cells somewhere in their course. In the case of the
vasodilator fibers in the chorda tympani and nervi erigentes, such cell stations have
been clearly demonstrated in the hilus of the submaxillary gland in the former nerve
THE CONTROL OF THE CIRCULATION 235
and in the hypogastric plexus situated on the neck of the bladder in the latter.
It is therefore commonly assumed that, although not recognizable by histologic methods,
such terminal cell stations must also exist in close association with all blood vessels
to which the vasodilator fibers run. Whether or not such peripheral cell stations exist,
there is a marked difference between the course of vasodilator and of vasoconstrictor
fibers.
The Vasomotor Nerve Centers
Our next problem is to trace these fibers farther into the central
nervous system, and find the location and study the characteristics of
the nerve centers from which they are derived. We must postulate the
existence of both vasoconstrictor and vasodilator centers, but since there
is no adequate evidence at the present time which enables us to locate
the latter, we must confine our attention to the vasoconstrictor centers.
These exist at two levels in the cerebrospinal axis: (1) in the gray mat-
ter of the spinal cord, and (2) in the gray matter of the medulla
oblongata.
The spinal, or as they are often called, the subsidiary vasoconstrictor
centers, are represented by certain cells of the lateral horn of gray mat-
ter in the thoracic portion of the spinal cord, from which the pregan-
glionic vasoconstrictor fibers above described are derived. The exact
location of the nerve cells composing the chief centers in the medulla has
not as yet been definitely made out; they undoubtedly lie near those of
the vagus center (see Hanson). The axons of the medullary cells de-
scend in the lateral columns of the spinal cord to end by synapses
around the cells of the subsidiary vasoconstrictor center in the lateral
horns.
The experimental evidence which indicates the existence of chief and
subsidiary centers is quite definite. Thus, if the spinal cord is cut at the
lower cervical region (below the phrenic nuclei, so as not to interfere
with the movements of the diaphragm), the arterial blood pressure falls
profoundly, because the pathway connecting the two centers is broken.
After several days, however, the blood pressure will gradually rise again.
If after this has occurred, the spinal cord is destroyed by pushing a wire
down the vertebral canal, the arterial blood pressure will again fall,
indicating that the vascular tone which had been reacquired after sec-
tion of the pathway between the main and the subsidiary centers must
have been brought about by the development in the subsidiary centers
of an independent power of reflex tonic action. This experiment there-
fore demonstrates that in the intact animal the subsidiary centers do not
by themselves discharge tonic impulses. In other words, the subsidiary
centers ordinarily serve • merely as transfer stations for the tonic im-
pulses coming from the chief center, but when these impulses no longer
236 THE CIRCULATION OP THE BLOOD
arrive, then a hitherto dormant power of tonic activity becomes devel-
oped in the subsidiary centers.
Independent Tonicity of Blood Vessels
Even after complete disconnection of the spinal cord from the blood
vessels, as by cutting of the splanchnic nerve to the abdomen or abla-
tion of that portion of the lower spinal cord from which the fibers to
the hind limb arise, the disconnected blood vessels, although at first
completely dilated, may later reacquire an independent tone of their
own, indicating therefore, that they must possess some neuromuscular
mechanism which can act independently of the nerve centers, and which
may be stimulated to activity by the presence of hormones in the blood.
The hormone was at one time thought to be cpinephrine (see page 745).
Epinephrine control is indicated in the effect produced upon arterial
blood pressure by stimulation of the great splanchnic nerve. Careful
analysis of the curve, shown in Fig. 29, shows that the rise is both im-
mediate and delayed; that is, the curve mounts immediately, then flat-
tens out a little, and then assumes a further rise. This delayed response
seems to depend upon the excretion of epinephrine into the blood, for it
does not occur when the suprarenal veins are occluded, and is much de-
layed by temporarily clamping the suprarenal veins on the same side
as that on which the splanchnic nerve is stimulated. It has been stated
by certain observers that, after occlusion of the adrenal veins, there is
a downward tendency of the blood pressure, which however develops
with extreme slowness; and that a distinct elevation of blood pressure
follows the removal of a clamp temporarily placed on the adrenal veins.
This rise is {^renounced if the splanchnic nerve is stimulated during the
occlusion of the veins. It must of course be understood that the imme-
diate rise in blood pressure following splanchnic stimulation is caused by
vasoconstriction in the splanchnic area itself, as is evidenced by the
fact that it does not occur, or is only very faint, when the abdominal
blood vessels are ligated prior to the stimulation of the splanchnic nerve.
Even after ligation of the adrenal veins and of the blood vessels of the
splanchnic area, stimulation of the splanchnic nerve may still cause a
slight rise in arterial blood pressure, possibly because some fibers may
run from the splanchnic to vascular areas not situated within the realm
of the splanchnic nerve — for example, the blood vessels of the lumbar
muscles.
CHAPTER XXVII
The activities of the vasomotor center are controlled partly by hor-
mones and partly by afferent impulses.
The Hormone Control
As with the respiratory center, the chief hormone, is the hydrogen-ion
concentration of the blood. When this is increased, as in asphyxia, the
vasoconstrictor part of the vasomotor center becomes stimulated, so
that the blood vessels are constricted and the blood pressure rises. Tak-
ing, as our criterion of hydrogen-ion concentration, the tension of the
carbon dioxide in the blood (see page 354), we may proceed to investi-
gate the relationship by observing the blood pressure during changes
in the carbon-dioxide tension brought about by causing the animal to
breathe atmospheres containing known percentages of the gas (Mathi-
son15). Thus, if a decerebrate cat is made to respire an atmosphere
containing 5 per cent or more of carbon dioxide, an immediate rise
occurs in the arterial blood pressure. That the inhaled carbon dioxide
acts by raising the hydrogen-ion concentration of the blood is indicated
by the fact that a similar rise in blood pressure can be obtained by intra-
venous injection of a weak solution of lactic acid (2 c.c. N/15) in a de-
cerebrate cat.
Instead of injecting the lactic acid, we may cause it to be produced
in the muscles of the animal itself by greatly diminishing their oxygen
supply. When a decerebrate cat, for example, is made to breathe an
atmosphere of almost pure nitrogen, there is, after a latent period of
about 30 seconds, a sudden rise in. arterial pressure. The existence of
this latent period in the latter case, as compared with its absence when
carbon dioxide is inspired, is owing to the time taken for lactic acid to
be produced in the muscles on account of the oxygen deprivation. It
is important to note in the above experiment that decerebrate animals
are employed so as to avoid the necessity of using anesthesia, under
which the results are much less definite. The fact that oxygen depriva-
tion causes excitation of the vasoconstrictor center has been known for
237
238 THE CIRCULATION OP THE BLOOD
some time, but the explanation that has usually been given has been that
it is due to a direct effect of oxygen want on the center.
The sensitivity of the medullary center towards the hydrogen-ion is
many times greater than that of the subsidiary centers in the spinal
cord. If an animal is kept alive by artificial respiration for some time
after cutting the cervical spinal cord, the subsidiary vasomotor centers
will, as we have seen, gradually acquire a tonic action, and the lowered
blood pressure will gradually rise again. If, when this has been attained,
the animal is made to breathe an atmosphere rich in carbon dioxide, a sud-
den rise in blood pressure will occur, but to produce it a very * much
greater percentage of this gas must be inspired than when the pathway
••; •* * * *;\. »• \: -•* 5 *Ill
between the chief and subsidiary centers is intact.' -Whereas 5 per'ceiit
carbon dioxide is sufficient to cause a rise of pressure in an animal hav-
ing its chief vasomotor center, it takes 25 per cent and upward to pro-
duce a like effect on a spinal animal; and similarly, although 2 c.c. of
N/15 lactic acid will stimulate the chief vasomotor center, it takes 5 c.c.
of N/2 to excite the spinal-cord centers.
The Nerve Control
However important hormones may be in maintaining a tonic stimula-
tion of the center, the more sudden changes in activity are mainly
brought about by afferent nerve impulses. The afferent impulses are
of two classes: (1) those causing a rise in blood pressure, called
pressor, and (2) those causing a fall in blood pressure, called depressor.
The effect produced on the arterial blood pressure by stimulation of
either pressor or depressor fibers is usually more or less evanescent,
especially in the case of the depressor fibers; and when the change fol-
lowing stimulation of the nerve passes off, the blood pressure always
returns to its former level. This indicates that the afferent impulses do
not affect the tonic control which the vasomotor center exercises on the
blood vessels. It has, therefore, been assumed by Porter16 that there are
really tAvo kinds of vasomotor centers: one concerned merely in the
bringing about of temporary reflex changes, the other concerned in the
maintenance of the vascular tone. It may be that the activities of the
former are primarily dependent upon afferent impulses, and the latter,
upon hormones. Justification for this view has been found in observa-
tions made on the effects of stimulation of pressor and depressor fibers
in animals under the influence of curare or alcohol. With the former
drug, stimulation of a nerve containing a preponderance of pressor or
depressor fibers produces double its usual effect, but the mean level of
the blood pressure apart from this effect remains unchanged. With the
latter drug (alcohol), on the other hand, the reflex response entirely
THE CONTROL OP THE CIRCULATION 239
disappears, although it immediately reappears when the alcohol effect
has passed off, and there is no evidence of a change in tone. The tonic
and the reflex mechanisms of the vasomotor center can not therefore be
identical.
At the present stage of our knowledge, it is only possible for us to
study the effect of stimulation of pressor and depressor fibers on the
vasoreflex center. Such fibers are contained in practically every sen-
sory nerve of the body, and it would appear that a fairly equal mixture
of both kinds of fiber exists in most of these nerves.
Pressor and Depressor Impulses. — Depressor impulses are alone present
in the cardiac depressor nerve. Sometimes as in the rabbit, this exists
as an independent nerve trunk, originating by two branches, one from
the superior laryngeal, the other from the vagus, and descending close to
Fig. 73. — Fall of blood pressure from excitation of the depressor nerve. The drum was
stopped in the middle of the curve and the excitation maintained for seventeen minutes. The line
of zero pressure should be 30 mm. lower than here shown. (From Bayliss.)
the vagus trunk, to end around the arch of the aorta. In other animals
the depressor is bound up with the vagus trunk from which it can some-
times be separated by careful dissection. The first prerequisite in inves-
tigating the cause of the changes produced by stimulation of these nerves
is the elimination of any chance of an alteration in heartbeat as a result
of simultaneous stimulation of afferent vagus fibers. This may be done
either by cutting both vagi or by administering atropine.
Stimulation of the central end of the cardiac depressor nerve in such
an animal causes an immediate fall in blood pressure, accompanied by an
increase in volume which can be demonstrated either in the hind limb or in
one of the abdominal viscera — evidence of general vasodilatation (Fig. 73).
When the central end of a sensory nerve, such as the sciatic, is acted
on by a stimulus of moderate strength, it will usually be found that the
arterial blood pressure rises and that the volume of the limb or of some
240 THE CIRCULATION OF THE BLOOD -
abdominal viscus becomes diminished — evidence of general vasoconstric-
tion. But when the sensory nerve is stimulated with extremely weak
faradic shocks, an entirely different result is likely to be obtained;
namely, a fall of blood pressure and an increase in volume of the limb
or viscus, indicating that in this manner we have stimulated depressor
fibers. By careful experimentation with quantitatively graduated elec-
trical stimuli, it has been found by Martin and others17 that on stimu-
lating an afferent nerve with weak shocks, a fall in blood pressure is
the first effect to be observed, and that this becomes more and more
marked as the strength of the stimuli is increased, until a certain opti-
mum is reached, after which the fall in blood pressure becomes less evi-
dent. When a certain strength of stimulation is exceeded, a rise instead
of a fall occurs. After this point additional increase in stimulation causes
more and more marked elevation of blood pressure through a very long
range of stimuli.
Stimulation of two afferent nerves at the same time usually produces
a greater reflex vasomotor change than the stimulation with an equiva-
lent strength of current of either nerve alone. That is to say, the effect
produced by stimulating the central end of both sciatics simultaneously
will be greater than that produced by stimulating either alone with double
the strength of stimulus.
As has been stated above, the reflex change in blood pressure is often
quite transitory in nature, although the stimulation of the pressor nerve is
maintained. When this decline has occurred, the pressor reaction can
often be renewed by shifting the stimulation to a second nerve. These
facts concerning the greater efficacy of combined stimulation of several
nerves are of considerable importance in connection with the general
question of reflex changes in blood pressure. For instance, many of the
pressor fibers found in the sciatic nerve are connected with the receptors
that mediate the sensations of the skin. When these receptors are
stimulated, as by heat or cold, reflex changes in blood pressure occur
(pressor reaction), (Fig. 74), and it is important to remember that
localized stimulation of the skin is less efficient in bringing about such
vascular changes than stimulation applied over large areas, even when
the local stimulus is intense and the general stimulus mild in character.
Jumping into a moderately cold bath will cause a much greater rise in
arterial blood pressure than plunging the hand into ice cold water.
Mechanism of Action of Pressor and Depressor Impulses. — When we
consider the exact mechanism by which these afferent impulses operate,
we have to bear in mind four possibilities: the reflex fall produced by
stimulation of a depressor afferent fiber may be due either to a stimula-
tion of the vasodilator part of the center or to an inhibition of the tone
THE CONTROL OF THE CIRCULATION
241
cf the vasoconstrictor part; and, conversely, a rise in arterial pressure
caused by vasoconstriction may be dependent either on a stimulation of
the vasoconstrictor part of the center or on an inhibition of the tone of
the vasodilator part. All of these changes have, as a matter of fact, been
shown to occur, at least under certain conditions, although the evidence
Fig. 74. — The effect of strong stimulation (heat) of the skin of the foot on the arterial blood
pressure and respiratory movements. Upper tracing, thoracic movement; lower tracing, arterial
blood pressure.
for the inhibition of dilator tone is as yet a little uncertain (see Pig. 75).
Without going into the subject in detail, we may nevertheless take
as an example of the methods by which the information has been ob-
tained, the experiment performed by Bayliss,18 showing that the vasodi-
lation which results from stimulation of the depressor nerve is owing
partly to removal of vasoconstrictor tone and partly to vasodilator
242
THE CIRCULATION OF THE BLOOD
stimulation. The volume of the hind limb of a curarized and vagotomized
rabbit increases when the central end of the cardiac depressor nerve is
stimulated. In order to determine whether this dilatation is due solely
to the removal of vasoconstrictor tone, the above experiment was repeated
on a rabbit in which the sympathetic chain had been cut below the level
of the second lumbar spinal roots. By such an operation all the vaso-
constrictor fibers to the vessels of the hind limb are severed, but the
vasodilator fibers, since they emanate through the sacral sensory roots,
are left intact. It was nevertheless found on stimulating the depressor
nerve that dilatation of the hind limb still occurred, thus indicating
Fig. 75. — Diagram showing the probable arrangements of the vasomotor reflexes.
A. Muscle of arteriole.
D. Vasodilator nerve fiber terminating on A and inhibiting its natural tonus, as indicated by -
sign.
D, Vasoconstrictor fiber also ending in A, but exciting it (+). These two kinds of fiber arise
from the dilator center (DC) and the constrictor center (CC) respectively.
F. Afferent depressor fiber, dividing into two branches, one of which (-) inhibits the con-
strictor center, while the other (+) excites the dilator center causing dilatation of the arteriole and
fall of blood pressure.
R. Pressor fiber exciting CC and inhibiting DC, and therefore causing vasoconstriction and rise
of blood pressure.
a, b, c, and d represent the synapses of the pressor and depressor branches with the efferent
neurons. (From Bayliss.)
that stimulation through vasodilator fibers must have taken place. Con-
versely, in another experiment, instead of the sympathetic chain, the
spinal cord was cut below the level of the second lumbar segment, thus
THE CONTROL OF THE CIRCULATION 243
severing the dilator but not the constrictor path, and again depressor
stimulation caused the volume of the limb to increase, indicating that
an inhibition of constrictor tone must have occurred.
Reciprocal Innervation of Vascular Areas
It must not be imagined that changes in the caliber of the blood ves-
sels occurring in one vascular area are necessarily occurring all over
the body. On the contrary, a most important reciprocal relationship
exists in the blood supply to different parts. After food is taken, for
example, more blood is required by the digestive organs than when they
are at rest, and this is insured by dilatation of their own vessels along
with reciprocal constriction of those of other parts of the body. On
account of the relatively great capacity of the abdominal vessels, their
dilatation during digestive activity is usually greater than the reciprocal
constriction of the other vessels, so that the diastolic blood pressure falls,
necessitating a more powerful cardiac discharge in order to maintain
the mean pressure. After taking food, the systolic pressure does not
as a rule fall so much as the diastolic, if it falls at all; and the pres-
sure pulse therefore becomes greater and causes a greater live load to
be applied to the vessels with each heartbeat. During the sudden strain
that is thrown on them, weakened arteries may give way, especially in
the brain.
Another example of reciprocal action of the vascular system is seen
in muscular exercise. The vessels of the active muscles dilate, while
those elsewhere constrict. The local dilatation in this case is, however,
not entirely at least a nervous phenomenon, being caused in fact, as we
shall see, by hormone action on account of the local increase in hydro-
gen-ion concentration (see page 414). There can be little doubt that
local irritants to the surface of the body, such as hot applications, lini-
ments, etc., act in the same way; they cause local dilatation of the super-
ficial and perhaps of the immediately underlying vessels and constric-
tion of those elsewhere in the body. Application of cold to local areas
of skin similarly causes local constriction accompanied by reciprocal
dilatation elsewhere. This action of cold is very marked in some parts of
the body, such as the hands, where by Stewart's method (page 283) it
can be shown, not only that the bloodflow of the hand to which the cold
is applied is greatly curtailed, but also that of the opposite side.
Experimental demonstration of reciprocal vascular innervation is fur-
nished by numerous experiments. If the central end of the great auric-
ular nerve of the ear is stimulated in a rabbit, a blanching of the ves
sels of the ear occurs at the same time as a rise in arterial blood pres-
244 THE CIRCULATION OF THE BLOOD
sure (Loven reflex). Similarly when the central end of one of the sen-
sory roots of the leg of a dog is stimulated, there is a rise in arterial
blood pressure and an increase in the volume of the limb.
THE INFLUENCE OF GRAVITY ON THE CIRCULATION
If the arterial blood pressure is measured in the arm and leg in a man
standing erect, a difference corresponding to the hydrostatic effect of
gravity will be found between the two readings. In comparison with
the high pressure normally existing in the arteries, this difference is,
however, of little significance. On the other hand, in the veins, where
the average pressure is low, gravity would cause serious embarrassment
to the circulation of blood were it not for the valves and the forces
which move the blood beyond them (page 214).
In erect animals the part of the circulation in which blood might stag-
nate as a result of gravity is the splanchnic area. Were such stagna-
tion to occur, the blood would not be returned to the right heart, so
that the arteries would not receive sufficient blood to maintain an ade-
quate circulation, particularly in the vessels of the brain.
Simple experiments devised by Leonard Hill19' 2S illustrate these prin-
ciples. When a snake, for example, is pinned out on a long piece of
wood and an opening made opposite the heart, this organ can be seen
to fill adequately with blood as long as the animal is maintained in the
horizontal position. When placed vertically, however, the heart be-
comes bloodless. If now the tail end of the animal is placed in a cylinder
of water so as to overcome the effect of gravity, the heart will be seen
to fill again with blood. Evidently in such an animal there is no mechan-
ism to compensate for gravity.
If a domestic rabbit with a large pendulous abdomen is held in the
vertical tail-down position, stagnation of blood in the splanchnic ves-
sels occurs to such an extent that in from fifteen to twenty minutes the
animal dies from1 cerebral anemia. If an abdominal binder is first of all
applied, the vertical position will not have the same consequences. This
experiment illustrates clearly the possible evil effects that gravity may
produce in animals in which no mechanism exists to compensate for it.
Placing an animal such as a dog under light ether anesthesia in the
vertical tail-down position produces an immediate fall in arterial blood
pressure, as shown in the tracing (Fig. 76), followed by a certain de-
gree of compensation even while the animal is still in the erect position.
The extent to which this compensation occurs varies with the depth of
the anesthesia. If the experiment is repeated after administering a large
dose of chloroform, not only will the initial fall be much greater, but
THE CONTROL OF THE CIRCULATION 245
subsequent compensation will be practically absent. The application of
these facts in the operating room will be self-evident.
Leonard Hill has shown that three factors are involved in the com-
pensating mechanism: (1) the tonicity of the abdominal musculature;
Fig. 76. — Aortic blood pressure, showing the effect of posture: A, vertical, head-up; B, hori
zontal; C, vertical, head-down; D, horizontal. (L.H.)
(2) the tone of the splanchnic blood vessels; (3) the pumping action of
the respiratory movements. The importance of the first-mentioned fac-
tor can be readily shown by making a crucial incision of the abdom-
inal walls in an animal in the erect position (Fig. 77), and that of
Fig. 77.— Tracing to show the effect of gravity on the arterial blood pressure. At A, the
animal was placed in the vertical position; at B, the abdomen was compressed; at C, a crucial
incision was made in the abdomen; at D, the pleural cavity was opened; at F, the animal was
returned to the horizontal position. (From Leonard Hill.)
the second factor by cutting the great splanchnic nerves, or the spinal
cord. After such an operation, even while in the horizontal position, as
we have seen, the blood pressure falls to a considerable extent. If the
animal is now placed in the vertical tail-down position, however, it falls
246
THE CIRCULATION OF THE BLOOD
to the zero line and the animal soon dies (Fig. 78). The influence of the
third factor is not so great as of the other two, but can be shown by the
increased respiratory activity which is likely to develop in the vertical
Fig. 78. — The effect of gravity on the aortic pressure after division of the spinal cord in the
upper dorsal region. By placing the animal in the vertical feet-down posture, the pressure fell
almost to zero, but on returning it to the horizontal posture, the circulation was restored. (From
Leonard Hill.)
tail-down position, the anemic condition of the respiratory center being
no doubt the cause of the increased respiration.
CHAPTER XXVIII
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA
Up to the present we have been considering the circulation of the blood
from a general point of view. There are certain organs and tissues, how-
ever, in which the general mechanism is altered in order to meet pecu-
liar requirements of blood supply. Thus, it is evident that the brain,
incased as it is in the rigid cranium, will be unable to contract and
expand as a result of vasoconstriction or vasodilation. On the other
hand, we know that the blood supply to this organ does vary con-
siderably from time to time. What is the nature of the mechanism by
which such changes are brought about? In the case of the liver the cir-
culation is peculiar on account of the fact that blood is carried to the
organ by two vessels, in one of which it is supplied under high pressure
and in the other, under low pressure. We must investigate the rela-
tionship of these two sources of blood supply. The circulation through
the coronary and pulmonary vessels must likewise receive special atten-
tion on account of the highly specialized functions of these organs.
THE CIRCULATION IN THE BRAIN
Anatomic Peculiarities
Serious curtailment of the blood supply to the brain is guarded against
by the existence of the circle of Willis. Besides the four main arteries —
the vertebrals and the two carotids — the spinal arteries contribute to
the blood supply of the circle, and consequently in certain animals, such
as the dog, the four main arteries may be ligated without causing death.
In man, however, ligation of both carotids is usually fatal. The free
anastomosis displayed in the circle of Willis is not maintained in the
case of the arteries which run from it to supply the bratti structure. On
the contrary, these vessels are more or less terminal in character; that
is to say, the capillary system produced by the different vessels does not
freely anastomose, so that the obstruction of one vessel, or an important
branch, is followed by death of the supplied area. The vessels which go
to the pia mater, however, break up into numerous smaller branches,
which freely anastomose before entering the brain tissue.
247
248
The venous blood is collected by the small, very thin-walled and valve-
less cerebral veins. These run together to form larger veins dis-
charging into the sinuses, the openings into which are kept patent by
the arrangement of dura mater around the orifices. The sinuses exist
between the dura and skull and are so constructed that they can not
be compressed, particularly those at the base of the brain. From them
the blood is conveyed mainly to the internal jugular vein, some of it
however escaping by the anastomoses existing between the cavernous
sinus and the opththalmic veins, and by the venous plexus of the spinal
cord. The most striking peculiarities of the veins are their patulous con-
dition and the absence of valves, so that any change in the blood pres-
sure in the internal jugular vein must be immediately reflected in that of
the venous sinuses. This explains why compression of the abdomen
Fig. 79. — Schema to show the relations of the Pacchionian bodies to the sinuses: d, d, Folds
of the dura mater, inclosing a sinus between them; v.b., the blood in the sinus; a, the arachnoidal
membrane; p, the pia mater; Pa., the Pacchionian body as a projection of the arachnoid into the
blood sinus. (From Howell's Physiology.)
causes venous blood to flow from an opening made in the longitudinal
sinus.
In considering the cerebral circulation, another factor that must be
borne in mind is the presence of cerebrospinal fluid. This is contained
in the subarachnoid spaces of the brain and spinal cord, these spaces, in
the case of the brain, being often considerably enlarged to form the
cisternse. The cerebrospinal fluid is also present in the ventricles of the
brain, which it will be remembered communicate with the subarachnoid
spaces through the foramen of Magendie, etc. It is unlikely that the
cerebrospinal fluid is of much importance in connection with the control
•of the blood supply to the brain tissue. It may be merely a lubricating
fluid; at least it is so small in amount (60 to 80 e.c. in man) as to be
apparently of little value in bringing about an alteration in brain volume.
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 249
Although normally so scanty, its secretion can become remarkably stim-
ulated under certain conditions as in fractures of the base of the skull.
Under these conditions in man, it may drain away at the rate of about
200 c.c. a day or more.
The fluid is apparently secreted from the choroid plexus, for when
the pathways by which the ventricles communicate with the subarach-
noid space are obstructed, it collects in the ventricles, producing internal
hydrocephalus. Under certain conditions its absorption is also very
rapid, as shown experimentally by the rapidity with which physiologic
saline is absorbed when it is injected into the subarachnoid space. This
absorption is believed to occur through the Pacchionian bodies, which
are minute sac-like protrusions of the arachnoid into the interior of a
venous sinus. The membrane that separates blood and cerebrospinal
fluid is extremely thin at these places (Fig. 79).
Physical Conditions of Circulation
On account of these anatomic peculiarities, the physical factors con-
trolling the circulation of blood to the brain are considerably different
from those obtaining in any other part of the body, with the possible
exception of the bones. In other vascular areas, we have seen that, when
dilatation or constriction of the vessels occurs, a marked increase or
diminution of the volume of the part becomes evident. Such a change
in volume is evidently impossible in the case of the brain because of
the rigid cranium in which it is contained. In fact, from a physical
point of view we must consider the blood vessels of the brain as pro-
jecting into a rigid case filled with incompressible material. Under
these conditions it is obvious that the vessels as a whole could neither
contract nor dilate without some increase or decrease in the volume of
the contents of the cranial cavity (Leonard Hill19).
Some have thought that the cerebrospinal fluid as it flows into or out
of the spinal cord might accomplish this alteration in the cranial con-
tents, but the relatively small amount of available cerebrospinal fluid,
the smallness of the openings between the brain and the spinal cord, and
the lack of experimental evidence that such changes in Volume of cere-
brospinal fluid in the spinal cord do actually occur, all stand in contra-
diction to such a view. However, although the vessels as a whole might
not contract or expand, yet some vessels, like the arteries, might con-
tract simultaneously with a corresponding dilatation of other vessels,
such as the smaller cerebral veins. In admitting the possibility of some
reciprocal relationship between arteries and veins, we must remember
that it is only before the well-protected sinuses are reached that a
change in the caliber of the veins would be possible. But it is difficult
250 THE CIRCULATION OF THE BLOOD
to see how such reciprocal dilatation and constriction could be of any
advantage except perhaps in causing certain areas to receive more
blood than others. A reciprocal relationship might also exist between
adjacent arterioles as well as between arterioles and veins; when, for
example, the arm center becomes active, it is conceivable that its
arterioles might dilate at the same moment that those of a neighboring,
less active center become constricted. Alterations obviously might oc-
cur without causing any perceptible change either in the volume of the
brain as a whole or in the condition of venous flow.
In consideration of these factors, most observers are agreed that the
total volume of blood in the brain must be constant at all times (Monro
and Kellie doctrine). Alteration of blood supply can, however, still be
brought about by changes in the velocity with which the blood traverses
the vessels. When more blood is required in the brain to supply the
increased metabolism which we must presume accompanies heightened
mental activity, it is not accomplished as in other parts of the body by
an increase in the capacity of the vessels as compared with those of
other vascular areas, but by a hurrying up of the circulation through
vessels whose caliber remains unaltered.
The main factors determining the velocity of bloodflow through the
brain must, therefore, be dependent upon changes occurring elsewhere
in the vascular system, a conclusion for which there is abundant experi-
mental evidence. Of the many ingenious methods that have been de-
vised to secure this evidence, we will cite but one in this place. Records
are taken of changes in: (1) the venous blood pressure of the brain by
connecting a cannula either with the vein immediately after leaving the
skull or, better still, with the torcular Herophili;. (2) the brain volume,
by connecting a very sensitive receiving tambour with a trephine hole
in the cranium so that its open end lies against the pia mater.* Al-
though, as we have seen, while incased in the rigid cranium the brain
volume can not change to any degree, yet this will occur when a
portion of the cranium is removed, so that pulsations correspond-
ing to those in the blood vessels will be observed; (3) the circula-
tory conditions elsewhere in the body, by taking arterial and
venous pressures and plethysmograms. The results in a normal an-
imal show the following points (see Fig. 80): (1) The tracings of
the arterial blood pressure (A}, the brain volume (C) and the intra-
cranial venous pressure (C) have exactly the same contour — that is,
the respiratory and the cardiac waves in all three of them are identical.
The venous blood as it flows into the jugular veins , also pulsates in
*This receiving tambour really consists of a brass tube of the same diameter as the trephine
hole, into which it is tightly fitted. The brass tube is closed at its inner end by thin rubber membrane,
and its outer end is connected with the receiving tambour.
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 251
unison with the artery. (2) Any change in the blood pressure of the
systemic venous system is immediately reflected in the blood pressure
of the sinuses of the brain and in the brain volume, (not well shown in
accompanying tracing). (3) A change never occurs in the vessels of
the brain which can not be accounted for by some change occurring
Fig. 80. — To show simultaneous records of the arterial blood pressure (A), the venous pres-
sure (B), the intracranial pressure (C), the pressure in the venous sinuses (D). The fall in ar-
terial pressure produced by stimulation of the cerebral end of the vagus will be found to cause
a fall of intracranial and cerebral venous pressure, accompanying that in the arteries, but a rise
in that of the venous system. (From Leonard Hill.)
elsewhere in the vascular system outside the cranial cavity. This re-
sult is important because it shows that there can not be vasomotor
nerve control of the brain vessels.
Taking into consideration not only the results of such experiments,
but also the peculiar physical conditions existing in the cranial cavity,
252 THE CIRCULATION OF THE BLOOD
we must conclude that changes in blood supply depend on changes in
the velocity of the bloodflow, and that such alterations in velocity are
dependent upon changes occurring in the aortic and more especially
in the vena-cava pressure. When the aortic pressure rises, more blood
will flow into the cerebral arteries and move along them at an increased
velocity, the increased pressure probably causing a moderate degree
of passive dilatation, to allow extra room for which the numerous
small cerebral veins become compressed. This compression of the veins
probably does not obstruct the greater flow of blood through them, be-
cause, taken as a whole, they are ordinarily much more capacious than
need be. On the other hand, if the aortic pressure should remain con-
stant, but that in the vena cava increase, then there would be obstruc-
tion to the passage of blood in the intracranial arteries, and conse-
quently a diminished velocity of flow.
Vasomotor Nerves
It might be inferred that, since the bloodflow through the cerebral
vessels is mainly dependent on vascular conditions elsewhere in the
body, there would be no need, as in the vessels of other vascular areas,
for vasomotor fibers. Histologists have, however, discovered the pres-
ence of such fibers, and it has become necessary for the physiologist to
find out if they are really of importance in .connection with the regula-
tion of the blood supply to the brain. Even if it is admitted that the
arterioles could not contract or expand as a whole without producing
local changes in venous pressure or cranial volume, it is yet of course
always possible, as has already been pointed, out, that one set of arte-
rioles might contract at the same moment that another set expanded.
That the vessels can undergo a process of constriction has been shown
by experiments in which the volume of outflow from the vessels of
the brain was measured in perfused preparations of brain. When
epinephrine was added to the perfusion fluid, curtailment of outflow
was observed to occur (Wiggers). Since this drug causes constriction of
vessels only when these are supplied with constrictor fibers (see page
736), the conclusion may be drawn that the cerebral blood vessels do
contain such nerve fibers. Nevertheless, the local vasomotor control of
the cerebral blood vessels can not have the significance in connection
with changes in blood supply that it has for other vascular areas (Hill
and Macleod20). No doubt nerve fibers are present in the cerebral
blood vessels, and presumably under certain conditions they are capable
of causing the blood vessels to undergo alterations in caliber, but it is
impossible to see of what real value this can ~be under normal conditions.
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 253
Intracranial Pressure
One word more with regard to what is known as intracranial pressure,
that is, the pressure in the space between the skull and the brain.
Under ordinary conditions it must be equal to that in the cerebral capil-
laries, and may be measured by connecting a sensitive manometer with
a tube screwed into the cranium as described above. It has been found
to vary from 0 mm. Hg in a man standing erect to 50-60 mm. Hg in a
dog poisoned by strychnine. It becomes increased, not only by com-
pression of the veins of the neck and by an increase in general arterial
pressure, but also in pathologic conditions, such as hydrocephalus. A
new growth in the brain, if it occupies more space than the tissue which
is destroyed, exerts pressure on all parts. of that region of the cranial
cavity, but this pressure may not be transmitted equally throughout
the cranial contents, for the falciform ligaments and the tentorium sup-
port a part of it, thus directing the spread of pressure along certain
pathways. The structures at the base of the brain, the optic nerves,
the veins of Galen and the Sylvian aqueduct are most affected in this
way. If the pressure is rapidly applied, however, it may rise through-
out the cranial contents. In such cases the pressure is, of course, cir-
culatory in origin, since immediately after death from cerebral tumor
the intracranial pressure is not found to be raised.
The major symptoms of cerebral compression are no doubt due to
anemia of the medulla oblongata, which may be the result either of
pressure applied locally in the bulbar region, where the presence of a
very small foreign body or only trivial tumor formation is sufficient to
destroy life, or of pressure transmitted from the cerebral cavity, in
which case, on account of the support offered by the tentorium, a much
larger growth is required to affect the medulla. Internal hydrocephalus
produced by blocking of the aqueduct of Sylvius and the veins of Galen
causes the greatest rise in intracranial tension, and may affect the me-
dulla, because the brain is driven downwards so as to pinch the bulb
against the occipital bone. .It must be emphasized that it is not the
pressure per se that causes the symptoms, but the attendant anemia,
the symptoms of acute cerebral anemia and of compression being iden-
tical (Leonard Hill19). To relieve the compression, trephining is the
common practice. The trephine hole should be as large and as near
to the source of compression (tumor, etc.) as possible.
CIRCULATION THROUGH THE LUNGS
The pulmonary or lesser circulation, as it is called, is quite different
from the systemic circulation. In the first place, because the pressure
254 THE CIRCULATION OP THE BLOOD
in the pulmonary arteries does not amount to more than about 20 mm.
Hg, or about one-sixth of that of the systemic arteries, the peripheral
resistance in the blood vessels of the lungs is much less than that of
the body in general. This lower resistance is owing partly to the large
diameter of the arterioles and the small amount of muscular fibers in
their walls, and partly to the fact that the capillaries are held con-
stantly in a somewhat dilated condition on account of the subatmos-
pheric pressure in the thorax (see page 306).
Another peculiarity of the pulmonary circulation is that the caliber
of the vessels is to a very large extent dependent upon the changes
that occur in the intrathoracic pressure with each inspiration and ex-
piration. They become dilated on inspiration and contracted on ex-
piration. The extent to which these respiratory changes affect the
amount of blood contained in the lungs, is very considerable. At the
height of inspiration it is computed that a little more than eight per
cent of the whole blood in the body is contained in the lungs, whereas
on expiration it diminishes to between five and seven per cent.
A third peculiarity is that the pulmonic blood vessels are not sup-
plied with vasomotor nerve fibers — at least with such as can readily be
demonstrated. It is said that, when the pulmonary vessels are per-
fused and the outflow measured, a diminution in the latter is found to
occur when epinephrine is added to the injection fluid — a result which
is, however, denied by certain investigators. Changes in the bloodflow
have not been observed to occur when the vagus or sympathetic nerve
fibers running to the lungs are stimulated. In short, the conclusion
which we must draw is much the same as that for the blood vessels
of the brain — namely, that although, as a result of the epinephrine ex-
periment, we must admit that a vasomotor supply may possibly be
present, yet it is one which can be of no significance under normal
conditions.
When there is obstruction to the outflow of blood from the' left ven-
tricle, as, for example, in cases of high aortic pressure, the blood is not
entirely discharged with each beat of the left ventricle, and therefore
dams back through the left auricle into the lungs. On account of the
marked distensibility of the pulmonary capillaries, a large amount of
this blood may collect there and thus make the lungs serve as a kind of
reservoir of the heart. When the capacity of this reservoir has, how-
ever, been overstepped, an increased peripheral resistance will come to
be" offered to the movement of blood in the pulmonary arteries, the
pressure in which will consequently rise and sooner or later interfere
with the discharge from the right ventricle, causing as a result a stag-
nation of blood in the systemic veins, and a consequent increase in vol-
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 255
ume of such viscera as the liver and kidneys. The same changes will
obviously also supervene when there is regurgitation of blood from the
left ventricle to the left auricle, as in cases of mitral insufficiency.
CIRCULATION THROUGH THE LIVER
.The liver is the only gland in the body receiving both venous and
arterial blood, the former being supplied to it at a very low pressure
by way of the capacious portal vein, and the latter at very high pressure
by the strikingly narrow hepatic artery. Except for the relatively
small amount of blood which is supplied to the walls of the blood vessels
and the biliary ducts, none of the hepatic artery blood mixes with that of
the portal vein until the vessels enter the hepatic lobules. Beyond this
point the two blood streams mix and the combined stream is drained
away by the sublobular and hepatic veins.
Methods of Investigation
To study the relative importance of these two sources of blood sup-
ply, and also to investigate the manner in which the latter is controlled,
the most satisfactory method has consisted in measurements of changes
in volume flow rather than in those of changes in pressure. The vol-
ume-flow measurement has been made either by connecting stromuhrs
(page 207) to the hepatic artery or portal vein, or by measuring the out-
flow of blood from the hepatic vein into the vena cava, first with both
inflow vessels intact, and then with one of them ligated. An objec-
tion to the first (the stromuhr) method is the possible interference with
bloodflow or blood pressure produced by inserting the stromuhr into
the entering vessels, and also the fact that simultaneous measurement
of the flow in both vessels can not be made satisfactorily.
To measure the outflow from the hepatic veins, the aorta is ligated
below the celiac axis and a wide cannula is inserted into the central
end of the vena cava below the level of the liver, a loose thread being
placed around this vessel just above the diaphragm. By pulling on this
thread the vena cava becomes obliterated, and the blood from the
hepatic veins is therefore diverted into the cannula, through which it
flows into one end of a vessel shaped somewhat like a sputum cup (the
receiver), the other end being connected by tubing with a piston re-
corder, from the movement of which the volume of blood flowing into
the receiver can readily be computed. To measure the flow of blood,
a clip on the tube of the receiver is removed af the same moment that
the thread around the vena cava above the diaphragm is tightened,
and when the receiver has filled with blood, this thread is again loosened
256 THE CIRCULATION OP THE BLOOD
and the receiver tilted up so that the blood flows at low pressure back
into the circulation. The receiver being of known capacity, the length
of time it takes the blood to fill it as determined by the piston recorder,
furnishes us with the necessary data from which to calculate the rate
of flow. The receiver is chosen of such a size that it takes only a few
seconds to fill, the diversion of blood into it not causing any material
fall in arterial pressure. The observations are repeated frequently.
Results. — By the use of these methods it has been found that the total
mass movement of blood to the liver of the dog varies between 1.46 and
2.40 c.c. per second for 100 grams of liver. Considerable changes may
occur in the arterial pressure without affecting the liver flow. When
the hepatic artery is occluded, the flow diminishes by about 30 per
cent, or conversely, when the portal vein is obstructed but the hepatic
artery left intact, by about 60 per cent, indicating that about one-third
of the total bloodflow through the liver is contributed by the hepatic
artery and two-thirds by the portal vein. Some blood, however, gains
the liver through anastomotic channels between it and the diaphrag-
matic veins.
The relative supply by the two vessels is subject to various condi-
tions. That through the hepatic artery, for example, may be very con-
siderably altered on account of vasoconstriction in this vessel, for its
walls can easily be shown to be liberally supplied with vasoconstrictor
fibers carried by the hepatic plexus. This can be demonstrated by
the rise in blood pressure which occurs in a branch of the hepatic artery
during stimulation of the plexus. On the other hand, alterations in the
bloodflow in the portal vein can not be brought about by active con-
striction or dilatation of the intrahepatic branches of this vessel, no
active vasomotor fibers having been demonstrated by stimulation of
the hepatic nerves, although, as in the case of the brain and lung blood
vessels, a certain amount of constriction may occur under the influence
of epinephrine.
The bloodflow through the portal vein is dependent on changes oc-
curring at either end of the distribution of the vessel, that is, changes
occurring in the liver itself or in the intestine. Of these factors the lat-
ter is no doubt the more important, an increase not only in portal blood
pressure but also in portal bloodflow being readily produced by dila-
tation of the splanchnic blood vessels; for example, as the result of sec-
tion of the splanchnic nerve. Alterations in portal bloodflow brought
about by changes in the caliber of the vessels in the liver itself are
partly dependent upon changes in the branches of the hepatic artery.
Let us consider briefly how this may be brought about. At the point
where the portal and hepatic arteries come together — that is, at the in-
PECULIARITIES OP BLOOD SUPPLY IN CERTAIN VISCERA 257
trahepatic capillaries — the pressure of the blood in them must become
equal, which means that in its course through the interlobular connec-
tive tissue, the branches of the hepatic artery must offer much resistance
to the blood flowing through them. This frictional resistance resides in
the hepatic arterioles, and since these are richly supplied with constric-
tor nerves, great variation in hepatic inflow becomes possible. These
changes will affect the degree of tension of the interlobular connective
tissue in which the arterioles lie. In this tissue, however, also lie the
thin-walled branches of the portal vein. When therefore the tension
of this tissue becomes greater, as a result, for example, of vasodilatation
in the hepatic artery, the portal vein radicles will become compressed
and the bloodflow along them impeded. Conversely, when vasocon-
striction occurs in the hepatic arteries, the congestion of the connective
tissue becomes diminished, the veins dilate, and the blood flows through
them more readily (Macleod and R. G. Pearce21). Experimental evi-
dence in support of the above view is furnished by observing the out-
flow of blood from the liver before and during stimulation of the he-
patic plexus. The first effect is an increase in the outflow, which very
soon returns to its original amount, even though the stimulation of the
plexus is kept up during the experiment. This return to the normal
flow must indicate either that the constriction of the hepatic artery has
not been maintained, or that it has been maintained but is accompanied
by a compensatory increase in the flow through the portal vein. As
a matter of fact, we know that the hepatic artery remains constricted
as long as the hepatic plexus is stimulated, indicating that the conges-
tion of the connective tissue in which the venules lie has become reduced
to such»an extent, as a result of the constriction, that these open up and
permit the blood to flow through them more readily. The initial in-
crease in outflow immediately following upon stimulation of the hepatic
plexus, is no doubt caused by the squeezing out of the blood already in
the hepatic vessels, and it is a result which is often observed in other
organs during stimulation of vasoconstrictor nerve fibers.
THE CORONARY CIRCULATION
We have already studied the effect produced on the heartbeat by in-
terfering with the flow of blood in the coronary vessels, and it remains
for us to study: (1) peculiarities in the bloodflow through them, and
(2) whether this bloodflow can be altered by dilatation or constriction
of the vessels brought about through nerves. With regard to the pecu-
liarities of bloodflow, it may be stated that there is said to be two periods
in each cardiac cycle during which an increase takes place in the mass
258 THE CIRCULATION OF THE BLOOD
movement of blood in the coronary vessels — namely, at the beginning
of systole, and again at the beginning of diastole. Nevertheless the
pressure pulse has the same contour in the coronary as in the systemic
circulation. (W. T. Porter.22) During systole the intramural branches
of the coronary artery are ^compressed and the blood pressed out of
them. This emptying of the vessels favors the flow of blood through
the heart walls.
Regarding the presence of coronary vasomotor nerves, there is at pres-
ent a certain amount of doubt. When strips of the coronary artery are
suspended in a solution of epinephrine, they undergo relaxation instead
of contraction. On the assumption that the action of epinephrine on
blood vessels is the same as that of stimulation of the vasoconstrictor
fibers, this result has been taken as evidence of the absence of such
fibers and the possible presence of vasodilator fibers. A somewhat
similar type of experiment has been performed by injecting epineph-
rine into the fluid used to perfuse the excised mammalian heart,
with the result that, when such injections are made into a heart that
is not beating, evidence of vasoconstriction is obtained, whereas when
injected into a beating heart, dilatation occurs. This latter result
may, however, be owing to the action of the epinephrine in stimulating
the cardiac contractions. Other observers, however, deny that the in-
jection of epinephrine into the coronary circulation has any influence
upon the outflow of the perfusion fluid. Taking the result of these
observations as a whole, we may at least conclude that epinephrine
does not produce the same marked vasoconstriction that it produces in
other blood vessels — a fact, which, as already stated, may be taken
advantage -of in bringing about the rise in coronary pressure .that is
necessary for successful resuscitation of the heart.
Attempts to demonstrate the presence of vasomotor 'fibers by electrical
stimulation of the vagus or sympathetic nerve have yielded results. which
are quite inconclusive, although some observers assert that the vagus
nerve carries vasoconstrictor fibers to the coronary vessels, and that
the sympathetic carries vasodilator.
CHAPTER XXIX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS*
In the following chapters a brief account will be offered of the clinical
use of the electrocardiogram, of polysphygmograms, and of bloodflow
measurements. This is done to show how physiologic technic is being
employed for the accurate investigation of cardiovascular disease.
ELECTROCARDIOGRAMS
To observe the electrical change produced by the spread of the excita-
tion wave over the heart from auricles to ventricles, it is not necessary
to place the electrodes directly on the heart, but, as already hinted, we
may follow the electrical change by leading off from electrodes applied
to the surface of the body. From such electrocardiographic tracings
extremely important facts concerning the propagation of the heartbeat
may be ascertained. In order to make an observation the hands and the
left foot are each placed in a solution of sodium chloride contained in
porous jars, immersed in larger vessels, containing a saturated solution of
ZnS04 and zinc terminals, t An arrangement like that in Fig. 81 may also
be used. By manipulation of suitable keys, the extremities may then be
connected with the electrocardiograph in the following manner: Lead 1,
right arm and left arm; lead 2, right arm and left leg; lead 3, left arm
and left leg. Through lead 1, the current acting on the galvanometer will
be that produced more especially at the base of the heart. Through lead
2, the current will pass through the long axis of the heart, and through
lead 3, it will pass mainly along its left border.
When any pair of leads is connected with the galvanometer, it is ob-
served that the string is deflected to one side owing to electrical cur-
rents arising from the skin. Before taking a record of the cardiac
movements of the string, it is necessary to compensate for this skin cur-
rent by introducing into the circuit in the opposite direction the re-
*A certain amount of repetition of. matter previously discussed has been found advisable in these
chapters for which the indulgence of the reader is requested.
tit is really unnecessary to use the so-called nonpolarizable electrodes. Glass vessels containing
20 per cent NaCl solution with the zinc plates dipping into them arc quite satisfactory.
259
260
THE CIRCULATION OP THE BLOOD
quired amount of current, called the compensating current, to bring the
strong shadow back to the zero or midposition. In order that the rec-
ord obtained may be quantitative in character, it is further necessary
that the movement of the string be standardized. This is done by as-
certaining to what extent the string moves when a current of known
voltage is sent through it and by altering the tension of the string so that
one millivolt of current causes an excursion of one centimeter of the
string shadow on the photographic plate. It would take us beyond the
Fig. 81. — Etectrocardiographic apparatus as made by the Cambridge Scientific Materials Co. Con-
tact electrodes are shown, but the immersion electrodes described in the context are preferable.
confines of this volume to go in any greater detail into the technic in-
volved in taking electrocardiograms, but it may be said that this is by
no means difficult, provided the instructions which are supplied with
the instrument are carefully followed. In practice the taking of elec-
trocardiograms is indeed quite a simple matter, and the extremely im-
portant information which they give us concerning the mechanism of
the heartbeat and the evidence of myocardial disease should make their
employment a universal practice in all cardiac clinics. Some of these
clinical applications are described elsewhere (page 266).
ELECTROCARDIOGRAMS
261
What particularly interests us here is the contour of the electrocardio-
gram in a normal person (Fig. 82). It will be observed that there are
three waves above the line of zero potential and two waves below it.
They have been lettered from before backward, P, Q, R, S, and T,
and in all such records when correctly obtained, the waves above the
line of zero potential indicate that the base of the heart is negative to
the apex. The exact cause of each wave has been ascertained by taking
simultaneously with the electrocardiogram a record of the mechanical
changes occurring in the heart during each cardiac cycle. Such records
-T
Fig. 82.— Normal electrocardiogram. Leads 1, 2, 3. Note that the height of the R deflection in
lead 3 equals the difference between the height of RI and 7?2-
have been secured by taking intracardiac pressure curves with the results
as shown in Fig. 83. The top curve represents auricular and the second
one ventricular pressure, whereas the lowest is an electrocardiogram.
It will be observed: (1) that the P-wave occurs just antecedent to con-
traction of the auricles; (2) that the small positive wave, Q, which is ab-
sent in these tracings, must occur just before the beginning of the con-
traction of the ventricles; (3) that the negative wave, R, occurs just be-
fore and during the early part of ventricular systole — that is, during
the presphygmic period; and (4) that the long upward wave, T, culmi-
nates at the moment the ventricle begins relaxing.
262
THE CIRCULATION OF THE BLOOD
Although such comparisons give us considerable insight into the cause
of several of the waves, there yet remain certain peculiarities of the
electrocardiogram to be considered. These are: (1) the cause of the
slight positive wave, Q; (2) the cause of the- positive wave, S; (3) the
cause for the period of equal potential at the base and apex during ven-
tricular systole indicated by the portion of the curve between S and T ;
(4) the cause for the negative wave, T. To solve these problems it is
necessary to compare electrocardiograms taken from the surface of the
body with those from electrodes placed directly on the base or apex of
the ventricle of the exposed heart.
Fig. 83. — Ivlectrocardiogram {dog) taken simultaneously with curves from auricle and ven-
tricle. It will be observed that wave P slightly precedes auricular systole and that wave R occurs
just before the presphygmic period starts in the ventricle. (From Lewis.)
The Ventricular Complex
In view of the nature of the electric change which occurs in a strip
of denervated muscle when a wave of contraction passes along it (page
188), the simplest interpretation of the ventricular part of the above
curve is that the contraction must pass into the ventricle at a little dis-
tance from the base, thus causing the latter, for a moment of time, to be
positive to the rest of the ventricle, and accounting for the slight down-
ward wave, Q. Immediately after this the base of the ventricle becomes
negative to the apex, giving us the marked upward wave, R, which
however lasts for but a short period of time, being followed by an inter-
val during which the base and apex are of the same electrical potential
(horizontal part of wave between R and T). Finally the base again be-
comes negative to the apex, thus accounting for the smaller upward
ELECTROCARDIOGRAMS 263
wave, T. The cause of the occasionally observed downward wave, S,
following R, is obscure.
The most significant fact in the electrocardiogram is therefore that
the base is negative to the apex at the beginning (It-wave) and again at
the end (T-wave) of the ventricular contraction. How may this be ex-
plained? When electrocardiograms are taken through electrodes placed
directly on the base and apex of the ventricle of the exposed heart, it
has been found that the contour of the electrocardiogram is like that
which is obtained from a strip of muscle when a wave of contraction
passes along it: it is diphasic in character (page 188), a result which
may be interpreted as indicating that the wave of contraction starts at
the base and ends at the apex. This rules out the explanation, at one
time suggested for the T-wave, that the wave starts at the base, then
proceeds to the apex, and finally ends at the base, following the disposi-
tion of the muscular fibers of the ventricle in a folded or loop form,
with the bend of the loop at the apex and the free ends at the base. Al-
though the explanation seemed at first to conform with the embryo-
logic fact that the heart is developed from a folded tube, it can not hold,
as has been shown by observing the course of the excitation wave se-
cured through electrodes placed at various points on the surface of the
exposed ventricle (page 194).
The explanation which is accepted by the majority of observers at the
present time is to the effect that the T-wave is caused by the longer con-
tinuance of the electric change at the base of the ventricle than at the
apex. To test this hypothesis the crucial experiment would evidently
be to see whether a T-wave could be induced in an electrocardiogram,
such as that of the frog ventricle, in which no T-wave exists, by hurry-
ing up the contraction process at the apex without affecting it at the
base. This can be done by local warming of the apex, or by applying
the ventricular electrode at varying parts' of the ventricle in an excised
heart beating in Ringer's solution of relatively high H-ion concentra-
tion. Mines showed that under these conditions a typical T-wave ap-
pears in the electrocardiogram, as shown in Fig. 84.*
The existence of the small Q-wave, indicating that the contraction
does not really start from the base, conforms with the observation that
the Purkinje system of fibers ends about the papillary muscles, which
therefore would be the first to contract, and with the observations of
Lewis, already alluded to above, on the appearance of the negative vari-
ation on the surface of the exposed heart.
The most important clinical application of the electrocardiogram is
"This tracing was found among those left by Professor Mines of McGill University, and for
permission to use it the author is indebted to the authorities of that institution.
264
THE CIRCULATION OF THE BLOOD
A. — Normal
B. — Apex cooled
C. — Apex warmed
Fig. 84. — Records of electrocardiogram and movement of ventricle of frog showing that when
the apex is warmed a typical T-wave appears in place of a wave in the opposite direction appear-
ing when the apex is cooled. (From Mines.)
ELECTROCARDIOGRAMS 265
undoubtedly in connection with the determination of the rate of trans-
mission of the excitation wave from auricle to ventricle; thus, the P-R
interval, as it is called, indicates the time taken for the impulse to
travel from the sinoauricular to the auriculoventricular node and bundle.
In delayed transmission this interval becomes abnormally long. Obvi-
ously also conditions of .heart-block, of auricular fibrillation, or of auric-
ular flutter will be immediately revealed by the electrocardiogram. The
interpretation of abnormalities in the contour of the ventricular portion
of the curve is, however, not so easy a matter, and should never be
undertaken unless curves from the three leads have been secured, for it
will be found that the corresponding electrocardiograms differ from
one another in detail; for example, the R-wave is usually most prominent
in lead 2, although sometimes it is more prominent in lead 3. T is always
upright in normal individuals in curves taken from lead 2, but it is not
infrequently inverted in those of lead 3, and may show partial inversion
in those from lead 1. The Q-R-S group is often of peculiar contour in
curves from lead 3. These variations are possibly dependent upon the
relative preponderance of the musculature in the left and right ven-
tricles, for it is evident that the amount of muscle included in the path-
way between the two leads will vary.
CHAPTER XXX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS (Cont'd)
CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY
The Electrocardiogram in the More Usual Forms of Cardiac
Irregularities
BY R. W. SCOTT
The principle of the application of the string galvanometer to the
study of cardiac irregularities has been indicated. It is our object here
to outline some of the more common forms of irregular heart action,
with a brief description of the abnormalities in the electrocardiogram
resulting therefrom. For the sake of comparison a normal electrocar-
diogram is shoAvn in Fig. 82. The cause and relationship of the various
deflections have been explained (see page 262).
Sinus Arrhythmia. — This irregularity is seen commonly in children
and young adults, and is without pathologic significance. The electro-
cardiogram presents the normal deflections and shows by the varying
spaces between the P deflections that the cardiac impulse has been gen-
erated at slightly irregular intervals.
Sinus Bradycardia. — The electrocardiogram in a simple case of sinus
bradycardia is usually normal, except that the deflections occur at an
unusually slow rate (Fig. 85). This indicates that the cardiac impulse
is built up at a slow rate, but when generated it evokes a normal auric-
ular and ventricular contraction.
The Extrasystole. — The extrasystole may be either auricular or ven-
tricular in origin. Occasionally a rare type is seen in which the im-
pulse arises in the junctional tissues between the auricle and ventricle.
When the focus of impulse production is at or near the sinoauricular
node, the resulting electrocardiogram complexes are practically normal.
If, however, the seat of impulse formation is removed from the S-A
node, the P deflection may be distorted or actually inverted, followed
by a normal Q-R-S-T complex (Fig. 86).
In the case of ventricular extrasystole, the cardiac impulse originates
in either the right or the left ventricle. This abnormal site, together
266
CLINICAL APPLICATIONS OP ELECTROCARDIOGRAPHY
267
Fig. 85. — Sinus bradycardia. Rate 32 per minute. Note the normal appearance of the electro-
cardiogram. P-R interval = .17 seconds.
Fig. 86. — Auricular extrasystole. Two auricular extrasystoles following two normal complexes.
Note the ectopic origin of the extrasystoles indicated by the inversion of P.
Fig. 87. — Ventricular extrasystoles arising in the right ventricle.
Fig. 88. — Ventricular extrasystole arising in the left ventricle.
268
THE CIRCULATION OF THE BLOOD
with the path which the impulse takes, produces a much greater differ-
ence of electric potential than is seen in the normal electrocardiogram.
When the impulse arises in the right ventricle near the base, the prin-
Fig. 89. — Paroxysmal tachycardia. Auricular origin. Note that the P deflection falls back on T.
Rate 200 per minute.
cipal R deflection is upwards in both leads 1 and 2. Arising near the
apex, the principal R deflection is up in lead 1 and down in lead 2. Two
extrasystoles both arising in the right ventricle are shown in Fig. 87.
Fig. 90. — Auricular fibrillation. Leads 1, 2, 3. Note the coarse fibrillation waves between the
R peaks, and the absence of any B deflections in relation to R. Also the unequal spacing of the R
deflections.
In the case of the left ventricle, a basal ' impulse gives a downward
principal deflection in lead 1 and up in lead 2. When the aberrant fo-
cus is located near the apex of the left ventricle, the principal deflec-
CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY 269
tion is down in both leads 1 and 2. Any one or several of the general
types of extrasystole may occur in the same patient. Fig. 88 shows
an extrasystole originating from the left ventricle.
Paroxysmal Tachycardia. — Electrocardiographic records taken in the
interval between the paroxysms may appear normal. During the tachy-
cardia the records normally show only two deflections, R and a combina-
tion of T and the succeeding P (Fig. 89). If the paroxysm is of auric-
ular origin, the P deflection may be inverted, indicating that the new
focus of impulse production is located at some other site than the sino-
auricular node. Rarely the new focus may be in the ventricles. Records
taken during the paroxysm may show a rapid succession of deflections,
simulating isolated ventricular extrasystoles.
Auricular Fibrillation. — The electrocardiogram in auricular fibrilla-
tion shows three distinctive features:
1. Absence of the P deflections typical of auricular contractions.
2. The ventricular complexes (Q-R-S-T waves) occur in irregular se-
quence and may vary in height.
3. The presence of small irregular oscillations best seen between the
ventricular complexes. A typical tracing of this condition is shown in
Fig. 90.
The dependence of the P-wave upon auricular contraction has been
indicated (page 261). Its absence in auricular fibrillation is accounted
for by the fact that the individual muscle fibers of the auricles contract
independently of one another, so that some fibers are in a state of con-
traction while others are relaxed. This renders impossible a coordinate
contraction of the auricle as a whole.
The multiple impulses from the fibrillating auricles reach the ventri-
cles and evoke a contraction provided the ventricle is not already in a
state of contraction (refractory period, page 178). These irregular
ventricular responses will of course produce unequal spacing of the
ventricular complexes in the electrocardiogram. The variations in the
height of the R deflections is thought to be due to the distortion caused
by the superimposition of the small waves representing auricular ac-
tivity. These small waves must occur throughout the whole cardiac
cycle, but are more or less masked by the ventricular complexes, appear-
ing as separate oscillations only during diastole.
Auricular Flutter. — Auricular flutter was discovered by the electro-
cardiograph, and it is practically impossible to make a diagnosis of this
condition without the use of the string galvanometer. The auricular
deflections are usually rhythmic and in the average case vary in rate
from 200 to 350 per minute. The initial deflection of P may be base
negative or apex negative — up or down — depending on the site of the
270
THE CIRCULATION OF THE BLOOD
origin of the auricular impulse (when arising from some other source
than the S-A node the impulse is said to be ectopic). Usually a regular
succession of P deflections can be traced throughout the record (Fig.
91).
Since it- is impossible for the ventricle to respond to all the impulses
coming from the auricles, a condition of partial heart-block obtains
(2:1 — 3:1 — 4:1, etc.). The ventricular complexes will occur regularly-
except when a 3:2 rhythm exists.
l?ig. 91. — Auricular flutter. Auricular rate 300. Ventricular rate 80. Note the inversion of the P
deflections.
Usually the ventricular complexes are such as to indicate that the
stimulus arose in the auricle (supraventricular). The height of the
individual deflections Q-R-S-T may vary, depending on the predominance
of a right or left ventricular hypertrophy.
Fig. 92.— Delayed conduction. Note the normal appearance of the electrocardiogram except for
the prolongation of the P-R interval, which measures .23 seconds.
Heart-block. — There are three degrees of severity in heart-block: (1)
delayed conduction, (2) partial dissociation, and (3) complete dissocia-
tion.
Any one of these conditions may be present in the same patient at
successive intervals.
DELAYED CONDUCTION. — When the conducting tissues of the heart are
so affected as to cause an abnormal prolongation of the P-R interval,
the condition is called delayed conduction. The ventricles respond to
each stimulus originating at the sinus node, but the time required for the
impulse to pass through the conducting tissues is longer than normal.
CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY
271
In a simple case the electrocardiogram may appear perfectly normal,
but when the P-R interval is measured accurately, it will be found to be
lengthened beyond the extreme limits of the normal (0.20 seconds) (Fig.
92).
PARTIAL DISSOCIATION. — In the typical case of partial dissociation the
Fig. 93. — Partial dissociation. Note the failure of ventricular response following the second P,
which has been preceded by two extrasystoles (x) of ventricular origin.
ventricles respond to the impulse coming from the auricle most of the
time, but occasionally fail to do so, when the condition is called "dropped
beat." The electrocardiogram records a P deflection but no ventricular
complex, showing that the auricles have contracted at their usual rate
but that the ventricles failed to respond to the stimulus coming from
the sinoauricular node (Fig. 93).
Fig. 94. — Complete dissociation. Note that the P wave spaces regularly and bears no definite re-
lation to the R wave of the ventricular complex. Auricular rate 72. Ventricular rate 40.
COMPLETE DISSOCIATION. — In a simple case of complete dissociation
the auricles beat independently of the ventricles; hence the P deflection
of the electrocardiograms bears no relation to the ventricular complex
(Q-R-S-T) (Fig. 94). The P deflections space regularly and are easily
made out when they fall during diastole of the ventricle. Occasionally
272 THE CIRCULATION OF THE BLOOD
the auricle will happen to contract during ventricular systole, causing a
distortion of the ventricular complex by the superimposition of a P
deflection. Except when this occurs the Q-R-S-T complex is the normal
supraventricular type. The P deflections occur more frequently than
the Q-R-S-T complex, showing that the auricles are beating more often
than the ventricles. The auricular rate in the average case of complete
heart-block is about 72, while the ventricular rate is much slower (35
to 40).
CHAPTER XXXI
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS (Cont'd)
POLYSPHYGMOGRAMS
In taking polysphygmograms, the following technic is usually followed;
Venous Pulse Tracings. — Direct the observed person to lie down with
his head slightly raised by a cushion and bent to the right side. Place
the receiver (thistle funnel) over the jugular bulb on the right side of
the neck. This lies immediately above the inner end of the clavicle.
Bring the style of the recording tambour to write with a minimal
amount of friction on the paper or drum. Since a venous pulse tracing
can not be interpreted without a simultaneous tracing from an artery,
now adjust the button of a receiving tambour over the radial artery and
adjust the style of its recording tambour so as to write on the drum in
the same perpendicular as the style of the venous tambour.
Tracings should be taken with the recording surface at a moderate
speed. Before disturbing, the relative positions of the writing points,
allow them to make vertical marks (with recording surface stationary)
at various parts of the tracings. These alignment marks permit of ac-
curate comparisons between the curves. Repeat the above, using the
carotid instead of the radial. A time tracing (l/5 sec.) should always be
taken simultaneously. The polysphygmograph is shown in Fig. 95.
To interpret the venous curve, make a vertical mark on the arterial
pulse tracing corresponding to the beginning of the pulse upstroke. If
this is done on the radial pulse tracing, measure one-tenth of a second in
front of it, and make a vertical mark to allow for the time lost in propa-
gation of the pulse from the heart to the radial artery.
This line 3 (corrected in case of radial pulse) corresponds to the be-
ginning of the sphygmic period of ventricular systole — i. e., to the open-
ing of the semilunar valves. Measure the distance from it to the near-
est vertical line that was made to indicate the relative position of the
writing points. Then measure off the same distance in the venous trac-
ing from the corresponding indicator line. This will fall at the begin-
ning of the small wave (c), which is due to the bulging into the auricles
of the closed auriculoventricular valves. (Fig. 96.)
273
274
THE CIRCULATION OF THE BLOOD
The auricular wave (a) occurs one-fifth of a second in front of c, and
may now be ascertained by measuring off this distance in front of c.
This is line 1.
Now measure the distance on the radial pulse tracing from the begin-
ning of the upstroke to the dicrotic notch. The distance between these is
the sphygmic period (E).
Fig. 95. — Polysphygmograph. This instrument records in ink on glazed paper two simul-
taneous tracings, i. e., radial pulse and one other, such as carotid, jugular, apex beat, etc., in addi-
tion to the time tracing. The ink tracings are both more convenient and permanent than smoked
paper tracings. The clockwork operates at variable speeds, permitting the taking of protracted
records at different speeds.
Measure off the same distance on the venous tracing from c. Line
5 will be found to fall just before a small wave (v), which is due to the
sudden opening of the tricuspid valves. This practically coincides with
the dicrotic notch on the radial pulse tracing. Sometimes a little wave
Fig. 96. — Normal jugular tracing. The spacing below shows the duration of the a-c interval.
(From E. P. Carter.)
occurs on the upstroke of wave v just where line 5 falls. This co-
incides with the closure of the semilunar valves. The distance between it
and wave v corresponds to the postsphygmic period.
The cause for the depression (marked x) following c will readily be
understood by referring to the intraauricular curve (Fig. 97), to which,
as already explained, the venous pulse tracing is qualitatively similar.
POLYSPHYGMOGRAMR
275
The rise in the curve following depression x is caused by the filling of
the auricle with blood. This goes on until v, when the tricuspid valves
open, allowing the blood to fall into the ventricle.
Fig. 97. — Reduced tracings from carotid, aorta, ventricle, auricle and jugular, to show the
general relationships of the various waves. An electrocardiogram is also shown. Note that the
jugular and auricular curves have the same contour, and that the depression (x) in them occurs
during systole of the ventricles. (After Lewis.)
To interpret the cardiogram, adjust receiving tambours to the radial and
open beat with both writing styles in the same perpendicular, and following
the other directions described under "venous pulse" mark on the
cardiogram: (See Fig. 98.)
Fig. 98. — Polysphygmograms including jugular, apex and radial tracings. Line 4 on the radial
tracing is first of all located. It is then transferred (by measurement from the alignment mark on
the right edge of the tracing) to the jugular and 1/10 second subtracted from it, giving line 3.
When this is similarly transferred to the apex tracing, it falls somewhere 'on the upstroke the be-
ginning of which is line 2.
276 THE CIRCULATION OF THE BLOOD
1. The beginning of the sphygmic period (E) (line 5).
2. The end of the sphygmic period (E) (line 5).
3. The auricular wave.
4. The beginning of ventricular systole (difference between 1 and 4
equals presphygmic interval).
5. The opening of auriculoventricular valves (lowest point in tracing).
Mark also on the tracing the exact moment at which the heart sounds
are heard.
It is important to make certain that the button of the tambour is ac-
curately over the apex beat, since otherwise a depressed or negative
wave may be inscribed at ventricular systole.
Simultaneous Arterial Pulse Tracings. — The velocity of the transmis-
sion of the pulse wave is calculated by measuring the time between the
systolic rise in the carotid and in the radial arteries, tracings of which
are taken by applying one receiving tambour to the carotid artery and
another to the radial artery.
Abnormal Pulses
The following is a brief description of the main character of abnormal
pulses:
The Ventricular Pulse. — In this no "a" waves are present in the
jugular tracing, the heart action being either regular or irregular. In
the former case, the absence of the "a" waves may depend on: (1) over-
filling of the right auricle, (2) increase in the heart rate, or (3) complete
heart-block associated with auricular fibrillation. When the heart is
irregular, the absence of the "a" waves signifies auricular fibrillation.
Delayed Conduction and Heart-block. — This causes a change in the
time relationship of the "a" and "c" waves in the jugular curve. When
the heart-block is of the -first degree, the "a-c" interval merely becomes
lengthened, but when it is of such degree that the normal impulse some-
times fails to be conveyed along the auriculoventricular bundle, isolated
•"a" waves can be detected. In the higher degrees of heart-block there
are regularly recurring "a" waves having no constant time relationship
to the ''c" waves. For the purpose of exact analysis of the curves in
suspected cases of delayed conduction, it is often advantageous to draw
vertical lines below the tracing representing the beginning of auricular
and ventricular systole. This has been done in the tracing reproduced
in Fig. 99.
The line joining these two verticals indicates the conduction time
or "a-c" interval. When it exceeds one-fifth of a second, there is
delay in the conduction time.
POLYSPHYGMOGRAMS
277
A tracing showing a higher degree of heart-block is given in Fig. 100.
Sinus Arrhythmia.— In this condition the radial pulse is markedly
irregular, but the "a," "c" and "v" waves of the jugular tracing occur
with the usual time relationship to one another, and there is no delay
in the "a"-" c" interval.
_
X
pjg. 99. — Delayed conduction time. First stage of heart-block. The A-C intervals measure more
than 0.2 second. (From E. P. Carter.)
Sinus Bradycardia. — The beat originates at long intervals in the
sinus; the "a-c" interval is normal, and the radial pulse very slow but
practically regular.
Premature Beats. — These may be either ventricular or auricular in
origin. In the former case the "a" waves on the jugular tracing space
regularly throughout, but the "c" waves at the point of disturbance
Fig. 100. — Dropped beats. Second stage of heart-block. (From F*. P. Carter.)'
coincide with the "a" waves, giving therefore a more pronounced wave.
This is due to a premature contraction of the ventricle occurring about
the time of the "a" wave, so that the latter finds the ventricle in a re-
fractory state (see page 178). The premature contraction is therefore
followed by a compensatory pause, which is evident on the tracing. An
example of such a case is given in Fig. 101. In doubtful cases the exact
278
THE CIRCULATION OF THE BLOOD
site of origin of the premature beats can be determined only by careful
measurement of the distances between the various beats of the ventricle.
Whenever an irregularity repeats itself and the duration of one cycle
of the arrhythmia accurately corresponds to another, the irregularity
1 I I I I
Fig. 101. — Premature beats (extrasystoles) ventricular in origin at PB. Compare the duration of
the intervals marked A and B' with those marked C and D. (From E. P. Carter.)
may be due to: (1) premature auricular or ventricular contractions;
(2) the occasional occurrence of dropped beats (a failure of ventricular
response) ; or (3) a high degree of heart-block with a wide variation in
the ventricular response. The important point to note here is that, no
matter how irregular such a tracing may appear, if the irregularity re-
peats itself it can not be due to auricular fibrillation.
Fig. 102. — Paroxysmal tachycardia. The paroxysms start at xx fallowing normal beats and
lasting for seven beats. The clue to "a," which falls with "v" after the first premature contrac-
tions, is found in the initial beat of the new rhythm. (From E. P. Carter.)
Paroxysmal Tachycardia. — When the rate of a regular pulse is sud-
denly altered but the change in rate bears a simple ratio to the previous
rhythm, the change may be due to (1) premature ventricular contrac-
tions which do not reach the radial, or (2) to the sudden development
POLYSPHYGMOGRAMS
279
of a two-in-one heart-block. When on the other hand, there is no exact
ratio between the slow and the rapid rate, the change is due to the sud-
den appearance or disappearance of paroxysmal tachycardia. The
paroxysms during which the auricle is beating very rapidly may last for
a variable time, such attacks persisting off and on for hours or even days.
The tracing in such a case is given in Fig. 102.
Fig. 103. — Auricular flutter. In this case the ventricular rate varied from 82 to 98 per minute.
(From E. P. Carter.)
Auricular Flutter. — It is impossible to diagnose the not infrequent
existence of this cardiac condition without the use of either the poly-
sphygmogram or the electrocardiogram. The jugular curve may be of
two types, one made up of rapid, more or less uniform waves, the other
of waves that are paired with a constant time interval between the pairs.
Fig. 104. — Auricular flutter. Note the relative rates of A and V, and also that the ventricular
rate is regular. (From E. P. Carter.)
All of the frequent beats of the auricle do not reach the ventricle in this
condition, so that the ratio between auricular and ventricular beats
may be 1:3 or 1:4. The condition must therefore not be confused with
heart-block, the main point of distinction being that in the latter condi-
tion the ventricular pulse is slow and the auricular about normal. The
280
THE CIRCULATION OF THE BLOOD
radial pulse may be regular or irregular. The cause for the failure of
every auricular beat to travel to the ventricle during auricular flutter
is partly the refractory condition of the bundle, and partly the refrac-
tory phase of ventricular contraction. The bundle may be considered
as a narrow bridge which will transmit the impulses across it only at a
certain rate. If the impulses arrive too rapidly, only some of them can
cross the bridge, and even of those that do cross, a limited number only
will find the ventricle in a condition of excitability because of the re-
fractory period (see page 178). Tracings showing auricular flutter are
^
. _
I s: K\ I w h* K* ha K\
V
Fig. 105. — Auricular fibrillation. Note the absence of all "a" waves from the jugular tracing, the
marked irregularity of the radial pulse, and the occurrence of "c" and "v" during the sphygmic
period. (From E. P. Carter.)
given in Figs. 103 and 104. In one of them the radial pulse is regular;
in the other, irregular.
Auricular Fibrillation. — The contractions of the auricle, as already ex-
plained, are entirely irregular, so that the jugular tracings show an en-
tire absence of all "a" waves,. the radial tracing being characterized by
the complete absence of a dominant rhythm and by great variation in the
length of the individual beats from one cardiac cycle to the next. This
irregularity does not repeat itself, and the long pauses are not simple
multiples of the shortest pause. Tracings from a case of auricular
fibrillation are shown in Fig. 105.
CHAPTER XXXII
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS (Cont'd)
THE MEASUREMENT OF THE MASS MOVEMENT OF THE BLOOD
Method. — The apparatus used for this purpose consists essentially of
a vessel containing a known quantity (3,000 c.c.) of water and a ther-
mometer from which a change of temperature of a hundredth of a de-
gree centigrade can be read. In order to diminish as much as possible
the loss of heat between the vessel and the outside air, the walls are
double, the space between being stuffed with broken cork. The top of
the vessel is closed with a thick cork plate, having suitable openings in
it for the hand or foot and for the thermometer and a stirrer (feather)
with which to keep the water in constant motion. The apparatus is called
a calorimeter.
After the hand or foot has been in the calorimeter, with the water a
few degrees below that of the body, for a certain time (ten minutes), the
temperature of the water will of course become raised, and the degree
to which this occurs, multiplied by the volume of the water in cubic
centimeters, will give in calories the amount of heat dissipated. By the
application of a very simple formula it is now an easy matter to calculate
how much blood must have passed through the blood vessels of the part
in order to give out the observed amount of heat; for, if we divide the
calories by the difference in temperature between the inflowing and out-
flowing blood of the part, the result must indicate the volume of blood, in
cubic centimeters, that has passed through it (since by definition a calorie
equals volume multiplied by difference in temperature). It remains to
explain the equation by which the results are arrived at. If Q equals the
amount of blood, H the calories of heat given out to the calorimeter, T
the temperature of the arterial blood and T' the temperature of the
H*
venous blood, then we have the equation : Q=™ — 7^7. It has been shown
*For the determination of H we must multiply the cubic centimeters of water plus the water
equivalent of the hand and calorimeter (because both of these will absorb some heat) by the dif-
ference in temperature plus the self-cooling of the calorimeter (because some heat is lost to the
air during the observation). The water equivalent of the hand is equal to its volume multiplied
by 0.8; that of the calorimeter must be determined for each instrument and is usually about 100 c.c.
The self-cooling of the calorimeter is determined by observing the fall in temperature for a period
equal to that of the actual observation without the hand in the calorimeter.
281
282 THE CIRCULATION OF THE BLOOD
by Stewart that T may be taken as the same as that of the mouth, or 0.5°
C. below that of the rectum, and T' as the average temperature of the
water in the calorimeter during the observation. To allow for the specific
heat of blood, the result is multiplied by-<r, the reciprocal of the specific
heat of blood.
Theoretically, then, the method is very simple, and there are no un-
usual technical difficulties in applying it. The only special precaution
is that the air surrounding the calorimeter should be kept fairly con-
stant in temperature, so that we may be enabled to allow in our calcula-
tions for the loss of heat from the calorimeter itself, this value being
obtained by observing the change of temperature in the calorimeter for
a certain period of time after the hand has been removed from it.
The Normal Flow
The results are calculated on the basis of grams of blood flowing
through 100 c.c. of tissue in one minute. The volume of the hand or foot
is ascertained by placing it in water contained in a small-sized irrigation
can, the tube of which is connected with a burette. The height to which
the water rises in the burette is noted, and after withdrawing the hand,
water is added from a graduate to the irrigation can until the same
height is reached on the burette. The number of cubic centimeters re-
quired gives the volume of the hand. In a normal, healthy individual
the average flow in the hand is from 12 to 13 gm. for the right hand,
and about half a gram less for the left. This difference between' the two
hands corresponds, of course, with their relative degree of development.
The average foot flow is much less, and varies according to whether the
patient is sitting up or lying down while the measurement is being made.
In a normal individual, while lying down, it was 5.11 gm. in the right
foot and 5.23 gm. in the left, per 100 c.c. of foot; but only 2.96 gm. for
the right and 4.1 gm. for the left foot, while sitting up. The results have
been found to undergo only a slight variation from month to month in a
given healthy individual, provided the air temperature during the dif-
ferent observations is the same and the person has been some time in the
room before the observations are begun. This precaution is especially
important if he is a dispensary patient and has been in the open air with
bare hands. The flow varies in different individuals both with regard
to absolute amount and the ratio between the two hands or feet. When
the total flow in the hands is compared with that in the feet, a ratio of
about 3 to 1 is usually obtained.
The Physiologic Causes for Variations in Bloodflow. — As above indicated,
the most marked of these is probably the temperature of the room. The
MEASUREMENT OF MASS MOVEMENT OF BLOOD 283
temperature of the water in the calorimeter has likewise a great influence,
and for the comparison of diiferent cases it is always important that the
room and calorimeter temperatures be stated alongside the results. Muscular
contractions, produced by moving the fingers in the calorimeter, cause a
marked increase in flow, accompanied by a diminished flow in the hand
that was at rest. A great diminution of flow results from constriction of
the arm of sufficient degree to obstruct the venous circulation ; and when
the constriction, as that caused by a blood pressure armlet, is increased to
between the systolic and diastolic pressures, extremely little blood flows
through the hand.
By immersing the opposite hand or foot in hot or cold water, the blood-
flow through the observed hand increases or decreases, respectively.
The change may be of a temporary character, or it may persist through-
out the whole period of immersion of the hand. These reactions are due
to a vascular reflex, and observations of its sensitiveness are of value in
the study of the effects of lesions either of the nerve or of the nerve
centers concerned in vascular reflexes.
Clinical Conditions which Affect the Bloodflow
Even in cases where there is plenty of other evidence of curtailment
of flow, the measurement may be of importance either for detecting
an alteration in the vascular reflex or, by comparison of the two
hands, for demonstrating the relative degree of alteration in flow. In
acute inflammatory conditions affecting one hand, there is an increase
in flow on the affected side accompained by a marked curtailment on
the other side. This indicates that an increased flow in the infected
area is accompanied by a reflex vasoconstriction elsewhere, particu-
larly in the symmetrically placed part of the opposite side of the
body. In cases of nonbacterial inflammation of the hand, as in gout,
no sign of vasoconstriction may be observed.
There are many clinical conditions in which Stewart's method re-
veals an alteration in bloodflow that would be unsuspected by the use
of ordinary clinical methods. It is for the investigation of these that
the method is of greatest value. The most important findings are as
follows:
Anemia. — The bloodflow in the hand may be much less than normal
in pernicious anemia and secondary anemia, and distinctly curtailed
in chlorosis. Since the minute volume of the heart is also increased
in these conditions, the vasoconstriction at the periphery will assist
in compelling more blood to pass through the lungs, so as to make up
for deficiency of blood.
284 THE CIRCULATION OF THE BLOOD
Fever. — Since changes in the cutaneous circulation probably con-
stitute the chief factor in the derangement of the temperature-regu-
lating mechanism in fever (cf. page 723), it is evidently of great ad-
vantage to be able to measure such changes quantitatively. This has
been done by Stewart in several cases of typhoid fever and in one case
of pneumonia. In general it was found that the flow in the feet never
exceeded the normal flow, and was usually much below it. This ten-
dency to vasoconstriction seems to be carried into convalescence. For
practical reasons the handflow has not been so extensively studied.
This hyperexcitability of the vasoconstrictor mechanism at the periph-
ery is most naturally interpreted as a defensive reaction of the or-
ganism by which an increased supply of blood is imported to those
internal organs which bear the brunt of the infection. When we con-
sider that in spite of this constriction of the periphery the blood pres-
sure is low and the pulse dicrotic, we must conclude that there is con-
siderable dilatation of other vascular parts, especially the splanchnic
area. A very practical application of these facts presents itself in con-
sidering the rationale of the cold-bath treatment for fever. If, for
example, we conclude that the cutaneous constriction is in the inter-
ests of an increase in the bloodflow to the organ on which the stress
of the infection falls, it will evidently be more rational to lower the
temperature by methods which will not diminish, and may even in-
crease, -the cutaneous constriction than to do so by causing the vessels
to dilate. In other words, the use of antipyretics seems to be contra-
indicated, since they diminish the body temperature by causing vaso-
dilatation at the periphery with a consequent withdrawal of blood
from the seat of infection.
Cardiovascular Diseases. — In cardiac cases the handflow is far more
apt to be markedly deficient where there is evidence of serious impair-
ment of the myocardium than in cases where a gross valvular lesion
exists but the heart action is strong and orderly. This indicates that
it is more serious for the force of the heart pump to be interfered
with than for its valves, particularly the mitral, to be leaky. Even
wjiere there is considerable venous engorgement, the flow may be lit-
tle diminished. In untreated cases of auricular fibrillation the blood-
flow is subnormal. After the administration of digitalis the bloodflow
in such cases is often promptly and decidedly increased.
As would be expected, arteriosclerosis is associated with a small blood-
flow, and the vasomotor reflexes are weaker than in normal persons.
In aortic aneurism, when the aneurism is of such a size as to cause
pressure on the subclavian artery or vein, there is a diminution in flow
of the corresponding hand, but aortic aneurism itself, although it may
MEASUREMENT OF MASS MOVEMENT OF BLOOD 285
cause -great changes in the character of the pulse beat, does not decid-
edly affect the mass movement of the blood. In aneurism of the sub-
clavian artery, the bloodflow may be much greater in the corresponding
than in the opposite hand, even though the amplitude of the pulse is
very obviously diminished and the difference between the systolic and
diastolic pressures (the pressure pulse) is much less on the affected
than on the normal side. By ordinary clinical measurements, there-
fore, false estimates of bloodflow are quite likely to be made. These
results are no doubt owing partly to vasodilatation brought about by
pressure of the aneurism on the brachial plexus and partly to the
lower resistance to the flow of blood into the dilated subclavian.
In Eaynaud's disease, as would be expected, the flow is small, the
diminution being more or less proportional to the duration of the
disease. The contralateral vasomotor reaction to cold is also pecu-
liarly intense.
In diabetic gangrene of the feet there is a very subnormal flow in both
the hands and the feet. The vasomotor reflexes are also feeble.
It is sometimes difficult to tell whether an observed curtailment of
flow is a nervous (reflex) effect or is due to some mechanical interfer-
ence. There are two ways by which the exact cause may be diagnosed:
(1) by observing the flow from day to day; if it remains unchanged,
any alteration must be dependent on mechanical causes; (2) by observ-
ing the change in flow brought about by altering the temperature of the
room or calorimeter and seeing whether the ratio between the two hands
remains unchanged or becomes altered. If the latter occurs, the in-
equality in flow must be due to nervous causes.
Diseases of the Nervous System. — The effect of neuritis on the flow
varies with the duration of the disease. In cases of early peripheral
unilateral neuritis there may be an increase of flow altering the ratio be-
tween the two hands with the greater flow on the diseased side. In
neuritis of long standing the flow is cut down, the greater flow occurring
on the healthy side. The changes here are probably due to anatomic
alterations in the lumen of the tube, perhaps a thickening of the intima.
In motor-neuron disease without any involvement of the sensory skin
nerves the flow seems to remain normal and the reflexes to be well-
marked. This indicates that involvement of the motor nerves does not
interfere with bloodflow to anything like the same degree as involvement
of the skin nerves.
Hemiplegia. — A deficiency of bloodflow of the paralyzed side is usually
observed, and the vasomotor reflexes are altered, the most usual change
being that vasoconstriction is more easily produced than vasodilatation.
286 THE CIRCULATION OF THE BLOOD
In some cases an abnormal tendency to vasoconstriction is a conspicuous
feature.
Tabes Dorsalis. — The flow is distinctly diminished, especially in the
feet, although also in the hands, and the vasomotor reflexes are feeble.
Sometimes there is inequality in the flow of the two hands, which how-
ever need not necessarily indicate a unilateral lesion of the cord in the
cervical region.
CHAPTER XXXIII
SHOCK
Shock may be due to a variety of causes. In general it may be de-
scribed as a condition in which there is more or less paralysis of the
sensory and motor portions of the reflex arc, along with profound dis-
turbances in the circulatory system, subnormal temperature, frequent
and shallow respiration, and more or less unconsciousness. Certain of
these symptoms may be considered as primary and others as secondary,
an important step in the investigation of this difficult and important
problem being to distinguish between the two groups. Before attempt-
ing to do this, however, it will be profitable to differentiate as sharply
as possible the various conditions in which one or another of the many
varieties of shock is said to occur.
The following varieties of shock have been described:
1. Gravity Shock. — This is caused by the stagnation of blood in the
splanchnic vessels and the consequent inadequate filling of the heart in
diastole. It occurs, when the erect position is assumed, in animals in
which the mechanism which ordinarily compensates for the tendency of
gravity to make the blood flow to the dependent parts is inadequate.
Thus, when a domesticated rabbit with a large pendulous abdomen is
held in the vertical tail-down position for any length of time, the animal
gradually passes into a shocked condition and may die in a short time
(20- to 30 minutes). Observation of the blood vessels of the ear or a
record of arterial blood pressure will show that the cause of shock in
this case has been a great curtailment of the blood supply to the upper
part of the body, and therefore to the nerve centers (Eig. 244). The
shock is entirely dependent upon the laxity of the abdominal muscula-
ture, for if a binder is applied to the abdomen, or if the experiment is
performed on a rabbit wrhose abdominal musculature is in good condi-
tion, gravity shock does not develop. Nor can fatal gravity shock be
produced in a dog, although in a deeply anesthetized animal a
marked fall in arterial blood pressure occurs when the vertical tail-
down position is assumed. In man, in whom compensation for the erect
posture is highly developed, shock from gravity occurs only when there
has been some other considerable upset in the circulatory mechanism
(see also page 245).
287
288 THE CIRCULATION OF THE BLOOD
2. Hemorrhagic Shock. — Free hemorrhage produces a typical condi-
tion of shock, but the extent to which different individuals react to the
same degree of hemorrhage varies considerably. The essential factor in
the production of hemorrhagic shock is of course similar to that of grav-
ity shock — namely, a deficient diastolic filling of the heart with blood.
Details concerning the effect of hemorrhage will be found elsewhere
(page 135).
3. Anesthetic Shock. — So far as blood-pressure reflexes are concerned,
an animal can be kept in a perfect condition when ether is administered
in just sufficient amount to produce light anesthesia. When larger
quantities of ether are employed, a typical condition of shock is almost
certain to supervene after a time. In such instances the arterial blood
pressure remains low and can not be restored even after an hour or two
of artificial respiration. There is, however, a difference between ether
shock and the variety which we shall discuss later under the title of
surgical s,hock : in the former, removal of the anesthetic causes all reflexes to
return, whereas in surgical shock most of these are subnormal. The danger
of anesthetic shock has been considerably diminished in the clinic by
the more careful administration of ether or by the use of other anesthet-
ics, such as nitrous oxide gas. A condition closely simulating shock
may also be induced in the earlier stages of the administration of anes-
thetics when these are badly given, but paralysis of the heart or of the
respiratory center is a usual contributory cause.
4. Spinal Shock. — Spinal shock is produced by section of the spinal
cord, but it is to be carefully distinguished from all other forms of shock
because of its local character, as it affects only those parts of the body
which lie below the level of the lesion in the cord. Above this level the
animal may be in a perfectly normal condition, except in cases where
the section has been at so high a level that it has severed the vasocon-
strictor pathway and thereby produced a fall in blood pressure from
vasodilatation. Even when this has happened the part of the animal
anterior to the spinal lesion is by no means in a condition of shock. Thus,
Sherrington observed in a monkey whose spinal cord had been cut far
forward that, although the posterior part of the body was in profound
spinal shock and the blood pressure very low, the animal amused him-
self by catching flies with his hands. A sufficient description of the con-
dition of spinal shock has been given elsewhere, but here it may be noted
that it consists essentially in a paralysis involving at first all of the re-
flex mechanisms, including the control of the sphincters, in the part of
the cord posterior to the section. In the course of a few days or weeks,
according to the position of the animal in the scale of development, the
reflexes gradually return, until ultimately in a couple of months — in a
SHOCK 289
dog, for example — they may all have reappeared. The cause of this
shock is no doubt the sudden interruption of the nervous pathways
which reflex action ordinarily takes in the higher animals (see page 803).
5. Nervous Shock; "Shell Shock." — Considerable attention has been
paid to the nervous shock that has frequently been observed in men who
have been subjected to the harrowing sights and the constant noise and
nerve strain incurred in modern warfare. The symptoms may appear
suddenly at the front or they may develop in men who have comported
themselves in an apparently normal manner until removed to the rear,
when they pass into a condition more or less simulating that of shock.
Severe conditions may also result to soldiers from injuries which in nor-
mal individuals would not in themselves be sufficient to produce sur-
gical shock. The characteristic symptoms in such cases are entirely
different frOm those of other forms of shock, and, as has been shown by
Elliot-Smith and T. H. Pear,25 the condition must be treated from the
neurologic or psychopathic point of view.
6. Surgical Shock. — It is this variety that is usually referred to when
one speaks of shock. It may be produced either by severe mechanical
injury to a healthy person or by extensive manipulation and rough
handling on the operating table. It is common in trench warfare, be-
ing therefore an important variety of "shell shock," which term must
be used only in a general sense. However produced, the symptoms of
surgical shock are very much the same. The patient lies in a quiet,
apathetic condition, caring little for what is going on around him, and
answering questions only when repeatedly and importunately questioned.
His skin, lips and gums are very pale and more or less cyanotic ; the skin
feels cold and is moist with sweat; the reflexes are greatly diminished,
and it is usually only after applying a very painful stimulus that any
movement of defense is elicited or resentment is shown on the part of the
patient. The postural reflexes are' also abolished, so that if a limb is
lifted it falls back limp and toneless. The pulse at the wrist is very
rapid, thin and almost imperceptible, .and the arterial blood pressure is
abnormally low. The respirations are frequent and shallow. The rec-
tal temperature is 1° C. or more below normal. The pupils are dilated
and react slowly to light. "When he can be induced to speak, the pa-
tient's voice is hoarse, and he complains of cold and numbness in the
extremities. The symptoms are not unlike those of cholera.
Experimental Investigations of Shock
For inducing shock experimentally, two general methods have been
employed: either rough manipulation of the abdominal viscera, or re-
290 THE CIRCULATION OF THE BLOOD
peated stimulation of large afferent nerves. Since the experiments are
usually performed on anesthetized animals, the effect of the anesthetic
has to be discounted in experimental work on the causes of shock.
The first step in such an investigation is naturally to classify the
symptoms into primary and secondary, for on the success of the classi-
fication must depend the outcome of further investigation into the
problem.
The earlier investigators were naturally attracted to the pronounced
fall in Hood pressure as the primary cause of shock. It is true that a
pronounced lowering will ultimately produce symptoms that are not
unlike those of shock, but on the other hand it can readily be shown that
this is a result only — a symptom and not a cause of the condition. It
was believed by Crile that the fall in blood pressure depended on a
universal dilatation of the blood vessels caused by exhaustion of the tone
of the vasoconstrictor center. It has been clearly shown, however, that
the tone of this center is practically normal in shock, and that the arte-
rioles are maintained not in a dilated but in a contracted state, indicat-
ing clearly therefore that the low blood pressure must be dependent
upon inadequate output of blood from the heart. The evidence for this
conclusion is as follows: (1) W. T. Porter26 and his collaborators have
shown that both pressor and depressor reflexes are perfectly normal
in a rabbit that is in a condition of extreme shock. It is particularly im-
portant that depressor effects were still obtained, since this indicates
that tonic activity of the center must still have been present. (2) The
blood vessels in a shocked animal are in a contracted state. On opening
a vess'el and observing the outflow of blood, an increase occurs when the
nerve to the blood vessel is cut. (3) This same fact has been shown by
Seelig and Joseph,27 who cut the vasomotor nerve proceeding to the
vessels of one ear of a white rabbit and thus caused a local paralytic
dilatation of the vessels. Intense shock was then produced in the animal
in the usual way, after which the blood pressure in the anterior part of
the animal was suddenly raised by applying a clamp to the abdominal
aorta just below the diaphragm. This increased blood pressure caused
the vessels of the denervated ear to become engorged with blood, but
not those of the opposite normal ear, which retained their tone (Fig.
106). (4) The volume of blood expelled by the ventricles has been
shown by Henderson28 to be distinctly diminished in the early stages of
shock, the lack of pronounced fall in blood pressure indicating that there
must be a compensatory constriction of the arterioles. Lastly (5), it
has been found by Morrison and Hooker29 that the outflow of blood
from the perfused organs of a shocked animal is less than that from the
Fig. 106. — Illustration showing the appearance of the blood vessels in the ears of a rabbit
"in a state of deep shock." The marked vasoconstriction is very plain in the left ear, the ves-
sels of the right ear being dilated because the cervical sympathetic, which carries the constrictor
fibers, has been cut. (From Seelig and Joseph.)
SHOCK 291
same organs under normal conditions. Furthermore, severing of the
nerve of such an organ causes an increased outflow.
To these various pieces of evidence of a constricted condition of at
least certain of the vessels in shock, may be added the less direct evi-
dence furnished by the pallor of the shocked patient and the indications
that the sympathetic nervous system, instead of being paralyzed, is
in an excited state, as shown by the sweating and the dilated pupils.
Furthermore, we know from the experiments of Pike, Guthrie and
Stewart30 on the resuscitation of the nerve centers after interference
with the circulation to the brain, that the vasomotor center is remark-
ably resistant to anemia. It can withstand this condition without losing
its tone or reflex activity better than any of the other cardinal centers.
Those who have maintained that a deficiency in the tone of the vaso-
constrictor and other nerve centers is responsible for shock have based
their evidence partly on histologic examination of nerve cells of shocked
animals, it being assumed that the chromatolysis shown by these cells
indicates an exhausted condition. The assumption is, however, entirely
unwarranted, and no regard is given to the well-established fact that
similar histologic changes may be produced by other conditions. It
is certainly safe to conclude that the changes in the nerve cells in shock
are the result and not the cause of the low blood pressure of this
condition.
Since the fall in arterial blood pressure occurs with contracted ar-
terioles, it must be dependent on a diminished discharge of Hood from
the heart. Interference with the heart action itself (independently of
the blood carried to this organ), or a deficiency in the filling of the ven-
tricles during diastole, — that is, a stasis of blood in the venous or cap-
illary areas, — are the possible causes for the diminished output. The
possibility that the heart action itself has been interfered with, as by
paralysis of the vagus mechanism, causing a rapid beating of the heart,
has been shown to be untenable by various experiments. After stimulat-
ing the central end of an uncut vagus nerve in the neck in shock, the
reflex vagus mechanism is still operative. Furthermore, when the arte-
rial blood pressure is artificially raised, either by epinephrine injection
or by cerebral compression, the heart promptly responds to the in-
creased blood pressure by contracting more slowly and vigorously.
Evidently, therefore, as the cardiac mechanism itself is normal, the de-
ficient discharge of blood must be dependent upon improper diastolic
filling. After this condition has set in, it becomes progressively worse
because of weakening of the heart muscles consequent upon the failing
blood supply through the coronary vessels.
The question therefore narrows itself down to the cause of the ineffi-
292 THE CIRCULATION OF THE BLOOD
dent return of venous blood to the heart. In the first place, let us see
whether shock can be produced experimentally in animals by mechanical
interference with the bloodflow in the vena cava. That such is the case
was shown by H. H. Janeway and Jackson,31 who found that mechanical ob-
struction of the inferior vena cava for a short time was followed by the usual
signs of shock. Such interference with the venous return to the heart
may also be produced by excessive movements of the thorax as a re-
sult of artificial respiration. That this in itself may cause shock is known
to all experimental investigators on the subject, although the interpre-
tation has not always been that which is given above. Yandell Hen-
derson32 thought that % the excessive ventilation caused a blowing off.
of carbon dioxide from the blood (see page 293), thus producing a
low tension of this gas in the blood (acapnia), which he believed to be
the responsible factor.
As in gravity shock, so in surgical shock, stagnation of blood in the
splanchnic area is common; the animal bleeds into his own (splanchnic)
blood vessels (capillaries and venules), because these have lost their tone.
As we have noted above, one of the most certain ways of producing
shock is by exposure and rough handling of the abdominal viscera. It
is therefore of importance to study the effects that can be noted on
the blood vessels of this area under such conditions. When the viscera
are first exposed to air, there may be a short period during which vaso-
constriction is evident. This is soon followed by a dilatation of the
arterioles in the exposed area, causing the capillaries and veins to be-
come markedly distended as during the first stage of inflammation. This
accumulation of blood in the mesenteric veins has been shown by Mor-
rison and Hooker to cause an increase in the weight of an isolated loop
of intestine as an animal passes into a state of shock.
Splanchnic engorgement alone does not, however, suffice to explain all
the loss of blood, and we are driven to conclude that the capillaries of
the tissues outside the abdomen must entrap much of it. As a matter of
fact, Cannon, and others, have found that concentration of the blood
occurs in these capillaries as indicated by comparisons of the corpuscles
and hemoglobin in blood drawn from veins and from capillaries. Nor-
mally the values are equal; in shock the capillary blood is much con-
centrated.
In so far as the circulatory disturbances are concerned, we may there-
fore sum up the conditions occurring in shock as follows: The blood
accumulates in the veins and capillaries — that is, in a part of the vas-
cular system that is beyond vasomotor control. The consequent with-
drawal of this blood from the circulation produces a diminution of the
bloodflow in the vena cava and consequently an inadequate filling of the
SHOCK 293
heart. The consequent curtailment in the systolic discharge does not,
however, at first cause any marked fall of arterial blood pressure be-
cause of a reciprocal constriction of the peripheral arterioles of the
body. Meanwhile, however, the stagnation of blood in the capillary areas is
progressively increasing, so that less and less blood remains available
for the systemic circulation. Consequently, after a while, in spite of
the arterial constriction, the blood pressure falls to the dangerous shock
level, and the secondary symptoms of -fall in temperature, dulling of the
reflexes, etc., supervene. Increasing viscidity of the blood also retards
its flow..
The fundamental question in the pathogenesis of shock concerns there-
fore the cause of the stagnation of circulatory fluid in the capillaries and
venules. Two hypotheses have been offered, one being that the stimulation
of afferent nerve fibers to the respiratory center causes excessive alveolar
ventilation with a consequent washing out of carbon dioxide from the
blood (acapnia), which causes a veno-capillary atonia, and the other,
that a bombardment of the vasoconstrictor and other nerve centers
by afferent impulses brings these centers into a condition of exhaus-
tion, which is the essential cause of shock. The acapnia hypoth-
esis may be at once dismissed, since, on the one hand, it has been
shown that in typical shock there is no deficiency of carbon dioxide in
the venous blood (Short),33 and on the other hand, conditions of shock
are often produced without excessive breathing.
Nor is there any evidence to support the view that shock- is caused by
fatigue of the cardinal centers as a result of excessive sensory stimu-
lation. In the first place, it has been shown by Mann34 that during han-
dling of the abdominal viscera the nervous impulses transmitted up the
spinal cord are much less marked than those transmitted when the cen-
tral ends of sensory nerves are stimulated by operative processes on the
limbs and joints, although shock is much more readily produced by the
former procedure. The method employed by Mann for detecting the
existence of these afferent impulses was that of Forbes and Miller, in
Avhich electrodes are placed on the brain stem in decerebrate animals,
and the current of action which accompanies the passage of nerve im-
pulses registered by a string galvanometer. Although this method is
simple and direct in principle, it has been found by Mann to require
great care in practice because of the fact that the slightest movement
of the head end of the animal produces deflections of the galvanometer.
If the further results of this investigation should show, as the early
ones have done, that shock may be produced in an animal without any
observed deflection of the galvanometer, it will disprove once and for all
294 THE CIRCULATION OF THE BLOOD
the theory that shock is dependent upon an impairment of a higher
nerve mechanism as a result of overstimulation by afferent impulses.
Cannon35 has recently suggested that the engorgement of the splanch-
nic blood vessels may be the result of a constriction of the portal rad-
icles in the liver, which dams back the blood in the portal circulation.
He points out that these radicles have vasoconstrictor nerve fibers, as
evidenced by the fact that the rate of flow of fluid through the per-
fused liver decreases during asphyxia, as well as when the hepatic
nerve plexus is stimulated electrically or when epinephrine is injected
into the portal vein. He argues that, since the blood vessels in other
areas of the body are constricted in shock, so also will be those of the
liver, with the result that the blood of the portal vein, in which ordinarily
there is a very low blood pressure (10 mm.Hg), will become dammed
back in these veins and therefore removed from the systemic circulation.
It does not seem to the writer, however, that this explanation is likely
to be the correct one, for, although it is true that vasoconstrictor in-
fluences have been shown to exist in the hepatic radicles of the portal vein,
yet, since it is only under special experimental conditions that this can
be done, they must be very feeble in nature. As we have seen else-
where, portal vasoconstriction can not be demonstrated by stimulation
of the hepatic plexus with stimuli which are sufficient to produce marked
constriction of the hepatic artery radicles (see page 255).
The engorgement of the splanchnic capillaries and venules is much
more likely to be dependent upon local influences acting on the vessels
themselves. When shock is produced by manipulation of the abdominal
viscera, it is easy to see how this local disturbance might be set up.
When shock is caused in other ways, as by violent stimulation of sen-
sory nerves, the atony of the splanchnic vessels is not so easily accounted
for unless we assume that it is a type of abnormal reciprocal vascular
innervation. For example, when stimuli are applied locally to sensory
surfaces under ordinary conditions, a distribution of the blood of the
body takes place, more being sent to the irritated region and less to
other parts of the body (see page 238). During the sensory stimula-
tion preceding shock, it is conceivable that this reciprocal innervation
acts in a faulty manner, causing at first a dilatation of the splanchnic
arterioles and thus allowing more blood to enter the splanchnic capil-
laries and venules, which being possessed of little tone are incapable
of responding by increased tonicity, so that they become overdistended
and the blood in them stagnates.
In any case there is no doubt that the initial change is the stagnation
of blood in these vessels, and when once such stagnation has occurred,
the process goes on spontaneously probably on account of the accumula-
SHOCK 295
tion in the stagnant blood of incompletely oxidized metabolic products,
which raise the hydrogen-ion concentration of the blood, and produce
a further relaxation of the muscle fibers in the vessel walls. That acid
has such an effect is well known (page 937). Dilatation or atonicity thus
progressively increases and is meamvhile further encouraged by the de-
privation of oxygen, for it has been shown that isolated artery strips do
not exhibit their usual tonicity when deprived of oxygen.
Treatment
Whatever may be the cause of the atony of the capillaries and
venules in shock, the existence of this condition indicates that the most
hopeful line of treatment is to cause the vessels to reacquire their tone.
It will be remembered that in gravity shock in a rabbit recovery may
be accomplished by the application of a tight binder to the abdomen,
or by placing the animal in a head-down position. Such measures ap-
plied in the case of man have not, however, been found of much value.
Pressure thus applied is evidently not brought to bear sufficiently on
the atonic vessels. Cannon has therefore made the interesting suggestion
that a hopeful procedure may consist in injecting directly into the ab-
domen a saline solution containing pituitrin, a hormone which, it will
be remembered, acts directly on involuntary muscle fiber.
Two other methods have been advocated for the treatment of shock —
namely, saline or blood transfusion and injection of epinephrine; but
neither method has proved of practical value. Epinephrine injections
do indeed temporarily raise the arterial blood pressure, but the subse-
quent condition of shock is possibly worse than that originally present.
After the injection of blood or saline solution containing gelatin or
mucilage, the blood pressure, although temporarily raised, very quickly
falls again. In this regard surgical shock differs from the shock follow-
ing severe hemorrhage, in which, as explained elsewhere, recovery of the
blood pressure as well as of the general condition of the animal can
be accomplished by transfusion with blood or with saline solution con-
taining mucilage or gelatin. This would indicate that there is some
essential difference between these two forms of shock (see page 140).
The only treatment of avail appears to be to keep the patient warm and
to remove causes of excessive pain.
Causes of Secondary Symptoms
It remains to consider the cause of some of the secondary conditions
developing in shock — namely, the disturbances in sensation and motion
and the fall in body temperature. All of these are undoubtedly depend-
296 THE CIRCULATION OF THE BLOOD
ent upon the low arterial blood pressure, although some authors have
suggested that the loss of sensation may be dependent upon an increased
resistance or block at the synapses of the receptor neurons (page 803).
This suggestion depends on the fact, demonstrated by Sherrington, that
repeated stimulation of the receptors of a reflex arc produces fatigue
of that particular reflex, and that this fatigue must be resident in the
synapsis and not in the motor neuron, since the same motor neuron
that participated in the fatigue can still be called into activity by afferent
stimuli transmitted to its nerve cell through other sensory pathways
(see page 825). It is thought that in shock the frequent afferent stimula-
tion produces synaptic fatigue and therefore dulls the sensory responses
of the animal. The researches of Mann above referred to, in which he
shows that shock may occur without any demonstrable afferent stimuli
in the brain stem, would seem, however, to negative the above hypothesis.
The raised threshold of sensory stimulation is no doubt an effect of the
low blood pressure. It has been shown, for example, by E. L. Porter36
that when the arterial blood pressure is maintained at a uniform level,
the threshold stimulus for spinal cord reflexes remains practically uni-
form, but becomes promptly increased when the arterial blood pressure
is made to fall. Why a lower blood pressure should have this effect is,
however, difficult to understand in the light of the researches of Stewart
and his coworkers, who, as remarked above, found that the cells of the
central nervous system may endure total anemia for many minutes and
still recover their physiologic condition. It may be, however, that the
low blood pressure affects the conductivity of the synapsis.
The muscular weakness is probably also dependent on low blood
pressure, for it has been found in animals that, when the arterial blood
pressure is lowered to about 90 mm. Hg, the muscles contract much less
efficiently than ordinarily. The fall in body temperature is dependent
upon the muscular inefficiency.
In conclusion, it should be pointed out that W. T. Porter, in the inves-
tigation of acute shock met with at the front, has found that, in many
cases at least, the circulatory disturbance is due to a condition of fat
embolism. The fat is derived from the marrow of long bones, such as
the femur, by injuries which smash the bones. Porter's observations
are at least very suggestive.
CIRCULATION REFERENCES
(Monographs)
Wiggers, C. J. : The Circulation in Health and Disease, Philadelphia, 1915.
Mackenzie, J. : Diseases of the Heart, Oxford Medical Publishers, ed. 2, 1910.
Lewis, Thomas: * Mechanism of the Heart Beat, 1911, Shaw & Son, Fetter Lane,
London.
SHOCK 297
Lewis, Thomas: Harvey Lectures, 1913-1914, J. B. Lippincott Co.
Lewis, Thomas: Clinical Disorders of the Heart Beat, P. B. Hoeber, New York, 1912.
Hill, Leonard: The Mechanism of the Circulation of the Blood, in Schafer's
Physiology, ii, 1900. Young J. Pentland.
Gaskell, W. H.: The Contraction of Cardiac Muscle, in Schafer's Physiology, ii,
1900, Young J. Peutland.
Flack, M.: Further Advances in Physiology, 1909. Ed. by Leonard Hill, E. Arnold,
London.
Porter, W. T.: American Text Book of Physiology, W. B. Saunders Co., 1900.
(Original Papers)
iMacWilliam, J. A., et al.: Heart, 1913, iv, 393; ibid., 1914, v, 153; Brit. Med.
Journal, Nov., 1914; VII Internat. Congress of Medicine, London, 1913, See.
II, Physiology.
2Hill, Leonard, F. E. S., et al: Proc. Eoy. Soc., 1914, B, Ixxxvii, 344; ibid., 1915, 3,
Ixxxviii, 508 and 516.
sErlanger, J.: Am. Jour. Physiol., 1916, xxxix, 401; ibid., 1916, xl, 82.
4Downs, A. W.: Am. Jour. Physiol., 1916, xl, 522.
sBayliss, W. M.: Proc. Eoy. Soc., 1916, Ixxxix, B, 380.
GKnowlton, F. P.: Jour. Physiol., 1911, xliii, 219.
?Milroy, T. H.: Jour. Physiol., 1917, Ii, 259.
sEyster and Meek: Heart, 1914, v, 119; ibid., 194, v, 137; Am. Jour. Physiol., 1914,
xxxiv, 368.
^Porter, W. T.: Art. on Circulation in an American Textbook of Physiology, W. B.
Saunders Co., 1900.
loBrodie, T. G.: Proc. Physiol. Soc., 1905, Jour. Physiol., 1905, xxxii.
"Stewart, G. N.: Heart, 1911, iii, 33.
i2Garrey, W.: Am. Jour. Physiol., 1912, xxx, 451.
isMines, G. E.: Jour. Physiol., 1913, xlvi, 188.
"Cohn, A. E.: Jour. Exper. Med., 1912, xvi, 732; Eobinson, G. Canby: Ibid., 1913,
xvii, 429; Cohn and Lewis, T.: Ibid., 1913, xviii, 739.
isMathison, G. C.: Jour. Physiol., 1910, xli, 416.
isPorter, W. T.: Am. Jour. Physiol., 1911, xxvii, 276; ibid., 1915, xxxvi, 418.
i^Martin, E. G., and co-workers: Am. Jour. Physiol., 1914, xxxii, 212; xxxiv, 220;
1915, xxxviii, 98; 1916, xl, 195.
isBayliss, W. M.: Proc. Eoy. Soc., 1908, Ixxx, B, 339.
"Hill, Leonard: The Physiology and Pathology of the Cerebral Circulation, J. and
A. Churchill, 1896.
2oHill, L., and Macleod, J. J. E.: Jour. Physiol., 1900, xxvi, 394.
2iMacleod, and Pearce, E. G.: Am. Jour. Physiol., 1914, xxxv, 87.
22p0rter, W. T.: Am. Jour. Physiol., 1898, i, 144.
23Hill, L., and Barnard, H.: Jour. Physiol., 1887, xxi, 323.
2*CarteT, E. P.: Jour. Lab. and Clin. Med., 1916, i, 719.
25Elliot-Smith, G., and Pear, T. H.: Shell Shock, Longmans, Green & Co., 1917.
26p0rter, W. T.: Am. Jour. Physiol., 1907, xx, 399.
27Seelig, M. G., and Joseph, D. E.: Jour. Lab. and Clin. Med., 1916, i, 283; also See-
lig and Lyon, E. P.: Surg., Gynec., and Obst., 1910, ii, 146.
28Henderson, Yandell: Am. Jour. Physiol., 1908, xxi, 155; also Mann: Bull. Johns
Hopkins Hosp., 1914, p. 210; Markwald, J., and Starling, E. P.: Jour. Physiol.,
1913, xlvii, 275.
29Morrison, E. A., and Hooker, D. E.: Am. Jour. Physiol., 1915, xxxvii, 86.
sopike, F. H., Stewart, G. N., and Guthrie, C. C.: Jour. Exper. Med., 1908, x, 499;
see also Dolley, D. H.: Jour. Med. Eesearch, 1909, p. 95, and 1910, p. 331.
siJaneway, H. H., and Jackson, H. C.: Proc. Soc. Exper. Biol. and Med., 1915, xii,
193; Erlanger, J.: Gesell, Gasser, Proc. Am. Physiol. Soc., Am. Jour. Physiol.,
1918, xlv.
298 THE CIRCULATION OF THE BLOOD
32Henderson, Y., and Haggard, W. H.: Jour. Biol. Chem., 1918, xxxiii, 333, 345-355-
365 (gives older references). See also Maeleod, J. J. E.: Jour. Lab. and Clin.
Med., (editorial), 1918, iii.
ssShort, Kendel: Lancet, London, 1914, p. 131.
3<Mann: Jour. Am. Med. Assn., 1918, Ixx, (511. Also Boston Med. and Surg. Jour.,
1917.
ssCannon, W. B.: Papers by Cannon and Collaborators in Jour. Am. Med. Assn., 1918,
Ixx, 520, 526, 531, 611, 618.
aeporter, E. L.: Proc. Am. Physiol. Soc., Am. Jour. Physiol., 1916, xlii, 606.
37Wiggers, C. J., and Dean, A. L.: Am. Jour. Physiol., 1916, xlii, 476; Am. Jour.
Med. Sc., 1917, clii, 666.
PART IV
THE RESPIRATION
CHAPTER XXXIV
RESPIRATION
For convenience, the physiology of respiration may be considered un-
der its mechanics, its control, and its chemistry.
THE MECHANICS OF RESPIRATION
Of the many factors concerned in maintaining the normal functioning
of the animal body, the respiratory act is probably the most important.
On this account and also because we are conscious of the respiratory
movements, the physiology of respiration has been studied from the
earliest times. Much of the earlier work naturally concerned itself
with the study of the air that enters and leaves the lungs at each respi-
ration— the ventilation of the lungs, as it may be called. Two obvious
properties of the respired air are: (1) its pressure and (2) its volume.
The Pressure of the Air in the Respiratory Passages — the Pulmonary
- or Intrapulmonic Pressure
This is readily measured by inserting a tube into one nostril and con-
necting the tube with a manometer; at each normal inspiration the
manometer registers a negative pressure of 2 or 3 mm. Hg, and at each
expiration, a positive pressure of about the same degree. Although
normally of small magnitude, the intrapulmonic pressure may become
very great when any obstruction is offered to the free passage of the
air. The greatest possible expiratory pressure can be measured by sim-
ply blowing into a mercury manometer, when it will be equal to that
which all the muscles of the thorax and abdomen can exert in compress-
ing the lungs. In a strong man it may amount to more than 100 mm.
Hg. Similarly, the greatest possible negative pressure on inspiration
may be measured by attempting to inspire through a tube connected
with a manometer. It represents the force with which the musculature
299
300 THE RESPIRATION
of the thorax and abdomen can open up the thoracic cage, and may
equal -70 mm. Hg. These measurements in themselves are not of much
importance, except as a measure of muscular development.
Intrapulmonic pressures that are intermediate between the two ex-
tremes will be acquired in the lower air passages in cases in which there
is partial obstruction of the upper respiratory passages, as in bronchitis,
spasm of the glottis, diphtheria, etc. During coughing also, the intra-
pulmonic pressure may become very high. In this act the thorax is first
filled with air by a deep inspiration; the glottis is then closed, and a
forced expiration is made. When a sufficiently high intrapulmonic pres-
sure is attained, the glottis opens and the sudden change in pressure
causes so forcible a blast of air that the offending foreign substance is
frequently carried with it out of the air passages. It is often assumed
that during coughing the sudden increase in pressure in the alveoli will
tend to cause their walls to rupture. This, however, is not the case.
The alveoli do not alone support the increase of pressure; they merely
act as the inner layer of a practically homogeneous structure com-
posed of lung, pleura and thoracic cage. When the tissues of the lung
are partially degenerated or atrophied, as in old people, then it is pos-
sible that a rupture may take place, but under ordinary conditions it
is not likely to occur.
Amount of Air in the Lungs
Measurements of the amount of respired air have recently assumed a
considerable interest on account of the various applications which can
be made of them in the study of lung conditions. The tidal air is that
which enters and leaves the lungs with each respiration (about 500 c.c.) ;
the complemental air is that which we can take in over and above an
ordinary tidal respiration (about 1500 c.c.) ; and the supplemental air,
is that which we can give out after an ordinary tidal expiration (about
1500 c.c.). Taking these three together, we have what is known as the
vital capacity. It is usually about 3500 c.c., and is represented by the
amount of air which we can expel from the lungs after as deep an inspi-
ration as possible. The vital capacity is diminished in certain pulmo-
nary diseases (see page 314). After all the supplemental air has been
expelled, there still remains in the lungs a large volume of air which
can not be voluntarily expelled. This is known as the residual air. To
measure it in a dead animal it is necessary to clamp the trachea, open
the thorax, remove the lungs to a vessel of water, and then allow the air
to collect from the opened trachea in an inverted graduated cylinder.
One part of the residual air is sometimes called the minimal air; it is
RESPIRATION
301
represented by that which is not expelled from the lungs of a dead
animal when the thorax is opened. In the collapse of the lungs thus
produced, the alveoli are not completely emptied of air, because some
becomes pocketed within them and is expelled only when the lungs are
compressed under water.
The volume of the residual air can readily be measured during life
by causing a person, after a forced expiration, to take two or three
breaths in and out of a rubber bag containing a measured quantity of
an indifferent gas such as hydrogen. Suppose the bag to contain at
the start 4000 c.c. of hydrogen, and after a few breaths 3000 c.c. of
this gas and 1000 c.c. of other gases (the total volume of hydrogen and
expired air in the bag being still 4000 c.c.) ; then the residual air will
Maximum inspiration
Ordinary inspiration
TIDAL AIR
Ordinary expiration
Supplemental air
Ifuxirnum expiration
Residual air
5QOC.C.Or30Cukin.
2000 120
or ,
c c cub. in
, Vital capacity
Capacity of equilibrium
Fig. 107. — Amounts of air contained by the lungs in various phases of ordinary and of forced
respiration. (From Waller.)
be 1333 c.c., for it is evident that after a few breaths the composition of
the expired air in the bag will be the same as that in the lungs. This
calculation is based upon the assumption that no hydrogen is absorbed
by the blood during the experiment, which is not strictly the case.
The amount absorbed is, however, so small in two or three breaths as to
make it permissible to disregard it. The measurement can also be made
by taking a few breaths in and out of a bag containing pure 02. By
ascertaining the proportion of nitrogen that collects in the bag, the
quantity of residual air can be calculated. We shall see later that the
measurement of the residual air during life has some practical impor-
tance in connection with the measurement of the bloodflow through the
lungs.
302 THE RESPIRATION
Alveolar and Dead Space Air
In addition to these moieties of respired air, we have to consider the
division of the air in the lungs into what is called alveolar air and
dead-space air. The former is the air which comes in contact with the
epithelium through which gas diffusion between the blood and the air
occurs, the latter being the air which fills the respiratory passages. The
dead space can not be defined anatomically with exactitude ; it is func-
tional rather than morphologic.
Measurement of the volume of the alveolar and dead-space air can be
made in an animal breathing under normal conditions by taking ad-
vantage of the fact that, while it is in the lungs, the air has added to
it C02 gas, which is present in the inspired air only in negligible traces.
The necessary data are: (1) the volume of the tidal respiration; (2) the
percentage of C02 in alveolar air; (3) the percentage of C02 in the tidal
air. Suppose the values to be 500 c.c., 6 per cent and 4 per cent, re-
4
spectively; then the volume of alveolar air must be 500 x-^ = 333 c.c.,
and the dead space 167 c.c. The measurement so made is accurate only
when certain precautions are taken. Because of the practical impor-
tance of this part of our subject we shall, however, defer its further
consideration until we have become familiar with the general features
of pulmonary physiology. Since the first air to move into the alveoli
at the beginning of inspiration is that present in the dead space, — the
last air expelled from the alveoli on the previous expiration, — it is of
no value in purifying the air' already present in the alveoli. If we take
a tidal inspiration as amounting to 500 c.c. and the functional dead space
as 150 c.c., it is plain that only 350 c.c. of the outside air gains the
alveoli, and that the subsequent expiration is composed of 150 c.c. of
outside air that had lodged in the dead space plus 350 c.c. of alveolar air.
These facts deserve a certain amount of emphasis because of their
practical importance in many phenomena connected with respiration.
One seldom thinks, for example, that out of the 500 c.c. of air inspired
with each breath, only 350 c.c. reaches the alveoli, where it comes in
contact with the 2500-3000 c.c. of air already present in this part of the
lungs.
There must therefore be a sort of interface somewhere in the alveoli
between the fresh outside air that comes in with each breath through
the bronchioles and the air which is more or less stagnant in the alveoli.
This interface must move backward and forward somewhat with each
breath, and a rapid diffusion of oxygen and of C02 must take place
RESPIRATION 303
across it between the inspired air and that in the alveoli. It is impossible
to fix any anatomic point at which the interface occurs.
The above described mechanism for the ventilation of the alveoli in-
sures the maintenance of slight but constant changes in the composition
of the air next the alveolar epithelium. It helps to prevent sudden varia-
tions in the amount of gases in the blood, particularly of C02. Should
such variations occur, irregular stimulation of the respiratory and other
important centers that are influenced by the amount of this gas present
in simple solution in the blood, would be the result. The mechanism
serves as a sort of mechanical buffer ~by diminishing the sudden changes
in gas concentration produced l)y inspiration and expiration.
Respiratory Tracings
The measurements of air for the determination of the foregoing val-
ues are made by the use of meters of various types. Sometimes, how-
ever, it is necessary to obtain an inscribed record of the respirations.
Fig. 108. — Pneumograph. The straps (b, b) are held around the thorax, and the tube of the
tambour connected by rubber tubing with a recording tambour.
This may be either qualitative or quantitative. A. qualitative record is
taken by attaching some sort of receiving tambour to the thoracic wall
(the best type is shown in Fig. 108), and connecting this with. a record-
ing tambour arranged to wrrite on a blackened surface. When it is
desired merely to count the respirations or to observe their regularity,
such a tracing is all that is required, but obviously it does not tell us
how much air has entered and left the lungs at each respiration. To
obtain a quantitative tracing, we must either connect a recording instru-
ment with the trachea or inclose the body of the animal in what is
known as a body plethysmograph. In observations on laboratory an-
imals the best type of recording instrument to connect with the respira-
tory passages is the Gad or Krogh pneumograph. A body plethysmograph
as used in the case of man is shown in Fig. 109. All these instruments
must of course be calibrated, which is done by pouring a definite num-
304
THE RESPIRATION
ber of c.c. of water from a graduate into a bottle with which the record-
ing instrument is connected by tubing. The displacement of the writing
point gives us the necessary data for standardization.
The Intrapleural Pressure .
The air which we have just been considering depends for its move-
ment in and out of the air passages upon changes occurring on the outer
aspect of the lungs in the space between them and the thoracic wall.
This is called the intrapleural space. It does not really exist as an
actual space in the living animal, for the visceral pleura which covers
the lungs is in accurate and intimate apposition with the parietal pleura
on the inner aspect of the thorax.
Fig. 109. — Body plethysmograph for recording respiration.
Priestley.)
(From J. S. Haldane and J. G.
If the thoracic walls are punctured in a living animal or in one which
has recently died, the air will rush into the thorax, the two layers
of pleura separate, and the lungs collapse, causing temporarily a space
to be formed between the two layers of pleura and indicating that a
certain subatmospheric or negative pressure must exist in the intact
thorax to prevent the lungs from collapsing. The degree of this nega-
tive pressure may- be measured by connecting a tube and a manometer
with the thoracic cavity. While the thorax is at rest, as in expiration
or immediately after death, this pressure amounts to about -5 milli-
meters.* On inspiration it increases to -10 millimeters. There are there-
fore two problems to be considered: (1) the cause of the negative pres-
sure in the quiescent thorax, and (2) the cause of the increase of the
negative pressure during inspiration.
"The minus sign indicates that the pressure is negative or subatmospheric. It is a suction pressure.
RESPIRATION 305
The Permanent Negative Pressure. — Let us start with the changes
that occur in the thorax when the first breath is drawn. While the an-
imal is still in utero, the lungs completely fill the thorax. When the
first breath is drawn the thoracic cage expands more quickly than the
lungs, so that the latter become stretched, the stretching force being
the air that is introduced into them from the outside through the tra-
chea and bronchial tubes. On becoming stretched the lungs fill the
increased space created in the thorax by the greater expansion of the
thoracic cage. This in itself, however, would not explain the cause of a
subatmospheric pressure in the intrapleural space. Another factor must
come into play — namely, the elastic tissue of the lungs, which by the
expansion will become stretched and, therefore, tend constantly to re-
lax to its previous condition and so exert a pull on the structures be-
tween it and the thoracic wall. It is this elastic recoil which we really
measure when we connect a manometer with the intrapleural space.
Throughout life the lungs remain of smaller size than the thoracic wall,
and therefore to fill the thoracic cavity they are constantly more or
less distended and the elastic tissue somewhat stretched. The lungs
are, however, not the only structures in the thorax which become ex-
panded; all thin -walled vessels and viscera, like the veins, the esopha-
gus, the auricles, etc., must also become opened out a little.
When the thoracic wall is punctured and the outside air allowed free
entry to the intrapleural space, differences in pressure no longer exist
on the inner and outer aspects of the lungs, so that they collapse into
the postmortem condition on account of the elastic recoil. If a puncture
in the thoracic wall of a living animal is immediately occluded, the
lungs will expand again, because the blood absorbs the gases from the
intrapleural space and recreates the partial vacuum required to expand
the lungs. This absorption of gas in the pleural cavity is usually quite
rapid; but if the pneumothorax, as the condition is called, is allowed to
persist for any length of time, the lungs will not become properly ex-
panded again.
The Greater Negative Pressure on Inspiration. — The cavity of the tho-
rax becomes increased in all diameters during inspiration, with the re-
sult that a greater space in the pleural cavity has to be filled. All the
thin-walled structures in the thorax therefore become still more stretched,
the lungs of course participating to the greatest extent because of the
entrance of outside air. The stretching of the elastic structures causes
a greater pull, or negative pressure, to be exerted in the pleural cavity.
Instead of being -5 mm. Hg, as in expiration, the intrathoracic pressure
now conies to be above -10 mm. Hg.
When any obstruction exists in the air passages, the changes in intra-
306 THE KESPIRATION
thoracic pressure produced by the movements of respiration become
more pronounced than under normal conditions. "When the thorax ex-
pands with the trachea blocked, the lungs are not able to open up suffi-
ciently to fill all the space so that there is excessive dilatation of the
veins, auricles and esophagus, as well as drawing in of the intercostal
spaces and bulging upwards of the diaphragm. If a manometer is con-
nected with the pleural space under these conditions, a very large
negative or suction pressure will be observed, amounting often to -70
or -80 mm. Hg. It is possible that under such conditions some space
might temporarily exist between the parietal and visceral layers of the
pleura, but it could not remain long, for it would very soon be filled
by fluid exuding from the blood vessels. In the opposite condition, in
which the respiratory passages are blocked and a forced expiration is
made, as for example in the first stage of coughing or during such acts
as defecation and parturition, the thoracic cage is compressed upon the
viscera, with the result that the air in the lungs assumes a positive
pressure, amounting often to nearly 100 mm. Hg. If a puncture wound
is made in the thorax under these conditions, the lungs instead of col-
lapsing will bulge out of the wound, for what is really occurring is
that the thorax is forcibly contracting on occluded sacs of air.
It is the alternating changes in intrapleural pressure that are respon-
sible for the changes in intrapulmonic pressure and these for the move-
ment of air in and out of the lungs with each respiration. In other
words, the thorax does not expand on inspiration because air rushes
in, as the Uninitiated imagine, but air rushes in because the thorax
expands.
The Influence of Intrapleural Pressure on the Blood Pressure. — The
movements of respiration produce effects on the vascular system that
are of considerable importance in maintaining the circulation of the
blood. If an arterial blood-pressure tracing is examined, it will be
observed that aside from the cardiac pulsations large waves exist on it that
are approximately synchronous with the respiratory movements, the
upstroke of each of these waves corresponding in general with inspira-
tion, and the downstroke with expiration (Fig. 22). These respiratory
variations in blood pressure might be due either to changes in heart
rhythm or to a purely mechanical cause. Regarding the first possi-
bility, it is indeed the case in most animals that the pulse is quicker on
inspiration than on expiration, but that this alone is not an adequate
explanation of the rise is shown by the fact that it still persists after
the vagus control of the heart has been eliminated, either by cutting
the nerve or by the action of atropine.
The cause must therefore be a mechanical one. Bearing in mind the
RESPIRATION 307
effects which we have seen are produced on the movement of air in and
out of the lungs by the changes in capacity of the thorax with each res-
piration, we naturally assume that the increase in blood pressure may
be due to the fact that on inspiration more blood is sucked out of the
systemic veins into those of the thorax, that this excess when it is pro-
pelled by the heart into the arteries raises the blood pressure, and that
on expiration the opposite condition obtains. That the movements of
the thorax on inspiration do accelerate the speed with which the venous
blood is traveling towards the heart can easily be shown by measure-
ments of bloodflow.
This explanation, however, does not suffice to account for all the
changes of blood pressure which occur in respiration, for if we take
very accurate tracings of blood pressure and of the respiratory move-
ments side by side, we shall find that, although, in general, the blood
pressure rises with inspiration, yet the beginning of the rise is consid-
erably delayed ; that is, immediately following the beginning of the
inspiratory act the arterial blood pressure continues for some time to
fall, and at the beginning of expiration it continues for some time to
rise (Fig. 22). Moreover, it will be found, if tracings taken from dif-
ferent animals are compared, 'that frequently the general. effect of ex-
piration is to cause more rise than fall, and of inspiration more fall
than rise. It will be found that these differences are dependent largely
on the type of respiration, whether thoracic or abdominal (Lewis).11
Let us consider first of all exactly what will happen in an animal
breathing entirely by the thorax (e.g., the rabbit). The first 'effect of
the inspiration is to cause the veins leading to the auricles, the auricles
themselves and the blood vessels of the lungs to become suddenly ex-
panded. More blood therefore will flow into them. For a moment or
two this blood will, however, tend to stagnate in the more capacious
vessels, and it will be some time until it finds its way to the left side
of the heart; therefore the initial effect of inspiration is a distinct fall
in arterial blood pressure. When the extra space created in the blood
vessels has been filled with blood, — that is, when inspiration has prac-
tically ceased, — the blood will flow on in increased volume to the left
side of the heart, and, therefore, raise the arterial blood pressure. On
expiration the first effect is that the diminishing negative pressure will
cause the thin-walled vessels mentioned above to constrict and thus
squeeze the blood inside them into the left side of the heart and raise
the pressure ; but the ultimate effect in the later stages of expiration
will be that the vessels, being constricted, will allow less blood through
them and the arterial blood pressure will fall.
Take now the case of abdominal respiration. 'In inspiration the dia-
308
THE RESPIRATION
phragm descends and crowds the viscera against the vena cava, with
the result that at first more blood is squeezed into the thorax and the
blood pressure tends slightly to rise. After this initial effect, how-
ever, the compression of the vena cava causes less blood to reach the
thorax, and the arterial blood pressure falls. The conditions will be
exactly reversed on expiration. The initial effect of thoracic inspira-
tion is, therefore, to make the arterial blood pressure fall, and the in-
itial effect of abdominal inspiration, to make it rise. The net effect
B.CHE ST.
C. ABDOMEN.
VJK/K/vJuA
D. CHEST.
Fig. 110. — Effect of abdominal and chest breathing on the pulse and blood pressure of man.
Abdominal inspiration raises the pressure and diminishes the amplitude of the pulse carve. Thoracic
inspiration less clearly lowers the pressure. Expiration has the opposite effects. (From Lewis.)
produced will be the algebraic sum of these two opposing influences
(see Fig. 110).
Another factor that comes into play in determining the effect of the
respiratory movements on the cardiac output acts through the changes
in the pericardial pressure. When this is lowered, as early in inspira-
tion, it encourages diastole, thus causing better filling and therefore
better discharge from the heart.
These considerations taken together make it easy to understand the
changes in blood pressure, particularly in the veins, which occur when
a forced inspiratory or expiratory movement is made with the glottis
closed. A forced expiration of this nature occurs during the acts of
RESPIRATION 309
defecation and parturition, as well as in the first stages of coughing; it
is also produced by blowing into a tube, or against some resistance.
On account of the positive pressure that is brought to bear on the veins
as they enter the thorax, the venous pressure suddenly rises, slowing
down the flow of blood through the capillaries and causing bulging of
the veins and, if the effect is sustained, cyanosis. On the arterial
side of the vascular system, after a momentary rise caused by the
squeezing out into the left side of the heart of the blood in the capil-
laries of the lungs, there is a more permanent fall in pressure due to
the fact that less blood is now getting from the right side to the left
side of the heart. After some time the pressure begins to rise again,
partly on account of the back pressure through the capillary vessels
and partly because of vasoconstriction as a result of asphyxial
conditions.
In the opposite condition, during a forced inspiratory movement with
the glottis closed or with the mouth attached to some tube through
which the attempt is made to suck air, the thoracic cavities open up
Avithout the lungs being able to occupy completely the extra space.
The dilatation of the veins and other thin-walled structures in the tho-
rax thus causes an immediate fall in both the venous and the arterial
pressure — in the venous, because the blood is sucked toward the large
vessels in the thorax and lungs, and in the arterial, because the blood is
now delayed in its passage from the right to the left side of the heart.
If this condition is maintained, the arterial pressure may recover some-
what, but that in the veins is permanently lowered.
CHAPTER XXXV
THE MECHANICS OF RESPIRATION (Cont'd)
VARIATIONS IN THE DEAD SPACE, THE RESIDUAL AIR AND
MID-CAPACITY, AND THE VITAL CAPACITY IN VARI-
OUS PHYSIOLOGIC AND PATHOLOGIC CONDITIONS
BY R. G. PEARCE, B.A., M.D.
Dead Space
Under ordinary conditions of breathing the dead space is fairly con-
stant in volume. Haldane5 and Henderson6 believe that it may be in-
creased by 400 per cent in maximal deep breathing, and that the in-
crease is due to the passive stretching of the lower air sacs. Although
such large variations in the capacity of the dead space has not been ob-
served by Krogh and Lindhard7 or by R. G. Pearce,8 it is undoubted
that moderate rhythmic variations may occur. Even in deeper breath-
ing (1500 c.c. or over), a slight increase, which with maximum breaths
may amount to 100 c.c., can be demonstrated. This is not surprising
when we remember that the walls of the bronchi and bronchioles are
made up largely of readily expansible tissue (elastic and smooth-muscle
fibers). As the respirations become deeper and the expanding force of
the inspiratory movements of the thorax becomes more pronounced, the
diameter of the bronchi and bronchioles will enlarge proportionately —
that is, the diameter or circumference will increase in direct proportion
to this force; but the area of the cross section of the bronchi (i. e., the
capacity) will increase as the square of the diameter. This depends on
the fact that the area of a circle is increased by 125 per cent when the
diameter is increased by 50 per cent, and by about 300 per cent when
the diameter is increased by 100 per cent.
The capacity of the dead space has a certain clinical significance.
Siebeck9 has estimated that the dead space may increase by 100 c.c. in
asthma, but others believe that the increase may be greater. One rea-
son for the discordant results lies in the fact that the percentage of
C02 found in the alveolar air obtained by the Haldane-Priestley method
has been used as one of the basic figures in the determination of the
310
THE MECHANICS OF RESPIRATION 311
capacity of the air passages. As, explained elsewhere (page 344), the pro-
longation of expiration required to obtain the sample of alveolar air by this
method gives figures that are too high even under normal conditions,
and it is plain that this error will be exaggerated in asthma, where the
expiration is greatly prolonged. An increase in the capacity of the
dead space must be accompanied by an increase in the respiratory vol-
ume if the alveoli are to be adequately ventilated. It has been thought
by some clinicians that the difficulty in asthma, emphysema and car-
diac decompensation may lie in part in an increase in the dead space.
Careful estimations of the dead space in these conditions, however,
fail to demonstrate any great variation.
An explanation of the fact that the dead space in emphysematous
patients has been found to be generally large when determined by the
Haldane-Priestley method (see page 340), and also for some of the clirir
ical phenomena accompanying the condition, may be as follows: In
emphysema the walls of the alveoli, especially about the lateral and
lower borders of the lungs, have lost their elasticity and fail to expand
or relax properly during the respiratory cycle. As a result the air in
these alveoli remains relatively unchanged except when forced respira-
tions are made. When a sample of alveolar air is taken directly, this
dead air is pushed out of the distended and diseased alveoli by the
forced respiration required in the direct sampling of the alveolar air.
Since the air in .these alveoli has been in contact with the blood enter-
ing the lungs, it has a high C02 content, which results, when compared
with the uniformly low 002 content found in the tidal air, in giving a
large figure for the dead space. Since the capacity of the dead space
is not increased, the blood in the normal alveoli is probably being super-
ventilated in order to compensate for the high G02 tension in the blood
entering the left heart from the diseased alveoli. However, the 02
content of the blood leaving the sound alveoli is practically normal (be-
cause superventilation can not cause it to take up more), and "can not
compensate for the low 02 content in the blood coming from the dis-
eased alveoli, the net effect being therefore a low tension of 02 in the
blood leaving the heart, which accounts for the cyanosis often seen in
emphysema (Pearce). A somewhat similar explanation can be given
for the cyanosis present in pulmonary edema, if we assume that all the
alveoli in this condition do not share alike in the edema (Hoover).
The Residual Air and Mid-capacity of the Lungs
During muscular exercise the residual air of the lungs is increased,
and the vital capacity decreased (Bohr). This causes the lungs to as-
312 THE RESPIRATION
smne a more inflated condition between breaths or, as it has been clum-
sily styled, a greater mid-capacity. These changes may serve as a
physiologic method for ^increasing the efficiency of alveolar ventilation
so as to meet the greater needs of the body. This is partly because the
pulmonary vessels become dilated and the bloodflow through the lungs
is favored, and partly because of the influence of the reserve and sup-
plemental airs on the tension of the arterial blood gases during the res-
piratory cycle. For example, if the lungs were completely depleted
of air during expiration, the blood leaving them at the end of this act
would be entirely venous. On the other hand, if the amount of air left
in the lungs at the end of expiration were above the normal amount,
each increment of C02 given off from the blood, or of 02 absorbed by
it would produce less change in the pressure of the C02 or 02.
The importance of these influences will be seen from the following
figures. If the residual and supplemental air amounts to 2000 c.c., and
the percentage of C02 in the alveolar air at the end of expiration is
5 per cent, then 100 c.c. of C02 must be present in the lungs. In a con-
dition of bodily rest about 20 c.c. of this gas is excreted during a res-
piratory cycle, so that if the breath were held during this period, the
percentage of C02 Avould rise from 5 to 6 per cent, and an inspiration of
400 c.c. would be required to bring the air in the lungs back to 5 per
cent of C02. On the other hand, if the residual and supplemental air
amounted to 3000 c.c. with 5 per cent of C02 in the alveolar air at the
end of the expiration, there wrould be 150 c.c. of C02 in the lungs at
the end of the expiration, so that holding the breath for the time of the
respiratory cycle would raise the percentage of C02 only to 5.66 (pro-
vided the production of C02 was the same as in the first case), and an
inspiration of 600 c.c. would be necessary to reduce it to the normal
expiratory figure. Or, putting it another way, the production of CO,
can be increased 50 per cent in the time of a respiratory cycle without
affecting the tension of gases in the lungs, provided the residual and
supplemental air and the volume of the respiration are increased 50
per cent. If only one of the factors is changed, however, then the bal-
ance of the respiration must be disturbed, and the greater variation
in the tension of the gases in the arterial blood must occur at the dif-
ferent phases of the respiratory cycle. Bohr and Siebeck have shown
that the residual air is invariably increased in emphysema and that the
mid-capacity of the lungs is likewise increased; and it would appear
from Siebeck 's data that a similar condition must be present in cases of
decompensated heart.
Patients suffering from dyspnea, particularly those suffering from
THE MECHANICS OF RESPIRATION
313
cardiac dyspnea, can not breathe as comfortably when lying as when
sitting. This condition is known as orthopnea. The advantage of the sit-
ting over the lying position for breathing can not be satisfactorily ex-
plained. The greater vital capacity in the upright position; the favor-
ing of the return of the venous blood from the cerebral vessels by
gravity; the increased caliber of the pulmonary vessels because of the
enlarged thoracic cavity (see page 318) ; and the increase in the reserve
air of the lungs — are all factors to be considered.
The Vital Capacity. — At one time it was thought that the vital capacity
of the lungs was related to their ventilatory capabilities, but for years
the determination of this value in patients has been considered unimpor-
tant. Recently Peabody and Wentworth10 have called attention to the
fact that patients with heart disease become dyspneic more readily than
do healthy subjects, and that this tendency seems to depend largely
on their inability to increase the depth of the respiration in a normal
manner. They find that this inability to breathe deeply corresponds to
a diminished vital capacity of the lungs as measured in a spirometer,
by the volume of the greatest possible expiration after the deepest in-
spiration. They believe that any condition which limits the possibility
of increasing the minute volume of air breathed must be an important
factor in the production of dyspnea.
In normal adults the following averages (Table I), were secured from
a large series of clinical cases. The subjects are grouped into two
classes, each group being subdivided according to height.
TABLE I
THE VITAL CAPACITY OF THE LUNGS OF NORMAL MALES
GROUP
NUMBER
STUDIED
HEIGHT IN
FEET AND
INCHES
NORMAL
VITAL
CAPACITY
C.C.
NUMBER
WITHIN
10% OF
NORMAL
HIGHEST
VITAL
CAPACITY
LOWEST
VITAL
CAPACITY
HIGH-
EST
%
LOWEST
%
NUMBER
BELOW
90% OF
NORMAL
I
II
III
14
44
38
6' +
Over 5'
8'/2" to 6'
5' 3" to
5' 8'/2"
5,100
4,800
4,000
9
41
31
7,180
5,800
5,080
5,030
4,300
3,450
141
121
127
99
90
86
0
0
1
THE VITAL CAPACITY OF THE L,UNGS OF NORMAL FEMALES
I
10
Over 5'
3,275
5
4,075
2,800
124
86
2
6"
II
13
Over 5'
3,050
9
3,425
2,660
112
88
2
4" to 5'
6"
III
21
5' 4" or
2,825
16
3,820
2,500
135
89
1
less
(Peabody and Wentworth.)
It would appear that in normal people the vital capacity is at least
85 per cent, and almost always 90 per cent or more, of the standard
adopted for each group. In elderly persons a slight decrease from these
standards may be expected.
THE RESPIRATION
TABLE II
THE RELATION OF THE VITAL CAPACITY OF THE LUNGS TO THE CLINICAL CONDITION IN
PATIENTS WITH HEART DISEASE*
GROUP
VITAL
NUM-
MOR-
SYMPTOMS
WORK-
REMARKS
CAPACITY
BER OF
TALITY
OF DECOM-
ING
%
CASES
%
PENSATION
%
%
I
90 -
25
0
0
92
Few symptoms ref-
erable to heart.
II
70 to 90
41
5
2
54
History of dyspnea
with exertion, yet
able to do moder-
ate work.
III
40 to 70
G7
17
89
7
Dyspnea with mod-
e r a t e exercise.
Few able to work.
IV
Under 40
23
61
100
0 .
Bedridden, with
marked signs of
cardiac insuf-
ficiency.
(Peabody and Wentworth.)
'Certain cases were tested several times and, owing to changes in the vital capacity they appear
in more than one group. In the "mortality" column they are included only in the lowest group into
which they fell. "Symptoms of decompensation" indicate dyspnea while at rest in bed or on very
slight exertion. Under "working" are included only those actually at work, and able to continue.
Many other patients in Group II were able to work, but they are not included as they were still in
the hospital.
Table II shows that there is a remarkably close relationship between
the clinical condition of cardiac patients, particularly as regards the
tendency to dyspnea, and the vital capacity of the lungs. Peabody and
Wentworth believe that the determination of the vital capacity affords
a clinical test as to the functional condition of the heart, since compen-
sated patients who do not complain of dyspnea on exertion have a nor-
mal vital capacity. Patients with more serious disease in whom dyspnea
is a prominent symptom, have a low vital capacity; and the decrease in
vital capacity runs parallel with the clinical condition. As a patient
improves, his vital capacity tends to rise ; as he becomes worse, it tends
to fall. In other diseases in which mechanical conditions interfere with
the movements of the lungs, the tendency to dyspnea corresponds closely
to the decrease in the vital capacity. The cause of the decrease in the
vital capacity of the lung in cardiac decompensation is difficult to ex-
plain satisfactorily. It may be the limitation in the movements of the
lungs produced by engorgement of the pulmonary vessels, by the weak-
ness of the intercostal muscles, the rigidity of the bony thorax,
emphysema, or accumulation of fluid in the pleural cavities.
In cardiac disease the air in the lungs at the end of a normal expiration
is usually increased. This is similar to the condition which attends exer-
cise, and is probably a physiologic adaptation to give optimum aeration
to the blood, as explained above.
CHAPTER XXXVI
THE MECHANISM BY WHICH THE CHANGES IN CAPACITY OF
THE THORAX AND LUNGS ARE BROUGHT ABOUT
BY R. G. PEARCE, B.A., M.D.
The changes that take place in the form and the dimensions of the
thorax during respiration are brought about by movements of the ribs,
diaphragm, sternum, and vertebras. The share which each plays must
be considered separately.
The Movements of the Ribs
The first seven pairs of ribs progressively increase in length, and are
attached directly to the sternum by cartilaginous bands. The eighth to
the twelfth pairs progressively decrease in length, and as far as the
tenth they are indirectly attached to the sternum by cartilages which join
the seventh. The eleventh and twelfth have their anterior ends free, and
may be considered a part of the abdominal wall and not an intrinsic part
of the thoracic cage.
Each pair of ribs, together with its articulating cartilage and vertebrae,
forms a ring, the plane of which is directed forward and downward.
The spinal articulations of the upper ribs differ from those of the lower
ones. In the former the articulations on the tubercle exist as convex
ovoid facets, which fit into corresponding hollow facets on the transverse
processes of the vertebrae, while the corresponding facets of the lower
ribs are flat. Each transverse process from above downward is tilted a
little more backward than the one above, so that the angle at which the
ribs are set to the spine increases from above downward. This manner
of articulation of the upper ribs with the vertebrae prevents any rotation
in the spinosternal axis, so that there can be no so-called bucket-handle
movement in this region (Keith). The articulation, however, allows the
neck of the rib to rotate in an axis approximately transverse to the body.
The angle which the shaft of the rib makes near its neck, together with
the arch of the shaft, which is directed downward and forward, has the
effect of causing the transverse rotation of the neck of the rib to be
315
316
THE RESPIRATION
converted into an upward movement, which is greatest in that part of the
shaft lying parallel to the axis of rotation of the neck (Fig. 111).
The upper ribs from the first to the fifth form a cone-shaped top to the
thorax, whereas the lower ones form a vertical series, each being situated
almost directly above its neighbor. The upper set is arranged for the
expansion of the conical upper lobe of the lungs, the lower for the ex-
pansion of the more or less cylindrical lower lobes. During inspiration
the anteroposterior diameter of the conical portion of the thorax in-
creases, because the ribs, together with the sternal connections, move
through progressively increasing arches, and each lower rib tends to over-
ride the rib just above. The maximal rise of the ribs from the first to the
Fig. 111. — A, first dorsal vertebra; B, sixth dorsal vertebra and rib. Axis of rotation shown in
each case.
tenth during inspiration shifts more and more from the anterior to the
lateral aspects of the thorax, because the angle formed by the shaft near
the neck of the rib approaches nearer to the articulating joints on the
vertebrae.
An examination of the shape of the first rib, its relationship to adjacent
structures and its movements, shows that it differs from the others in
its respiratory function. The first pair of ribs and the manubrium sterni
are bound closely together by short, wide costal cartilages, and form a
structural unit which Keith1 calls the thoracic operculum. This lid is
articulated behind with the first thoracic vertebra by a joint, which is
more nearly transverse than that of the rest of the costal series ; and in
front with the manubrium, which is also articulated with the clavicles
THE MECHANICS OF RESPIRATION 317
above and with the body of the sternum below. The freedom of move-
ment at the angle which the mamibrium makes with the sternum at this
joint is related to the type of breathing. When the lower portion of the
sternum is elevated during inspiration, the movement of the joint is not
free, but when the sternum is retracted, the movement at the angle may
amount to 16°. Lack of movement of the sternal manubrial joint has
been considered by some physicians as one of the predisposing causes of
pulmonary tuberculosis. During inspiration, the first rib and its anterior
attachments are raised by the scaleni, and serve as a point towards which
the second, third, fourth and fifth ribs are elevated. During expiration,
they are depressed toward the lower ribs, which form a more or less
fixed base.
The combined effect of these influences is to produce a motion of the
upper ribs which is described by the clinician as being undulatory. This
movement is more apparent in the upper part of the thorax, because
here the relative difference in the length of the ribs is greatest. Hoover
attributes a certain diagnostic significance to loss of the undulatory
movement, diminution in the extensibility of the underlying lungs causing
it to become less or to disappear. The phenomenon is elicited by placing
the tip of the ring finger on the second rib in the midclavicular line, the
tip of the middle finger on the third rib midway between the midclavicu-
lar and anteroaxillary line, and the tip of the index finger on the fourth
rib in the midaxillary line. The patient is then instructed to make a
moderately rapid and deep inspiration. The finger on the third rib will
be observed to move farther than that on the second rib, and the finger
on the fourth rib will move farther than that on the third. The move-
ment of each rib from above downward succeeds and exceeds that in
the rib just above.
When there is a moderate degree of impairment in the ventilation of
the upper lobe, the three ribs move in unison and through the same dis-
tance, so that the undulatory movement is lost although the ribs involved
may exhibit a considerable excursion. The undulatory movement is also
impaired by any disease which encroaches on the air spaces, invades the
interstitial tissue of the lung, or displaces the lung as in the case of an
enlarged heart or a distended pericardial sac. Another possible factor
in this phenomenon is that any inflammatory process in the lung or ad-
jacent tissue will produce a reflex inhibition of the muscles of the ribs,
and thus limit the expansion of the thorax.
The axis of movement of the lower ribs, as of the upper ribs, accurately
corresponds with that indicated by their articulation with the vertebrae,
because the muscles attached to them, as well as the diaphragm, influence
their movements to a large extent. Anteriorly the lower ribs from the
318
THE EESPIRATION
sixth to the tenth are joined to the sternum by the cartilages which unite
the sixth, seventh, eighth, ninth, and tenth, so that any movement in
which the ribs are raised is accompanied by an anterior movement of the
sternum (Fig. 112). The ribs are so articulated to the spinal column that
the inspiratory act causes the lateral and anterior part of each rib arch
to move forward and outward more than the one above it.
In natural breathing in the . standing or sitting posture there is a
slight extension of the spine during inspiration. This serves to increase
all diameters of the thorax and its absence is undoubtedly an important
Fig. 112. — Lower half of the thorax from the 6th dorsal to the 4th vertebra, seen from the
front, c, ensiform process; d, d' , aorta; e, esophagus; /, aperture in tendon of diaphragm for
passage of vena cava inferior; I, 2, .?, trilobate expansions of tendinous center of diaphragm; 4, 5,
costal portions, right and left, of diaphragm muscle; 6, right crus of diaphragm; 8, 9, internal
intercostal muscles, which are absent near the vertebral column, where it joins 9 and p, the ex-
ternal intercostals; 10, 10, subcostal muscles of left side. (From L,uschka.)
contributory factor in reducing the vital capacity of an individual when
lying on the back. Figures given by Hutchinson for the effect which
posture has on the vital capacity are of interest because of their bearing
on the cause of orthopnea. In the same individual he found the following
vital capacities:
Standing 4300 c.c.
Sitting 4200 c.c.
Supine 3800 c.c.
Prone 3620 c.c.
THE MECHANICS OF RESPIRATION
The Action of the Musculature of the Ribs
319
In a general way, the external intercostal muscles may be considered
as a broad extension of the scalene muscles over the thoracic walls, with
the ribs as intersections. The scaleni serve to fix the position of the
Fig. 113.- — Intercostal muscles of 5th and 6th spaces. A, side view; B, back view; JV, 4th
dorsal vertebra; V, 5th rib and cartilage; /, /, M. levatores costarum, 2, 2, external intercostals;
3, 3, internal intercostals, exposed by removal of the external muscles. In A, there are no external
intercostals in the intercartilaginous spaces; in B there are no intercostals near the vertebral
column. (From Allen Thomson.)
first rib so that it forms an anchorage for the action of the external
intercostal muscles in raising the lower ribs. They also raise the upper
three pairs of ribs along with the manubrium and sternum.
The function of the intercostal muscles has been the subject of much
Fig. 114. — Hamberger's schema to demonstrate the functional antagonism of internal a'nd ex-
ternal intercostals.
When the ribs ac and bd pass into the inspiratory positions ag and bf, the intercostal space
dilates (bh is greater than ab) ; the sternum gf moves away from the vertebral column ab (bf is
greater than be) ; the fibers of the external intercostals ak shorten (ak is greater than a/) ; and
those of the internal intercostals r k lengthen (ck is greater than /#), The reverse occurs when
the inspiratory position is taken. (From lyuciani's Human Physiology.)
320
THE RESPIRATION
debate, and can not be said to be definitely settled. The direction of the
fibers in the internal intercostals indicates that they are expiratory in
function, since they can not shorten in the inspiratory position; while,
on the other hand, the fibers of the external intercostals can not shorten
in the expiratory position, and hence must be considered inspiratory in
character (Fig. 113). In 1751 Hamberger showed that mechanically this
is the case, and gave the schema shown in Fig. 114.
The function of the intercartilaginous muscles, however, must be
inspiratory, as is shown in Fig. 115.
Fig. 115. — Schema to demonstrate that the function of the internal intercartilaginous intercos-
tals is identical with that of the external interosseous intercostals.
The ribs and costal cartilage may be regarded as rods bent at the angles acd and bef, in
which the articular points c and e represent the symphysis between the bony and the cartilaginous
parts on which traction is made. During inspiration the fibers of the intercartilaginous muscles,
which have the direction gh, move the sternum df away from the vertebral column ab, like the
hbers of the external intercostals, which run in the direction kl. During this double action the
angles c and e must be decreased, because the muscles of the upper intercostal spaces work simul-
taneously, and the entire thorax is slightly elevated during inspiration. From this scheme it is
apparent that the external intercostals and the intercartilaginous muscles must be the same. (From
Luciani's Human Physiology.)
The Action of the Diaphragm
It is possible, however, that the main function of both the intercostal
muscles is to regulate the tone of the intercostal spaces and so prevent
their suction inwards when the negative pressure in the thorax increases
(i. e., suction becomes greater). The ascent of the ribs, while producing an
increase in the anteroposterior and transverse diameters of the thorax,
would decrease the vertical diameter if this was not counteracted by the
fixation of the lower ribs and the descent of the diaphragm. The periph-
eral edges of the diaphragm are attached behind to the lumbar vertebrae,
.laterally to the lower edges of the six lower ribs and their cartilages,
and in front to the tip of the ensiform cartilage. The fibers converge to
THE MECHANICS OF RESPIRATION 321
enter the central tendon, and the lateral sheets are pressed upward by
the intraabdominal positive and intrathoracic negative pressures, so that
they form a dome-shaped vault, with the liver in the right side and the
stomach and the spleen in the left.
During expiration the lateral edges of the diaphragm are in contact
with the parietal pleura of the thoracic cavity, forming what are known
as the pleural sinuses. During inspiration the fibers of the diaphragm
shorten; this straightens out the arch of the diaphragm and pulls the
lateral edges of the diaphragm aAvay from the parietal pleura, thus open-
ing up the pleural sinuses, into Avhich the lungs descend. Usually the
opening up of the sinuses is accompanied by a slight retraction of the
external chest wall, Avhich is known as Litten's diaphragm phenomenon.
The descent of the diaphragm may produce a movement of from 10 to
15 mm. on each side, which accounts for a rather important fraction of
the volume of air exchange by the lungs. The central portion of the
diaphragm does not move much in normal respiration, but in forced
respiration its movement may be considerable.
Because of its attachments to the lower six ribs, the contraction of the
diaphragm tends to pull the margins of the ribs towards the median line,
but under normal conditions this movement is opposed by the action of
the external intercostals in raising the ribs and expanding the horizontal
diameters of the thorax, and by the lower vertebral muscles, which fix
the position of the loAver ribs.
The relative part which the diaphragm and the external intercostal
muscles play in the widening of the lower part of the thorax is of some*
importance from the standpoint of diagnosis. It has generally been held
that the contraction of the diaphragm produces a widening of the lower
part of the thorax, because in its descent it presses upon the abdominal
viscera and so distends the abdomen and pushes out the lower ribs.
That this might occur seems not improbable, but Hoover2 has recently
shown by experimental and clinical observations that the flaring in the
costal margins seen in normal inspiration depends on other factors. He
calls attention to the fact that the contraction of the intercostals raises
the ribs and increases the angular divergence of the subcostal borders.
This widening of the angle made by the costal margins at the tip of the
sternum is very pronounced in paralysis of the diaphragm while in
paralysis of the intercostal muscles, the costal borders are drawn towards
the median line and the subcostal angle is decreased. This shows that
the diaphragm must tend to diminish the angle.
The line of traction of the diaphragm is a straight one joining the cen-
tral tendon with the edge of the ribs. When the diaphragm forms a
well-defined arch, it exerts its traction at a disadvantage, and .the ex-
322
THE RESPIRATION
Fig. 116. — Diagram to show the effect of high and low positions of the diaphragm on the
costal angle.
Line 1. Normal position of diaphragm. Costal margins move out during inspiration.
Line 2. High position of diaphragm. Normal outward movement of costal margins accentuated.
Line 3. Low position of diaphragm. Costal margins move in during inspiration.
Line 4. Very low position of diaphragm. Costal margins move out during inspiration.
Line 5. Actual line of traction of diaphragm. (From T. Wingate Todd.)
THE MECHANICS OF RESPIRATION
323
Fig. 117. — Diagram to show the effect of clinical displacements of the diaphragm on the costal
angle.
Line 1. Normal position of diaphragm. Costal margains move out during inspiration.
2. Position of diaphragm in general cardiac enlargement. Costal margin from ensiform to ninth
rib moves toward median line.
3. Position of diaphragm in left-sided cardiac enlargement. Left costal margin is fixed or
moves in during inspiration.
4. Position of diaphragm in right-sided cardiac enlargement. Right costal margin is fixed or
moves in during inspiration.
5. Costal margin. (From T. Wingate Todd.)
324 THE RESPIRATION
ternal intereostals have the mastery and cause the costal borders to
spread. When the arch of the diaphragm is depressed, as in pleurisy
with effusion, emphysema, and empyema, the line of traction and the
line of the muscular fibers of the diaphragm correspond more closely,
so that the diaphragm is able to use its full force against the intercostal
muscles, with the result that the costal border moves towards the median
line. The curves of the different fibers of the diaphragm vary greatly;
the arch is much less marked in the portion attached to the costal margin
near the median line than in that attached in the axillary line. For this
reason the anterolateral part of the diaphragm requires less depression
to give it a horizontal position than is required for parts occupying a
more lateral position. A small pericardial effusion or an increase in the
size of the heart may therefore depress the diaphragm sufficiently to give
it mastery over the intereostals in the front portion, so that the costal
border may here move towards the midline, while the lower borders
move in a perfectly normal manner (see Figs. 116 and 117).
During forced breathing several muscles are brought into play, among
the most important of which are the scaleni, sternomastoid, trapezius,
pectorals, rhomboids, and serratus magnus.
There has been considerable debate as to Avhether expiration is normally
an active or a passive process. Undoubtedly the expiratory phase under
normal conditions does not require the same muscular effort as does that
of inspiration, but there are many observations which indicate that ex-
piration is partly under muscular control. The abdominal musculature,
for example, increases in -tone during expiration, so as to bring about a
rise in the abdominal pressure, with the result that the relaxed diaphragm
is pushed up into the thoracic cavity. To this extent at least, expiration
is accompanied by increased muscular activity.
Before leaving the subject of the diaphragmatic movements, reference
must be made to the recent observations of Lee, Guenther and Meleney3
bearing on the general physiologic properties of the diaphragmatic
muscle. They point out that most skeletal muscles in the living body
contract with varying degrees of intensity and at irregular intervals,
between which relatively long periods of rest occur, but the diaphragm
from birth to death performs a continuous succession of brief contrac-
tions of fairly regular rhythm and uniform extent, alternating with brief
periods of rest. Its muscle fibers, together with those of the other
respiratory muscles, therefore hold a unique position among skeletal
muscles, which suggests a crude analogy with that of the heart. They
have compared the physiologic properties of the diaphragm with those
of the extensor longus digitorum, the sartorius, and the soleus, and found
THE MECHANICS OP RESPIRATION 325
that the diaphragm is composed of a much more efficient muscular tissue
than that of the other muscles.
The Effects of the Respiratory Movements on the Lungs. — The chancres
produced in the dimensions of the lungs by the inspiratory expansion of
the thoracic cavity are not uniform, since different parts of these struc-
tures are not equally extensible. From an anatomic standpoint, the
lungs may be divided into three zones: (1) The inner or root zone contain-
ing the bronchus, artery and vein, and their main subdivisions. The
large amount of fibrous tissue in this region offers great resistance to
any expanding force. (2) The intermediate zone, containing the vascular
and bronchial ramifications radiating towards the surface of the lungs,
with pulmonary tissue implanted between the rays. This part of the
lungs has varying degrees of extensibility, the pulmonary tissue having
the most and the vascular and bronchial the least. (3) The outer zone,
perhaps 25 to 30 mm. in depth, composed of pulmonary tissue and
equally extensible throughout (Keith1). The expansion of the lung is
accomplished by a moving apart of the less extensible rays of tissue so
as to permit the expansion of the more extensible pulmonary tissue be-
tween them. Keith compares the mechanism to that seen in the opening
of a Japanese fan.
Because the lung expands in the direction of least resistance, study
of the inflated dead lung does not reveal the normal expansion brought
about by the thoracic movements. In the living body expansion is more
limited in some regions than in others. Of the five areas which may be
distinguished on the surface of the lungs, three are in contact with rela-
tively immovable parts of the chest wall, and therefore can not be ex-
panded directly. These are: the mediastinal, in contact with the pericar-
dium and the structures of the mediastinum; the dorsal surface, in contact
with the spinal column and the posterior aspect of the thoracic cage, and
the apical surface. The motions of the first pair of ribs and the manu-
brium expand chiefly the anterior and ventrolateral part of the apex
of the lung, and have only an indirect influence on the dorsal part of the
apex — i. e., the part lying directly in front of the necks of the first and
second ribs, the most common site of pulmonary tuberculosis. The two
surfaces of the lungs which are directly expanded are the diaphragmatic
and the ventrolateral or sternocostal. Meltzer4 found that the negative
pressure in the thorax during inspiration Avas least along the relatively
stationary Avails of the thorax, and greatest in the regions nearest the
diaphragm. From this he concludes that some of the expanding force
is lost as it passes through the lungs to the surfaces of indirect expansion.
Many observers have claimed that the expansion of the lung does not
take place throughout instantaneously and equally. This is illustrated
326 THE RESPIRATION
by the fact that, in the region immediately surrounding a localized con-
solidation, a fluid has increased resonance, which would not be the case
if the relaxation produced was equally distributed throughout the lung.
The root of the lung, which has generally been regarded as more or
less fixed, undergoes in normal breathing a definite forward, downward
and outward movement, and the heart shares in this movement (Keith).
The movements of the lower ribs and diaphragm are responsible for the
expansion of the lower lobes and dorsal portion of the upper lobes of the
lungs, whereas the movement of the upper five ribs expands the anterior
portion of the upper lobes. The relative infrequency of pleuritic fric-
tion-sounds and pain over the upper lobes as compared with their fre-
quency over the lower lobes is explained by the fact that the expansion
of the upper lobes is accomplished with little displacement of the pleural
surfaces, whereas in the lower lobes expansion is accompanied by a glid-
ing of the lungs across the ribs.
CHAPTER XXXVII
THE CONTROL OF THE RESPIRATION
The participation of such widespread groups of muscles in the respira-
tory act demands that some mechanism be provided to insure its adequate
control. With every inspiration, for example, the muscles of the alse
nasi act so as to cause dilatation of the nares, the vocal cords are ab-
ducted, and the intercostal muscles, along with the scalenes and the
diaphragm are contracting while the muscles of the abdominal wall are
relaxing; and all these events occur at exactly the proper time so as to
bring about the most efficient opening up of the thoracic cavity. Evi-
dently there must be some mechanism to insure this perfect control. This
is effected through the nervous system.
THE RESPIRATORY NERVE CENTERS
The efferent fibers to the various groups of muscle originate in their
respective motor neurons, which in most cases are situated in the gray
matter of the spinal cord. The harmonious action of these motor neu-
rons, or subsidiary centers, is brought about by the transmission to them
of impulses from a higher 'or master center placed in the medulla ob-
longata, the pathway of transmission between this master center and the
subsidiary centers being in the lateral columns of the spinal cord.
The evidence that the chief respiratory center is in the medulla is fur
nished by observing the effects produced on the respiratory movements
by serial destruction of the cerebrospinal axis from above downward.
By this method the approximate position of the center is found, its exact
location being then determined by punctiform destruction or stimulation
of the supposed locus of the center. If we destroy the cerebrum from
before backward, piece by piece, we shall find that no marked effect is
produced on the respirations until we arrive at about the middle of the
medulla, when immediate paralysis of the respiratory movements occurs.
If we now proceed to puncture various areas on the floor of the fourth
ventricle in another animal, we shall find an area called the noeud vital,
located about the tip of the calamus scriptorius, destruction of which
causes immediate cessation of respiration. It is believed that the center
resides in the group of nerve cells known to neurologists as the fasciculus
solitarius. It is bilateral.
327
328
THE RESPIRATION
The subsidiary centers are entirely dependent upon the master center
for their harmonious action, as is shown by the fact that if the phrenic
motor neuron — which is situated in the cervical spinal cord between the
fourth and sixth spinal segments — is isolated from the medulla by a
lateral hemisection of the cord just above the fourth segment and by
mesial section of the cord opposite the center, the corresponding half of
the diaphragm no longer participates in the inspiratory act (see Fig. 118).
The chief center on either side of the midline of the medulla is con-
nected with the motor neurons of both sides of the spinal cord, as is
proved by the follo\ving experiment. When the central end of the vagus
nerve is stimulated, the respiratory center becomes excited and the respi-
rations more pronounced, the participation of the muscles on both sides
of the body being equal in extent. If IIOAV we bisect the medulla down the
Fig. 118. — Diagram to show cuts required for isolation of the phrenic center.
midline and repeat the stimulation of one vagus, the muscles on both sides
will still participate in the increased respiration, which they will likewise do
if the cervical cord is bisected or hemisected but the medulla left intact
(Fig. 119). The simplest interpretation of these results is that commis-
sural fibers connect both halves of the respiratory center in the medulla
and that each half is also connected with the motor neurons of both sides
of the spinal cord. Often, especially in young animals, a hemisection of
the cord causes cessation of the movements of the diaphragm on the same
side; but this paralyzed side at once begins to contract again when the
phrenic of the opposite side is cut, probably because the respiratory
impulse descending from the chief center, on finding its way along the
motor center of the same side of the cord blocked, is forced to follow the
crossed path. The crossing in the cord is believed to take place at the
same level as that at which the subsidiary center is located (W. T.
Porter12).
THE CONTROL OF THE RESPIRATION
329
The question now arises as to how the chief center functionates. Is it
purely reflex in the sense that it depends for its activity entirely on the
transmission to it of nervous impulses from elsewhere, or is it automatic
in the sense that it can work independently of such impulses? The au-
tomaticity of the heart makes it seem not improbable that the center
which controls the co-ordinate action of the respiratory muscles would
also have an inherent or automatic power. The activity of such an auto-
matic respiratory center would, of course, be subject to great variation
as a result of changes in the composition of the blood supplying it, and
the fact that it was automatic would not remove it from the influence of
nervous impulses. Indeed it is possible to conceive of the automaticity
of the center as being of a low7 order, with its normal functioning
dependent upon afferent nerve impulses. Its automaticity might, then,
Medulla
Spinal cord
& root*
C.3
. Fig. 119. — Diagram to show certain positions in the medulla and upper cervical cord, where
sections may be made without seriously disturbing the respirations. Sections made separately will
not disturb the respiration, nor interfere with the effect of vagus stimulation. If, both sections
are made at once, however, breathing will be seriously interfered with on the side of the
hemisection, and this side will not respond to vagus stimulation.
be merely a factor of safety called into play only when the influences
ordinarily controlling the center were for some reason removed.
The question which at present confronts us, however, is whether the
center may or may not act automatically. Many experiments have been
undertaken to test this point, the nature of all of them depending upon
the isolation of the center as completely as possible from afferent nerve
paths. The most successful experiment has been performed as follows:
The influence of the higher nerve centers was removed by cutting across
the peduncles of the cerebrum or the pons. The influence of afferent im-
pulses traveling up the spinal cord was removed by completely severing
the spinal cord below the level of the phrenic nerves and sectioning all
the posterior or sensory spinal roots of the cervical cord above the level
of this section. The vagi were also cut to remove the impulses traveling
330
THE RESPIRATION
by them to the respiratory center. By such an operation the only lower
respiratory neurons left intact are those of the phrenic nerve, so that the
respiratory movements that alone are possible are those in which the
diaphragm participates and the muscles of the alse nasi and larynx. It
was found that the animal after the operation went on respiring, though
imperfectly, and that the respirations soon became more marked and
asphyxial in character, indicating that the blood was not becoming
Diaphraym.
Fig. 120. — Diagram to show where cuts are made to isolate the chief respiratory center from
afferent impulses.
properly aerated and that the chemical changes occurring in it were
acting directly on the center, stimulating it to greater activity. The
conclusion seems warranted that the respiratory center can act auto-
matically, for the only possible afferent nerves left in the above prepara-
tion were those carried to the center by the fifth nerve (Fig. 120).
That the respiratory center is extraordinarily sensitive to changes in
the composition of the blood flowing through it is a fact that has been
known for a long time, but it is only within recent years that the exact
THE CONTROL OF THE RESPIRATION 331
nature of this control and the remarkable sensitivity of the center towards
it have been thoroughly established. We shall return to this important
subject later. Meanwhile we shall proceed to examine the manner in
which the center is affected by sensory impulses transmitted to it.
The afferent nerve fibers going to the respiratory center may conven-
iently be divided into two groups: those which act on it only occasionally,
and those which act on it more or less continuously.
Th6 Occasionally Acting Impulses
To the first group belong afferent nerves from practically every part of
the body. That impressions from the skin affect the respiratory center
is well known by the increased breathing caused by applications of cold
water. The influence of these afferent impulses is often very marked,
and is frequently taken advantage of in stimulating a newborn infant to
take the first breath. Stimulation of the terminations of the fifth nerve
in the mucous membrane of the nose, as by inhaling a pungent odor,
immediately inhibits respiration. To these occasionally acting afferent
impulses may be added the impulses that are conveyed to the respiratory
center from the higher nerve centers of the cerebrum. These impulses
are largely voluntary in nature, and enable us to hold our breath at will.
Some of the cerebral impulses are however also involuntary, their exist-
ence being seen by observing the respirations of an animal before and
after sectioning the pons or peduncles. The respirations for a time at least
become distinctly affected, but they later return with perfect regularity.
They may become very irregular, however, if the vagi as well as the pons
are cut. Other experimental evidence of the existence of cerebral respir-
atory fibers is furnished by cerebral localization experiments. During
stimulation of the cerebral cortex, for example, a marked effect on the
respiratory movements is often noted.
Respiratory rhythm, unlike that of the heart, has often to be modified
in order that the respiratory mechanism may be used for other purposes
than the ventilation of the lungs. This alteration in rhythm may take
the form of a mere inhibition, such as the act of swallowing; or the
respiration may be altered, as in phonation and singing. More consid-
erable alteration in the expiratory discharge occurs in coughing and-
sneezing, and still more in the acts of micturition, defecation, and parturi-
tion. We must conclude therefore that the rhythmic stimuli sent out
from the respiratory center are so weak that stimuli from other sources
may instantly inhibit or change their form at any stage of the cycle.
332 THE RESPIRATION
Stimulation of the endings of the glossopharyngeal nerve inhibits res-
piration, which explains the holding of the breath that occurs in swal-
lowing.
The Continuously Acting Afferent Impulses
The continuously acting afferent impulses are transmitted to the chief
respiratory center by the vagi and their branches, the superior laryngeal
nerves. If the vagus nerves are cut or their continuity severed by
freezing a portion of them, the respiratory movements become markedly
slower. Evidently, the vagus nerves in some way hurry up the respira-
tory movements. Again, if the central end of either vagus is stimu-
lated with the ordinary interrupted f aradic current, a profound effect-
on the respiratory movements is usually observed. This effect is how-
ever not strictly predictable. Usually there is a quickening of respira-
tion, and if the stimulus is a strong one, there may be a standstill of the
thorax in the inspiratory position. On the other hand, if the central
end of the nerve is stimulated with other types of stimuli, as by slow,
weak faradic shocks or by the stimulus produced by the closure of an
ascending voltaic current, the effect may be to stimulate expiration
rather than inspiration. Such results would seem to indicate that the
vagus contains two kinds of afferent fibers to the respiratory center, one
kind stimulating inspiration, the other, stimulating expiration.
Supposing that such fibers exist, the next question is, how do they
become stimulated at their terminations -in the lungs? The most nat-
ural assumption is that the mechanical distention and collapse of the
alveoli which occurs with each respiratory act, serves as the stimulus —
an hypothesis to which support is offered by the observation that, when
air is bloAvn into the lungs so as to distend the alveoli, the animal im-
mediately makes a forced expiratory movement, Avhereas when the air
is sucked out, the thorax assumes the inspiratory position.
Of the many methods that have been employed to produce disten-
tion of the alveoli, the best is undoubtedly that recently employed by
Haldane and Boothby.13 The person or animal is made to respire through
a tube in which is inserted a three-waj^ stopcock, which communicates
either with the outside air or with a side-tube leading to a spirometer
or bag containing air under slight pressure, so that when the stopcock
is turned breathing takes place against a definite positive pressure.
Such a method is obviously much more physiologic than one in which
the air-tube is suddenly clamped at the end of inspiration and the lungs
left in a distended condition.
The term used to designate the cessation of breathing is called apnea.
The extent to which it occurs varies very considerably in different an-
THE CONTROL OF THE RESPIRATION 333
imals and, in the case of man, in different individuals. Thus, when a
man is made suddenly to breathe into compressed air, the apnea often
lasts for about half a minute, the pause being then broken by a deep ex-
piration followed by a further pause, then again an expiration, and so
on with progressively shorter pauses. Disregarding for the present
any influences which changes in the composition of the air in the lungs
or of the gases in the blood might have in producing the apnea, we may
consider the possibility that it is the result of afferent fibers in the
vagus. This is an old view, but the most recent experimental evidence
does not lend support to it. It was shown by Boothby and Berry,14 for
example, that a similar apnea, though indeed of shorter duration, could
be produced in dogs in which the pulmonary branches of both vagus
nerves had been severed two months previously. The apnea is, there-
fore, not a reflex of the vagus, and must be interpreted as due to nerv-
ous impulses passing to the respiratory center from some other part of
the nervous system, perhaps from centers higher up, or to stimuli trans-
mitted to the respiratory center possibly through afferent fibers in the
respiratory muscles.
The formerly very popular theory that respiration is controlled au-
tomatically by alternate distention and collapse of the alveoli, acting
through the afferent fibers of the vagus nerve on the respiratory center
in such a way as to bring the opposite act with each expiration and
inspiration, must, therefore, be abandoned. But we can not deny that
the vagus plays a most important role in the control of the function of
the respiratory center, for apart from the effect which we have seen to
follow the severence of continuity of the nerve, there is the important
observation of Alcock and others13 that when nonpolarizable electrodes
are placed on the vagus nerve and connected with a galvanometer, a
current of action occurs toward the end of each inspiration in quiet
breathing; and when the respirations are forced, a current of action
appears during both inspiration and expiration. Another reason for
believing that the vagi have some important function to perform in con-
nection with the control of respiration is the fact, observed by F. H. Scott,16
that in an intact animal, when atmospheres containing increasing percent-
ages of carbon dioxide are respired, the respirations become both deeper
and quicker, whereas in one whose vagi have been cut the carbon diox-
ide causes only a deepening of the respirations. From this result it
would appear that the vagi exert an influence on the rate of the respira-
tions but not on their depth, this effect, as we shall see later, being de-
pendent primarily on changes in the composition of the blood supplying
the respiratory center. It is probable that both controlling agencies act
together, the one serving to maintain the center in a proper state of
334 THE RESPIRATION
excitability, and being active to a greater or less extent all the time;
while the other acts only occasionally on the "tuned up" center. There
is, of course, no doubt that it is through the nerves that the occasional
alterations of respiration occur. They appear also to have a certain
influence on the rhythm, for Stewart, Pike and Guthrie17 observed that,
after resuscitation from acute brain anemia, the respirations when they
returned were of the same rhythm as that of the artificial respirations
employed during the resuscitation.
The usually accepted hypothesis as to the mechanism by which the
nerve impulses hasten the respiratory movements is that an afferent
impulse is transmitted to the respiratory center towards the end of each
inspiration, which has the effect of inhibiting the inspiratory discharge
from the center and thus cutting short the act of inspiration so that ex-
piration automatically supervenes. This explanation is in agreement
with the fact that quiet inspiration involves activity on the part of the
respiratory muscles, whereas expiration is usually almost entirely pas-
sive, being due to the return to a resting position of the stretched and
displaced structures. On the other hand, in forced respiration and in
certain animals under normal conditions, expiration becomes active, in
which event a current of action becomes evident in the vagus nerve dur-
ing the expiratory phase.
The superior laryngeal branch of the vagus should really be classified
as one of those nerves that have an occasional influence on the respiratory
center, its particular function being in connection with the act of cough-
ing. When a foreign body irritates the mucous membrane of the larynx,
the nerve fibers transmit impulses to the respiratory center which ex-
cite a violent expiration and at the same time cause the glottis to close.
The closure of the glottis lasts, however, only during the first part of
the expiration; it then opens, with the result that the sudden release of
intrapulmonic pressure causes the expulsion of the foreign substance
in the air passages.
CHAPTER XXXVIII
THE CONTROL OF RESPIRATION (Cont'd)
THE HORMONE CONTROL OF THE RESPIRATORY CENTER
Just as the rhythmical activity -of the heart is readily influenced by
changes in the composition of the blood supplying it, so also is that of
the respiratory center. In the case of the heart it is the cations — cal-
cium, potassium and sodium — that have the most pronounced effect,
whereas in the case of the respiratory center it is largely the relative con-
centration of hydrogen and hydroxyl ions — the H-ion concentration
(CH) of the blood. This influence can be shown in a general way by
injecting acid or alkaline solutions into the peripheral end of the carotid
artery of an anesthetized animal, or better still of one that has been
decerebrated. Acid injections stimulate the respiratory activity; alka-
line injections tend to depress it. When the acid or alkaline solutions
are injected intravenously in other parts of the body, so that they be-
come thoroughly mixed with the blood before the respiratory center is
reached, the effects are not nearly so pronounced, because the buffer in-
fluence of the blood has time to develop (see page 36).
From the results of such injection experiments, however, one could
not draw the conclusion that under normal conditions the activity of
the respiratory center is affected by measurable changes in CH of the
blood, for, as we have seen, constancy of CH is one of the most remark-
able properties of the animal fluids. To justify the conclusion that the
respiratory center is affected by changes in CH, it is necessary to observe
the behavior of some easily measurable acid or alkaline constituent of
the blood that undergoes changes in amount that are proportional to an
alteration in CH. In order to understand what this acid or basic
substance may be, it will be advisable to recapitulate the main factors
concerned in maintaining CH at a constant level. This value is obviously
dependent upon the balance between basic and acid substances, so that
any variations which it undergoes must be caused by changes in the
relative amount of one of these. Changes in base may occur, exoge-
nously, by altering the alkali content of the food, or, endogenously, in
various ways but particularly by variations in the amount of ammonia
produced during the course of metabolism of protein. Thus, when sud-
den demands are made by the organism for an increased amount of base,
335
336 THE RESPIRATION
the amino groups — split off from the ammo bodies— become converted
into ammonia instead of into the neutral substance, urea. But the chief
variations seem to concern acids rather than the basic substances. These
acids may be divided into three groups: fixed inorganic' acids, represented
by phosphoric; fixed organic acids, represented by lactic; and volatile
acids, represented by carbon dioxide. Of these three groups, the first
shows the least tendency to change, and the third, the greatest. Changes
in the second group (fixed organic acids) are effected partly by excretion
through the urine and partly by oxidation into volatile acid. The sud-
den and rapid changes in the third group are brought about by the dif-
fusion of the C02 of the blood into the alveolar air. Gross changes in
the acid content of the blood are therefore mainly effected through al-
teration'in the excretion of the fixed acids, whereas sudden changes are
effected by excretion of the volatile acid. It is important to note here
that the fixed organic acids do not participate to any great extent in
the makeup of the acid content of normal blood: they appear only under
unusual conditions, as in dyspnea. The variations in CH that ordinarily
affect the activity of the respiratory center are therefore dependent
upon changes in the volatile acid, a direct measure of which is found
in the tension of CO* in the blood. The correlation between CH of the
blood and respiratory activity must be a very close one if CH is to be
maintained.
The Laws of Gases. — In order to understand the principles upon which
alterations in C02 tension are dependent, it will be necessary for us to
review briefly some of tne gas laws. Among these laws the first in im-
portance is the law of pressure, which states that, other things being
equal, the pressure of a gas is inversely proportional to its volume ; if
a gas occupying a certain volume is compressed by a pump so that it oc-
cupies one-half of its previous volume, its pressure will become doubled.
The second is the law of partial pressure, which states that the partial
pressure of a gas in a mixture of gases, having no action on one another,
is equal to that which this particular gas would exert did it alone oc-
cupy the space occupied by the mixture. Thus, atmospheric air consists
roughly of 79 volumes per cent of nitrogen and 21 of oxygen ; the par-
21
tial pressure of the oxygen is therefore equal to y^r x 7^0 mm. Hg,
this last figure being the barometric pressure of air at sea level. The
third is the law of solution of gases, which is to the effect that the amount
of gas which goes into solution in a liquid having no chemical attraction
for the gas, is proportional to the partial pressure of gas. If water is
exposed to air, the amount of oxygen which it dissolves will be the same
as if the water had been exposed to oxygen at a pressure equal to that
THE CONTROL OF THE RESPIRATION 337
of the partial pressure which it produces in air. The same will be
the ease with the nitrogen of the air. The actual amount of gas which
becomes dissolved in the fluid, pressure and temperature being constant,
depends partly on the nature of the gas and partly on the nature of the
fluid. For example, the solubility of oxygen in water is considerably
different from that in a neutral oil; or, taking the same solvent, nitro-
gen and C02 do not dissolve to the same extent in water. It becomes
necessary, therefore, in calculating what amount of a particular gas
will dissolve in a particular fluid to use a figure known as the coefficient
of solubility of the gas — that is, the amount of gas taken up by a unit
volume of fluid at standard temperature and pressure; for example, to
say that the coefficient of absorption of nitrogen in water at 0° C. is
0.0239 means that, at this temperature and at normal barometric pres-
sure, 1 c.c. of water will dissolve 0.0239 c.c. of nitrogen when exposed to
a pure atmosphere of this gas. Obviously, then, if water were exposed
to 79 per cent of an atmosphere of nitrogen (as in air) the amount which
79
ivould become dissolved in each c.c. would be -rx 0.0239 = 0.0189 c.c.
In solutions containing no chemical substances with which the gas can
enter into combination, it is evident that the tension of the gas will be
proportional to the amount of gas that can be displaced or pumped out
from the fluid. On the other hand, when a chemical compound is formed,
the combined gas will exercise no direct influence on the tension, so that
this will be independent of the amount; in such cases separate methods
will have to ~be used for the determination of amount and tension. Let
us take the case of puffc water exposed to an atmosphere of C02: the
amount of C02 which goes into solution will depend entirely on the
pressure. If a trace of alkali is dissolved in the water, however, some
of the C02 will become combined to form carbonate, so that a much
larger quantity of C02 will be displaceable from the solution (as by
adding a mineral acid to it) than corresponds to the tension of C02 in
the atmosphere surrounding it. Since blood contains alkali the condi-
tions are analogous with those of a weak alkaline solution.
The Tension of CO, and 02 in the Arterial Blood. — If we were to
pass blood at body temperature in a very thin film over the walls of a
confined space containing a mixture of gases one of which was C02, it
is evident that the percentage of C02 in the atmosphere contained in
this space would remain unchanged only when the tension of this gas in
the blood was the same as that in the confined atmosphere. If, on the
other hand, the tension of C02 in the blood should correspond to a per-
centage that is higher than that in the atmosphere, then C02 would dif-
fuse from the blood, and at the end of the experiment an analysis of the
338
THE RESPIRATION
atmosphere in the space would show that the C02 percentage had been
raised. If the blood contained a lower tension than that corresponding
to the percentage of C02 in the space, some of the C02 would diffuse
into the blood, and its percentage in the atmosphere would be lowered.
By successively exposing blood to gas mixtures that contain slightly
different percentages .of C02, we should ultimately find one with which
the free C02 in the blood was in perfect equilibrium, and we should be
able to state that the tension of this gas in the blood was equal to a
certain percentage in the atmosphere surrounding the blood (see Fig.
121).
Many forms of apparatus based on the above principle have been in-
vented for the examination of the tension of the gases in the blood.
The most accurate is that devised by Krogh,18 the principle of wrhich
cot
1
f.S e,t
C0a 5.1 »T
cot
IT
Eig. 121. — Diagram to show principle for measurement of the tension of CQz in blood. The
CC>2 tension of blood is supposed to be 5.75.
differs slightly from that just described in that a bubble of air is
exposed to a relatively large quantity of blood, so that after a time
actual equilibrium of gas tension becomes established between the bub-
ble and the gases of the blood. This apparatus is shown in Figs. 122
and 123. It consists of a graduated tube of narrow bore sur-
rounded by a water jacket. To the upper end of the graduated tube
a small syringe is attached. The lower end of the graduated tube ex-
pands into a thistle-shaped bulb, closed below by a cork, through which
is inserted a tube (inflow tube) ending near the top of the bulb in a
fine opening and connected outside with an artery. An outflow tube is
also connected with the thistle-shaped bulb.
At the beginning of the experiment the thistle-shaped bulb and the
graduated tube are filled with physiologic saline. By means of the
syringe a small bubble of air is then introduced, so that it lies at the
THE CONTROL OP THE RESPIRATION
339
junction of the thistle-shaped bulb and the graduated tube. As the blood
is allowed to enter through the inflow tube, it is ejected in a fine stream
around the bubble of air, which moves about in the stream. The blood
displaces the saline out of the bulb into the side tube. After the bub-
ble has been subjected to the influence of the blood for some minutes,
the gases in it come into perfect equilibrium with those in the blood.
The percentage of 02 and C02 in the bubble will therefore correspond
to the tension of these gases in the blood. The analysis is effected by
drawing the bubble into the graduated tube by means, of the syringe, .
Fig. 122. — The gas analysis pipette for the microtonometer sh
see context. (From A. Krogh.)
Fig. 123 "'
bub '
Fig. 123.
own in Fig. 123. For description
context. (From A. Krogh.)
Fig. 123. — Microtonometer, to be inserted into a blood vessel. The small circle represents the
ble of air. For further description see context. (From A. Krogh.)
measuring its capacity, transferring it into a bulb containing KOH,
which absorbs the 002, then taking it back into the capillary tube and
again measuring. The shrinkage obviously corresponds to the amount
of C02. The bubble is then transferred into potassium pyrogallate solu-
tion, where the 02 is absorbed.*
The Tension of C02 and 02 in Alveolar Air. — Having seen how we
may determine the tension of the gases in blood, we must now consider
*Since thei above was written, a more efficient tonometer devised by the late T. G. Brodie has
been described by O'Sullivan (Am. Jour. Physiol., Sept., 1918).
340 THE RESPIRATION
the method by which the tensions of these gases in alveolar air can be
determined. The simplest and until recently the most accurate method
is that of Haldane and Priestley.19 This consists in having an individual,
Avith his nostrils clamped', breathe quietly through a piece of hose pipe
about a meter long, which has at the mouth end a short side-tube lead-
ing to an evacuated gas-sampling bulb of about 50 c.c. capacity.* After
the subject has become accustomed to breathing through the tube, he
is asked to make a forced expiration and at the end of it to close the
mouthpiece with his tongue. At this moment the operator opens the
tap of the sampling tube, allowing the air from the tubing through
which the individual has made the forced expiration to rush in and fill
it. This sample represents the air from the alveoli (see page 302), and
is analyzed for percentages of C02 and 02. Since each normal inspira-
tion dilutes the alveolar air somewhat, it is necessary, for constant re-
Fig. 124. — Apparatus for collection of a sample of alveolar air by Haldane's method. It is
better to use a mouthpiece than a mask.
suits, to make two analyses of alveolar air from each subject, one taken
at the end of a normal inspiration and the other at the end of normal
expiration. The average of the two results is taken as the composition
of the alveolar air.
On account of the difficulty in securing intelligent cooperation in the
application of this method, particularly Avith children, others have been
devised. One of the simplest is that of Fridericia, which is a modifica
tion of the Haldane-Priestley method, the apparatus for which is shown
in the figure (Fig. 125), and the manipulation of which is outlined in
the legend. Another is to take a mixed sample of the very last portion
of several normal expirations. On account of the extended use which is
being made of measurements of alveolar air composition, both in lab-
*In place of the gas-sampling tube it is much more convenient and equally accurate to employ one
of the modern ground glass piston syringes (L,uer). The piston should, of course, be well smeared
with a good mineral grease.
THE CONTROL OF THE RESPIRATION
341
Fig. 125. — Fridericia's apparatus for measuring the CO™ in alveolar air. The person expires
forcibly through the tube with the stopcocks as in I. A is closed and the tube placed in water to
cool the air, after which B is turned as in II. The entrapped column of air equals 100 c.c. A
solution of caustic alkali is now sucked into C with stopcocks as in II. B is then turned as in
I but with A still closed, and the alkali solution allowed to enter b, after which B is turned off,
the excess of alkali solution in C allowed to run out and the burette shaken. The burette is
then submersed up to a in a cylinder of water, with B as in III. After allowing for cooling,
the level at which the water stands gives the per cent of CO».
2093
in. inspired
air
Co*
Fig. 126. — Curves to show the relationship between the O2 and CO2 tensions in alveolar air
(dotted lines) and arterial blood (continuous lines). It will be observed that the tension of CO2
in blood is slightly above that in alveolar air, but that the reverse relationship obtains for Oa. In
the upper part of the curve the O2 tension in the alveolar air was experimentally altered, causing
a corresponding alteration in the O2 tension of the blood. This result is of practical significance
in connection with O2 alterations in gas poisoning, pneumonia, etc. (From A. and M. Krogh.)
342
THE RESPIRATION
oratory and in clinical work, a special chapter has been devoted to the
subject, giving in detail the more recent methods devised by R. Gr. Pearce.
Lastly, it should be noted that several observers believe that a more
reliable estimate of the alveolar tension of C02 (and of 02) can be made
by analyzing a sample of ordinary expired air and calculating the per-
centages of C02 and 02 in the alveolar air by allowing a constant dead-
space capacity of 140 c.c. (Krogh, etc.).
If we compare the C02 tension of arterial blood, as measured by the
Krogh method, with that of alveolar air, we shall find that there is a
remarkable correspondence, indicating, therefore, that, when the arterial
2.9
0,05
Co,
2.9
fa. a spirettair
220 30
Fig. 127. — Same as Fig. 126, except that in this case the tension of CO2 in the alveolar air was
experimentally altered. (From A. and M. Krogh.)
blood leaves the alveoli, its partial pressure or tension of C02 is exactly
equal to that in the alveolar air. This is shown in the accompanying
curves of experiments performed by Krogh. The dotted line in these
curves represents the tension of C02 or 02 in alveolar air, and the con-
tinuous line, these tensions in arterial blood. Close correspondence
will be observed between the C02 curves even when sudden changes in
alveolar C02 were induced by artificial means. In the case of the 02
tensions, however, that of the blood is always lower than that of the
alveolar air, the differences being especially marked when the 02 ten-
sion in the alveoli is raised (Pigs. 126 and 127).
Tension of C02 in Venous Blood. — If we examine the C02 tension of
the venous blood coming to the lungs, we shall find that it is distinctly
THE CONTROL OF THE RESPIRATION 343
higher than that in the alveolar air. The earliest method for measuring
it consisted in passing a lung catheter into the right bronchus and then
blocking the passage above the open end of the catheter by inflating a
rubber collar or ampulla. The renewal of air in the right lung is thereby
prevented, and a sample of the stagnant air can be removed and analyzed.
In such a case, however, the blood will have circulated several times
round the body, and with only one lung operating the risk is incurred
that more C02 is being discharged into the blocked lung than cor-
responds to the tension of C02 of venous blood under normal conditions.
Much more practical methods are those of Haldane, Yandell Hender-
son and R. G. Pearce, which are much the same in principle. In Pearce 's
method, the person first of all inspires from a gas meter containing a
gaseous mixture with about 10 per cent of C02. Immediately after fill-
ing the lungs, he makes a rapid forced expiration into a tube provided
with a valve having four openings. This valve is turned through a
complete circuit during the expiration, so that four fractions of the ex-
pired air can be collected in rubber bags connected with side tubes
opening opposite the four openings in the valve. The first fraction will
contain a little less than 10 per cent C02, the second distinctly less,
while the fourth will contain the same as the third, indicating that equi-
librium between the C02 of the alveolar air and the blood must have been
attained. This figure therefore gives us the tension of C02 in the venous
blood of the lungs. In Henderson's method the rebreathing is per-
formed into gas receivers containing 6 per cent C02.
These results then indicate that the whole process by which C02 is
exchanged in the lungs is dependent on the law of gas diffusion ; the gas
diffuses from a place of lower to a place of higher pressure, and does
so until equilibrium is attained.
CHAPTER XXXIX
THE CONTROL OF RESPIRATION (Cont'd)
THE ESTIMATION OF ALVEOLAR GASES
BY R. G. PEABCE, B.A., M.D.
Methods such as that of Haldane and Priestley, which calculate the
mean percentage composition of the alveolar air by analysis of a sample
taken from the end of -a prolonged forced expiration, give values which
are too high for C02 and too low for 02. There are several reasons
for this: (1) In the time taken for the prolonged deep expiration an
appreciable amount of C02 will be given off by the blood to the alveolar
air, and oxygen will be absorbed — that is, the sample will not contain
the same percentages of C02 and 02 at different stages of expiration.
(2) The portion of the tidal air which reaches the alveoli dilutes the
alveolar air and thus causes the amount of C02 given off by the blood to
vary during the different phases of respiration. If we bear in mind that
the tensions of CO2 in the alveolar air and in the blood leaving the lungs
are always the same (page 343), and that the entire fa.ll in C02 tension
in the alveolar air occurs during inspiration, then it is clear that the
blood in the pulmonary capillaries must have a maximum tension and
load of C02 at the end of expiration, and a minimum tension and load
of C02 at the end of inspiration. Accordingly, the average of the per-
centage of C02 and 02 at the end of inspiration and expiration, as de-
termined by the Haldane-Priestley method or by any of its modifications,
must fail to give the correct mean tension of these gases in the alveolar
air during expiration. The error which makes the C02 higher than it
should be, makes the percentage of 02 less than it should be. These in-
fluences taken along with the fact, which will be shown later, that the
evolution of C02 from the blood is relatively more rapid at low than at
high tension of C02, indicates that the blood in the pulmonary capil-
laries during inspiration must contribute a greater part of the C02
excreted during a respiratory cycle than that in the pulmonary capil-
laries during expiration, and moreover that a greater part of the C02
excreted must be evolved at a tension which is below the mean tension
of the C02 present in the entire time of the expiration. We conclude,
therefore, that the average tension of C02 in the alveolar air, determined
344
THE CONTROL OF THE RESPIRATION 345
by the actual tension under which the gas is evolved from the blood, is
less than the average tension of C02 in the alveolar air during the time
of a respiratory cycle.
In the case of 02 the conditions are different. While the diluting
effect of the alveolar tidal air is marked in altering the amount of C02
given off during the different phases of a respiration, it can have little
influence on the amount of 02 taken up by the blood under normal con-
ditions. This is evident from a study of the dissociation curve of hemo-
globin (page 383), which shows that at tensions above 65 mm. Hg the
hemoglobin is practically saturated with 02. .Since the tension of 02
in the alveolar air under normal conditions is greater than 65 mm.
(95-100 mm.), the rate of absorption of 02 must be practically maximal
during the respiratory cycle — that is, it will not change at different
phases of it.
While the relationship of the alveolar gases is continually changing
at different stages of the respiratory cycle, their mean relationship for
periods including several respirations or for complete respirations is
more or less constant, being controlled by the type of the metabolism,
and mathematically expressed by the respiratory quotient (page 547).
The average relative percentages of the two gases in the alveolar air
must therefore be the same as in the tidal air. In the alveolar air col-
lected by the Haldane method, however, the above factors cause the
respiratory quotient to be less than that in the tidal air.
These points have been insisted upon because much of the knowledge
of the gaseous exchange between the blood and the air in the lungs, as
well as the control of respiration, has been built upon data obtained by
the Haldane-Priestly method, and in considering this work, which we
shall do in subsequent pages, it is advisable that we be aware of the
limitations of the method employed. The method has been an invaluable
one for opening up a hitherto entirely unexplored field of research, but
now, the pioneer work having been done, we must employ methods
which will enable us to explore 'more exactly.
An Accurate Standard Method for Normal Subjects. — The most accu-
rate method, and one free from many of the theoretic errors present in
the others, depends on the relationship found to exist between the dilut-
ing effect of the air in the dead spa.ce (see page 302) and the known per-
centage composition of the alveolar air in expirations which are of vary-
ing depths but of equal and normal duration and which follow normal
inspirations (R. G. Pearce).
In this method the subject is made to breathe through valves, which automatically
separate the inspired from the expired air. The expired air is led into a tube con-
nected with two spirometers by two three-way stopcocks. The spirometers are of the
346
THE RESPIRATION
Gad-Krogh type, one being capable of holding ten liters, and the other one and a half.
The exact time during which air enters is recorded by the small spirometer by means of
a grooved dial on the axis of the lid, on, which a thread works over a system of pulleys,
and any movement is accurately recorded by a writing point on the smoked paper of a
drum. The spirometers are connected so that the air current may be directed in the
three following ways: (1) through Cocks 1 and 2 outside; (2) directly through both
cocks into the large spirometer for the purpose of collecting a series of expirations;
and (3) through Cock 1 directly into the small spirometer for catching a single expira-
tion. In all experiments the first filling of the spirometer is rejected, so that the dead
space of the spirometers is filled with air of approximately the same composition as in
the succeeding expirations. The time is marked in seconds by a time clock. The respira-
tory movements are recorded by a pneumograph. (Fig. 128.)
The subject is brought into respiratory equilibrium by having him breathe through
the valves for a period of time before the observation. The respiratory movements
during this time are recorded while the cocks are in Position 1. When the observation
is started, the cocks are turned into Position 2 during the time an inspiration is being
Fig. 128. — Arrangement of meters and connections of Pearce's method for measurement of COs
of alveolar air in normal subjects.
made, so that the expirations which follow may be collected in the large spirometer.
After about ten respirations (a counted number) have been collected, the cocks are
turned to Position 3 during an inspiration, and a single deep expiration is collected
in the small spirometer. In order that the time of this may be the same as the normal
expiration, it is necessary to quicken it. This is more or less a chance procedure, but
with a little training, the operator can close the stopcock with sufficient accuracy to
interrupt the deep expiration, at the end of the normal expiratory time. Should
there be any gross variation from the normal expiratory time, the sample must be col-
lected again. Not infrequently the inspiration immediately preceding the expiration
into the small spirometer is varied involuntarily by the subject on account of his being
aware that the following expiration has to be deepened and quickened; this can be
partially overcome by giving1 him the signal to breathe out deeply after he has actually
begun to expire.
Determinations are made of the average volume of the tidal air (c.c. air in large
spirometer divided by number of breaths), of the volume collected from the deep ex-
piration, and of the percentage composition of the tidal air and that of the deep
expiration. A criterion for determining whether or not the procedure has been carried
THE CONTROL OF THE RESPIRATION 347
out correctly is the respiratory quotient (ratio of CO., excreted to O2 absorbed). For
reasons which are set forth above, the quotients should be approximately equal in the
air collected in the large and in the small spirometers; if they are not so, the condi-
tions of the method have not been correctly carried out.
Since the dead space and the average composition of the alveolar air under these
conditions may be considered constant, the percentage composition of the deep expira-
tion will differ from that of the mixed sample of several normal expirations in propor-
tion as the dead space exerts a greater diluting effect in the small than in the large
expiration. This being the case, the data obtained can be combined algebraically to
give either the capacity of the air passages or the percentage composition of the
alveolar air.
Let A = amount of air in large expiration (small spirometer),
Ai = amount of air in small or normal expiration (tidal air),
B = the percentage of CO2 or O2 in the expired air of large expiration,
Bi = the percentage of CO2 or O2 in the. expired air of small expiration,
x = the capacity of the dead space,
y = the average percentage of C(X or O2 in the alveolar air; then,
A x B = (A - x)y and Ai x Bi = (Ai - x)y.
Solving this for x, y remaining constant under the same physiologic conditions, we
A x Ai x (B-Bi) J
have: x — , the dead space. Or solving for y, we have:
AxB-AixBi
y = . * I 12^ i, the mean percentage of CO,, in the alveolar air. In case the
A- Ai
dead space for O2 is desired, B and Bi must be made to equal the O2 absorbed.
Clinical Method. — The use of the kymograph and pneumograph, and
other complicating factors, make the method as just described quite im-
practicable for clinical procedure, but the use of the same apparatus
with the following modification will yield satisfactory results for most
clinical purposes. The patient is made to respire through the valves for
a short time, after which the observer collects a single expiration in a
small spirometer by turning the stopcock from Position 1 to 2. A sam-
ple of this is taken for analysis, and the spirometer is again emptied
and a series of successive samples of deeper expirations taken. This is
done by directing the patient, after he has started to breathe normally
into the spirometer, to breathe more deeply. The amount of air col-
lected in each expiration is controlled by the observer by closing the
stopcock when the desired volume is obtained. By this means one can
collect several expirations differing from one another by increasing
amounts but all occupying the same time. The samples of the various
expirations are collected in a series of numbered sampling syringes, and
the gaseous composition of each is determined. When the percentage
of C02 or 02 in each expiration is plotted on cross section paper on the
ordinates, with the volume of the expirations in c.c. on the abscissae, a
hyperbolic curve should be obtained. Any marked deviation from such a
curve indicates that some error has been made in taking a sample, and
348
THE RESPIRATION
this observation should be discarded. The different observations are
then combined in the formula given on page 347. The determination
of the C02 percentage of expired air is so simple that a number of speci-
mens of varying depths of expiration can be taken and thus many points
on the curve determined. For the most accurate results it is in general
best to compare only those expirations which differ from one another
by at least 0.3 per cent in C02 and by at least 200 c.c. in volume. This
depends on the fact that the diluting effect of the dead space in reduc-
ing the percentage of C02 in the expired air from that in the alveolar
air is greater in relatively small expirations. If more exact work is de-
sired, the 02 content can be determined on each specimen, the respiratory
quotient calculated, and only those expirations which show the same
respiratory quotient combined.
In the table each observation is compared with each of the others in
all possible combinations. •
PrR CENT
ALVEOLAR CO.,
DKAD SPACE
OBSERVA-
EXPIRED
CO., IN
EXPIRED
1
2
3
1
2
3
TION
AIR
AIR
1
450
3.10
2
637
3.66
4.99
170
3
750
4.00
5.34
189
4
960
4.28
5.30
5.48
5.27
189
183
214
5
1120
4.30
5.11
5.15
4.92
161
140
184
6
1440
4.40
5.16
4.98
4.82
171
127
17"!
General average for CO2 in alveolar air, 5.13.
General average for dead space, 172. Dead space in valves in this experiment was
about 30 c.c.
Another method which has been suggested for clinical purposes is
that of Plesch; this consists in having the subject breathe several times
in and out of a small bag. It is assumed that after such respiration
the composition of the air in the bag will become the same as that in the
alveoli. Although this is no doubt true, it has been shown that the
method is fallacious, because the C02 tension determined in this way
is not that of the arterial blood alone, but is the average between it and
that of the venous blood.
THE CONTROL OF RESPIRATION (Cont'd)
THE NATURE OF THE RESPIRATORY HORMONE
The practical importance of the observations described in the foregoing
chapters in the investigation of the relationship between CH of the
blood and respiratory activity will now be plain, and it remains for us
to consider the physiologic evidence that such a relationship exists. In
the first place, let us consider the behavior of the acid-base equilibrium
'during conditions of abnormal breathing- — hyperpnea and dyspnea.*
As C02 accumulates and 02 becomes used up in a confined space, the
breathing becomes intensified. In searching for the exact cause of this
effect, we must first of all ascertain whether the hyperpnea is due to the
deficiency of 02 or to the accumulation of C02. Many of the experi-
ments bearing on these problems can be more satisfactorily performed on
man than on laboratory animals, because anesthesia is not necessary and
the subjective symptoms experienced are of great value in the inter-
pretation of the results. If an individual is placed in a large air-tight
chamber (2000 liters' capacity), and the depth and rate of breathing ob-
served as the C02 accumulates and the 02 becomes used up in the air of
the chamber, no distinct change in respiration will be observed until the
C02 percentage of the air has risen to almost 3. Above this point, how-
ever, the hyperpnea becomes more and more pronounced, until finally,
when the C02 percentage has risen to about 6 and the 02 percentage has
fallen to 13.5, it becomes unbearable (dyspnea). From the results of the
foregoing observation alone we could not, however, decide whether the
excitation of the respiratory center is due to the deficiency of 02 or .to
the increase of C02. If the experiment is repeated with the difference
that the C02 as it accumulates is absorbed by soda lime, no hyperpnea
will develop even when the 02 is as low as in the previous experiment.
We may conclude, therefore, that in the first experiment C02 accumulation
must have acted as the respiratory stimulus.
The same conclusion is arrived at as a result of observations on indi-
viduals caused to breathe in a more confined space as into a rubber bag
of about 225 liters' capacity. Under these conditions hyperpnea de-
*Hyperpnea means slightly increased breathing; dyspnea, labored breathing, but yet with suffi-
cient ventilation to maintain life; asphyxia, the results of insufficient breathing.
349
350 THE RESPIRATION
velops more rapidly than in the large cabinet, and a higher percentage
(10 per cent) of C02 can be tolerated. That in this case also deficiency of
02 is not responsible for the hyperpnea can be shown by repetition of the
experiment either with an excess of 02 in the bag or with absorption of the
C02 by soda lime. In the former case hyperpnea will develop as usual,
while in the latter it will not supervene until the percentage of 02 has
fallen below 10, when cyanosis becomes marked. In fact, some people
become cyanosed and unconscious, and collapse under these conditions
before there is any respiratory disturbance. A peculiarity of the effect
of 02 deficiency is that the person may be unaware of the seriousness
of his condition ; indeed he may be somewhat stimulated. The conclusion
may be drawn that deficiency of 02 per se can serve as a respiratory
stimulus only when it is so extreme as to cause other serious symptoms.
This conclusion does not rule out an important influence of 02 deficiency
in increasing the excitability of 'the center towards C02. Under ordi-
nary conditions, however, the center is far more sensitive towards plight
changes in the C02 percentage.
There is an obvious reason why the adjustment of pulmonic ventila-
tion should not depend upon changes in 02 supply to the respiratory cen-
ter. If it were so, many other tissue activities and other nerve centers
would suffer from the 02 deficiency before there was time for the breath-
ing to become stimulated sufficiently to make good the loss of 02. As a
matter of fact, headache, dizziness, nausea and even fainting are almost
certain to be caused whenever any muscular exercise is attempted in an
atmosphere containing a deficiency of 02 but no excess of C02 (cf. moun-
tain sickness). An adequate 02 supply of the body is, therefore, insured
by changes in C02 tension of the blood.
Quantitative Relationship between C02 of Inspired Air and Pulmonary
Ventilation. — These results suggest, as the next step in the investigation
of our problem, the determination of the quantitative relationship be-
tween the C02 percentage of the respired air and the amount of air
breathed (pulmonic ventilation).* That there is such a relationship has
been most successfully demonstrated by R. W. Scott, who used for his
purpose decerebrate cats.f The trachea was connected, through a T-tube
provided with valves, with tubing leading to a large bottle and a Gad-Krogh
spirometer, so that the animal breathed out of the bottle into the
spirometer, these two being also connected together. The spirom-
*A distinction is somewhere drawn between pulmonic ventilation and alveolar ventilation, the
former being the total amount of air that enters and leaves the lungs, and the latter, that which en-
ters and leaves the alveoli. This distinction is based on the assumption that the capacity of the dead
space may vary considerably from time to time, which, as pointed out elsewhere, is erroneous. For
practical purposes pulmonic ventilation is the s'afer value to give.
fDecerebrate animals must be used in these experiments, since anesthetics markedly depress the
activity of the respiratory center.
THE CONTROL OP THE RESPIRATION
351
eter was made to record its movements on a drum, so that an accurate
record of the depth and frequency of the respirations was secured. Sam-
ples of air were removed from the bottle by ground-glass plunger syringes
at frequent intervals during the time that the animal was respiring into
the tubing.
joe
400
300
/oo
Fig. 129. — Composite curve obtained from the data on sixteen experiments, showing the re-
spiratory response to COa in the decerebrate cat. Abscissae = percentage of COa in the inspired
air. Ordinates = the percentile increase the tidal air per minute. (From R. W. Scott.)
The results are given in the accompanying curve (Fig. 129), which shows
that there is a perfect correspondence between the C02 percentage in the
air of the bottle and the pulmonary ventilation. Moreover, when the
bottle was filled with 02 instead of air to start with, the same results
were obtained, showing that the C02 accumulation alone was responsible
for the hyperpnea. In these cases the percentage of 02 remaining in the
352 THE RESPIRATION
system after hyperpnea had become extreme, was far above that at which
direct excitation of the center from 02 deficiency is possible.
Experiments of a similar type had previously been performed by Por-
ter and his pupils,21 but their object was not so much to show the close
parallelism between the C02 content of the respired air and the pulmonic
ventilation as to demonstrate the changes produced in the sensitivity of
the respiratory center in pneumonia.
Possibility that C02 Specifically Stimulates Center. — After showing
that C02 acts as an excitant of the respiratory center, the question arises
whether we are justified in the assumption that has been made tentatively
that the action depends on the raising of the CH of the blood, or whether
it may be a specific action of the HC03 anion itself. Many attempts have
been made to decide this question experimentally, the general principle
of the experiments being to determine whether CH of the blood runs
parallel with the C02 content of the respired air and with the hyperpnea.
Using the gas-chain method (page 31), Hasselbalch and Lundsgaard22
found that the hyperpnea produced in rabbits by breathing in C02-rich
air runs approximately parallel with the increase in the CH of the blood,
but on account of the experimental difficulties encountered they could not
decide whether changes in CH are alone responsible for the effect. These
authors had previously demonstrated that changes in CH can be induced
in blood removed from the body by alterations in the C02 tension within
the physiologic limits. An increase of one millimeter in C02 tension
Avas found to cause an increase in CH of 0.0065 x 10 7 (see page 27).
R. W. Scott's experiments, above referred to, have, however, yielded
more definite results. By using the colorimetric method for determining
CH of the blood (see page 32), it could be readily shown, as is evident
from the table (col. 8 in table), that a marked rise in CH became evident
when the inspired air contained 5 per cent or more of C02. That this
rise was due to increase in the C02 tension was shown not only by finding
a greater percentage of C02 (col. 15) in the blood, but also by being able
to demonstrate that when C02-free air was bubbled through the blood
removed during the dyspnea, CH immediately returned to the normal,
which it also did when the blood removed after the animal had breathed
for a few minutes in outside air (col. 16). The C02 content likewise re-
turned (col. 17). Had the increase in acidity been caused by nonvolatile
acids — lactic, for example — these results, particularly the latter, could
not have been obtained.
Although there is therefore no doubt that the CH of the blood ma"
be raised because of an increase in C02 in solution in the blood plasma —
a C02 acidosis, as we may call it (s*ee page 354) — this does not prove that
the stimulation of the respiratory center is brought about solely by CH.
THE CONTROL OF THE RESPIRATION
353
The increase in the carbonate ion — HC03 ion — itself might also serve as
a stimulus. That such is actually the case was demonstrated by
finding that, if the CH of the blood was first of all lowered by injecting
alkali intravenously, hyperpnea still developed in proportion as the CO.,
accumulated in the inspired air; and that CH of the blood, when the
hyperpnea was at its highest, was below that of normal blood. Some
other factor than CH must obviously be responsible for this result. This
must be the HCO, anion.
THE EFFECT OF REBREATHING CARBON DIOXIDE ON THE MINUTE VOLUME AND ON THE
H-ION CONCENTRATION AND TOTAL CARBONATE CONTENT OF THE ARTERIAL
BLOOD IN THE DECEREBRATE CAT
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*PH is the actual value given in the table. This is inversely proportional to CH.
Further corroboration of the claim that the HC03 anion has a specific
stimulating effect on the respiratory center that is independent of CH,
has been furnished by Hooker, Wilson and Connett.23 These authors
succeeded in keeping the centers of the medulla alive by perfusion with
defibrinated blood through the blood vessels of the brain, and found
that, although the respiratory movements of the diaphragm became de-
pressed with a decrease and excited with an increase in CH of the per-
fused fluid, a greater activity of the center was produced when this con-
tained a high tension of C02 than with another fluid of the same CH
but with a low tension of C02. We conclude that, although the CK is the
important respiratory hormone, the carbonate ion (HC03 anion) also has
a stimulating influence.
354 THE RESPIRATION
Relationship Among Acidosis Conditions, Alveolar C02 and Respir-
atory Activity. — It will be plain that variations in the respiratory hor-
mone, whatever this may be, are associated with changes in the C02
content of the alveolar air. Closer examination has shown, however,
that this relationship is by no means always so simple as in the instances
just described, where increased respiration was found to be associated
with an increase in alveolar CO,. There are many cases where the re-
verse relationship obtains — namely, decreased alveolar C02 and hy-
perpnea. The whole question is very closely linked with that of the con-
trol of the reaction of the body fluids and with the etiologic factors in
acidosis. When it is fully answered, many obscure clinical conditions in
which respiratory disturbances occur will be much better understood than
they are at present. On account of its great importance, considerable
attention will be devoted in the next few pages to some of the researches
which have been made bearing on the relationship between the C02 of
the alveolar air and the various modified types of breathing that can be
produced experimentally or become developed under altered physiologic
conditions.
We shall consider these conditions in the following order: (1) Con-
stancy of the alveolar C02 under normal conditions and during moderate
variations in barometric pressure. (2) The quantitative relationship
between an artificially induced increase in alveolar C02 tension (as by
breathing C02-rich air) and the increased respiration. (3) The results
of these observations will demonstrate a very precise relationship to exist
between alveolar C02 tension and respiration, but if we proceed to repeat
the latter observations under conditions where the accumulation of C02
in the inspired air is accompanied by oxygen deficiency (as by breathing
in a confined space), we shall see that the relationship no longer holds,
indicating that the oxygen deficiency has caused something to happen
which disturbs it.
We shall find that the disturbing factor is accumulation of unoxidized
acids in the blood, and this will naturally lead us to study the conditions
in which such acids might develop; namely, (4) Breathing in rarefied
air (mountain sickness). (5) Apnea. (6) Muscular exercise.
In succeeding chapters, we intend to review the work in these fields in
considerable detail, partly because of its very important bearing on the
general question of the control of the respiratory center and partly be-
cause of the light the observations throw on the nature of the mechanism
involved in the adjustment of the CH of the blood and tissues.
As we have seen, much work concerning the physicochemical principles
involved in the control of the reaction of the blood has been contributed
during recent years by physical and biological chemists, but much of this
THE CONTROL OF THE RESPIRATION 355
work in our judgment fails to pay sufficient regard to the extraordinarily
complicated conditions existing in the animal body, and more particu-
larly, to correlate the purely physicochemical data with the numerous
observations that have from time to time been recorded by physiologists
regarding the behavior of the respiratory center. Physical chemists have
recently, for example, gone so far as to define acidosis as a condition in
which there is a diminution in the bicarbonate content of the blood in-
duced by the discharge into it of fixed acids. This is going too far, for
it fails to recognize acidosis due to an increase in the C02 of the blood.
r TT CO ~\
It is the molecular ratio ,T *^A which determines the tension of C02.
[NaHCOg J
When C02 is added to the blood, either experimentally by respiring the
gas, or naturally OAving to muscular exercise or to pathologic conditions in
which there is a deficient excretion of C02, as in heart disease, the ten-
dency of the equation to change, by increase of the numerator, is pre-
vented partly by stimulation of the respiratory center, which gets rid of
C02, and partly by an increase in the denominator. The respiratory
center is so sensitive to slight increases in CH that it becomes excited
before a sufficient increase in H2C03 has occurred to disturb the normal
r H CO ~\
ratio HT tT^A When fixed acids are added to the blood the denom-
L NaHCOg.J
inator of the equation, NaHC03, is lowered and consequently CH rises,
and increased respiration occurs, lowering H2C03 and thus reestablishing
the ratio.
CHAPTER XLI
THE CONTROL OF RESPIRATION (Cont'd)
THE CONSTANCY OF THE ALVEOLAR C02 TENSION
UNDER NORMAL CONDITIONS
Since a close relationship exists between the alveolar C02 tension and
the respiratory activity, it is to be expected that the two would bear a
strict proportionality to each other, and since the breathing under normal
conditions does not vary much, the C02 tension should also be constant.
Many observations show this to be the case. The tension is remarkably
constant from day to day and even from month to month in the same
individual, provided the physiologic conditions are the same. A slight
seasonal variation is said to exist, a rise in the temperature of the en-
vironment of the individual causing a slight depression in the C02 ten-
sion, while a fall in temperature causes a slight rise (Haldane). These
changes are independent of any demonstrable change in rectal temper-
ature and, therefore, are probably due to the influence of the temperature
on the skin.
Since it is the number of molecules of C02 in a given volume of alve-
olar air (i. e., the partial pressure or tension) that is of importance, it
is only when the barometric pressure is the same that the percentage of
C02 in the sample of alveolar air can be constant. To allow for this,
all results are reduced to standard barometric pressure (760 mm. Hg).
If the barometric pressure is lowered, there will have to be a higher
percentage of C02 in the sample in order that there may be the same
tension of this gas in the air of the alveoli ; and vice versa when the bar-
ometric pressure is raised. The equation by which this tension, ex-
pressed in millimeters of mercury, is determined is: 100:760: :a:p, where
a is the percentage actually found in the air of the sampling tube and p
the tension. A correction must also be introduced in this equation to
allow for the vapor tension of the air in the alveoli, for of course H20
molecules will behave like C02 molecules in causing a partial pressure.
When reduced to this standard, it has been found that the tension of
C02 in the alveolar air remains constant under the different barometric
conditions that obtain at the top of a mountain or at the foot of a deep
mine. This is shown in the following table:
356
THE CONTROL OF THE RESPIRATION 357
(1)
(2)
(3)
BAROMETRIC
CO, ACTUALLY FOUND
PARTIAL PRESSURE
PRESSURE
IN DRY ALVEOLAR
OF CO, IN MOIST
(MM. HO)
AIR
ALVEOLAR AIR AFTER
(PERCENT)
CALCULATING FOR
BAROMETRIC PRESSURE
Top of Ben Nevis
646.5
6.62
5.23*
Oxford
755
5.95
5.53
Foot of Dolcoath Mine
832
5.29
5.48
Compressed air cabinet
1260
3.52
5.64
*The figures in this column are
arrivpd at hv tf
B' - A x P'
P. when P — figures in
B
last column; B' — figures in first column; A = aqueous tension of alveolar air; P' = figures of
second column; B = barometric pressure at sea level. A is obtained from tables giving the aqueous
tension at different temperatures.
Changes in the frequency of breathing that are within physiologic
limits have no influence on the tension of alveolar C02, provided that
exactly the same time is taken in performing the forced expirations
during which the samples of alveolar air for analysis are removed.
The Degree of Sensitivity of the Respiratory Center to Changes in the
C02 Tension of the Alveolar Air
This can be determined by observing the alterations produced in the
volume of air that actually enters the alveoli (alveolar ventilation) dur-
ing breathing in atmospheres containing different percentages of C02.
In man an increase of from 0.2 to 0.3 per cent in the alveolar C02 is
sufficient to double approximately the alveolar ventilation ; or, more pre-
cisely, an increase of ten liters in the air entering and leaving the alve-
oli per minute is caused by raising the alveolar C02 tension by from 2.2
to 3.1 mm. Hg (Douglas, etc.)24.
THE ALVEOLAR C02 TENSION DURING BREATHING IN A
CONFINED SPACE
We have already employed similar experiments in ascertaining whether
C02 accumulation or 02 depletion is responsible for the hyperpnea pro-
duced under these conditions. We concluded for the former, but now
on closer examination we shall see that, although our conclusion was
correct, the deficiency in 02 also has an indirect effect on the respiratory
center. This is revealed by the fact that the tension of the C02 in the
alveolar air does not increase in proportion to the observed increase in
pulmonary ventilation. We must conclude that the decrease in 02 has
some effect. How may this be explained? Two possibilities exist: (1)
that the 02 want has caused organic acids to accumulate in the blood
and so raise the CH ; and (2) that in the absence of a certain tension of
358 THE RESPIRATION
02 the excitability of the center is raised (i. e., its "threshold" lowered),
so that it becomes stimulated by CH, to which ordinarily it does not re-
spond. We shall now proceed to examine the experimental evidence
bearing on these possibilities.
By examination of the alveolar air "of an individual confined in a pneu-
matic cabinet in which the barometric pressure is gradually lowered,
it has been found that although the C02 tension remains constant for
a considerable range (cf. page 356), it begins to fall when the barometric
pressure has reached about 550 mm. Hg. At this pressure the tension
of 02 in the alveolar air will be 62 mm. instead of its normal of about
105 at atmospheric pressure. Below it the alveolar C02 tension quickly
falls, and at the same time hyperpnea becomes evident, although the
person himself may be unaware that he is breathing more deeply. If
this experiment is repeated with the difference that, as the pressure is
lowered, an excess of 02 is introduced into the chamber, the hyperpnea
does not supervene until a barometric pressure has been reached that is
distinctly lower than when no excess of 02 is present, and at the same
time the C02 tension in the alveolar air remains unchanged. The ex-
planation of this result is that by lowering the 02 tension in the alveolar
air and, therefore, in the blood and tissues, oxidative processes become
depressed so that unoxidized acids, such as lactic, accumulate in the
blood and by adding their effect to that of the C02 serve to raise the CH
of the blood. As a result, the respiratory center becomes excited, hy-
perpnea supervenes, and the volatile CO, is removed from the blood into
the alveolar air. On supplying 02 artificially, this failure of proper
oxidation does not set in and breathing goes on normally-
There should be a stage in the above experiment during which the
CO, tension of the alveolar air is increased — namely, when the fixed acids
first appear and decompose the carbonates of the blood. This stage has,
however, not been detected. When a person is kept in such a chamber
for some time at a pressure which causes a diminution in the alveolar
C02 tension, the tension does not immediately return to its normal level
when atmospheric air is again breathed, indicating that the fixed acids
are only slowly got rid of.
The second hypothesis — namely, that the 02 deficiency directly raises
the excitability of the respiratory center — has many advocates, among
them Lindhard,25 who found that, Avhen the percentage of 02 in the alve-
olar air was raised, a higher percentage of C02 was necessary to cause
an increase in the ventilation of the lungs, and conversely, that a distinct
increase in the excitability of the center occurred when the inspired air
contained less than the normal percentage of 02. Although it is ad-
mitted by Haldane and his school that such alteration in the excitability
THE CONTROL OF THE RESPIRATION 359
of the respiratory center to the CH of the blood may occur after long-
continued exposure of the center to the changed tension of 02, yet they
deny that such alteration can occur as a temporary condition. These
workers found that, in order to raise the pulmonic ventilation by 100
per cent, the increase in the alveolar C02 tension required was practically
the same (0.3 per cent) when the inspired air contained 20 per cent of
O2 as when it contained 54 per cent.
In the observations already referred to on the decerebrate cat, R. W.
Scott20 has secured some evidence that would seem to support Haldane's
contention. He found that the response of the respiratory center to the
percentage of C02 in the respired air was exactly the same whether the
latter contained a low (13-14) or a high (30 and over) percentage of 02.
The possibility that the excitability of the respiratory center is affected
directly by the 02 tension is to be considered as one of the most im-
portant problems awaiting solution.
Even if it may have a certain influence on the excitability of the re-
spiratory center, 02 deficiency per se can serve as a direct stimulus of
the center only when it is of extreme degree. Much light has been
thrown on the relationship of 02 to respiratory activity by observing
the respirations during breathing in and out of rubber bags through
soda lime absorption bottles of sufficient size to remove the C02. We
have already seen that even the general results of such observations
(page 349) show clearly how much more potent a respiratory stimulant
is accumulation of C02 than deficiency of 02. More particular investi-
gation in which the alveolar air is analyzed bears out these conclusions
and at the same time indicates the exact conditions under which organic
acids become -developed.
With a very small bag (a few liters' capacity) hyperpnea of a dis-
tressing type but without cyanosis supervenes in a few minutes, and the
alveolar air contains perhaps as low as 6 per cent 02 and 4 per cent C02.
Of still greater interest and significance, however, is the fact that the
ratio between the volume of C02 excreted and of 02 absorbed (respira-
tory quotient) during the hyperpnea is raised considerably above unity,
indicating that an. excessive excretion of C02 must be occurring. This
result is explained by assuming that the deprivation of 02 causes large
quantities of fixed acids to be produced, and that these expel C02 from
the blood more quickly than the 02 is absorbed. In corroboration of
this explanation, it has been observed that, after outside air is breathed
for some time following the above experiment, the respiratory quotient
becomes very low, so that C02 must now be accumulating in the blood.
If the above experiment is repeated Avith a larger bag (about 200
liters), so that the 02 falls slowly, the breathing can be maintained for
360 THE RESPIRATION
a much longer period without any evident symptoms of hyperpnea, even
though the 02 percentage in the alveolar air may fall as low as in the
previous experiment, and there are marked symptoms of 02 want, such
as cyanosis, twitching of the muscles of the hands, lips, etc. The re-
spiratory quotient does not become abnormal in this experiment indicat-
ing that no expulsion of C02 from the blood can have occurred as in
the previous experiment. The cause for the virtual absence of hyper-
pnea in this experiment is no doubt that the more gradual reduction in
02 of the alveolar air and therefore of the blood did not bring about the
accumulation of -lactic acid at a rate that was greater than that at which
the C02 was got rid of into the alveolar air.
BREATHING IN RAREFIED AIR; MOUNTAIN SICKNESS
In considering the part played by fixed organic acid in the control
of the CH of the blood, the most important results have been secured
by observations on the condition of individuals living at high altitudes.
As is well known, under these conditions certain symptoms are likely
to develop, the condition being known as mountain sickness. The great
interest which physiologists have taken in this subject has been owing,
not so much to the importance of the observations in connection with
the condition itself, as to the light which they throw on the mechanism
of respiratory control and on the cause for abnormal types of breathing.
More or less hyperpnea, especially on exertion, soon appears in a
rarefied atmosphere, and the alveolar C02 tension assumes a value con-
siderably below the normal. For example, at sea level the minute vol-
ume of air breathed in one individual was 10.4 liters, and the alveolar
C02 tension 39.6 mm. Hg. After being some time on Pike's Peak, where
the barometer registers only 459 mm. Hg, Douglas26 found the minute
volume of air to be 14.9 liters, and the alveolar C02 tension 27.1 mm. Hg.
At first sight the above statement may seem to contradict one pre-
viously made, to the effect that the alveolar C02 tension remains constant
at different barometric pressures. This applies, however, to the imme-
diate effects, whereas we are now considering the later effects. The im-
portant point is: How are we to reconcile with the above hypothesis the
fact that a diminution in the alveolar C02 tension should be accompanied
by hyperpnea? A solution of the seeming contradiction will not only
be of importance in connection with our present problem, but will assist
us in the investigation of the clinical conditions of hyperpnea, in which
likewise a diminished C02 alveolar tension is often observed. Mountain
sickness may indeed be considered as an intermediate condition between
the physiologic and the pathologic.
From what we have learned we should expect the above result to be
THE CONTROL OF THE RESPIRATION
361
dependent upon an increase in the nonvolatile acid content of the blood
That such is really the case has been conclusively shown both by titra-
tion methods and by observing the dissociation curve of hemoglobin,
which, as will be explained later (see page 386), may be made to serve
Fig. 130. — The horizontal interrupted lines represent the mean normal alveolar CO2 and O^
pressures at sea level (i.e., Oxford and New Haven); the thick line, alveolar CO2 pressure; and
the thin line, alveolar O* pressure. (From Douglas, Haldane, Henderson, and Schneider.)
as an index of the H-ion concentration of the blood. The exact chemical
nature of the nonvolatile acids that accumulate in the blood is not as yet
known. Two types of acid can be thought of, either unoxidized organic
362 THE RESPIRATION
acids, of which lactic acid may be taken as the representative, or inor-
ganic substances, like the acid phosphates. That it is not lactic acid is
shown by both direct and indirect evidence. The direct evidence has
been furnished by Ryffel, who was unable to find any increased per-
centage of this substance either in the urine or in the blood of persons
who had been living for some time in the famous Regina Margherita
hut on Monte Rosa.27 The indirect evidence has been furnished by ob-
serving the time that it takes after the individual has started breathing
the rarefied air for the alveolar C02 tension to fall, as well as that re-
quired to bring about the recovery to the normal when he descends to
sea level. The following curve, which is self-explanatory, will illustrate
these points.
Thus, on Pike's Peak, where the barometric pressure is 459 mm. Hg,
the C02 tension after an initial fall took about seven days before it
came to its permanent level for that barometric pressure, and fourteen
days elapsed after descending from the mountain before the sea-level
tension had been regained. The slow nature of these changes, when com-
pared with, the rapid changes observed in the experiment with the bags
already alluded to (page 358), shows clearly tliat lactic acid can not be
responsible for the increase in H-ion concentration in mountain sickness.
By exclusion it would appear that the increase in CH is the result of an
excess of fixed inorganic acid (H3P04) in the blood dependent on a dis-
proportionate excretion of bases by the kidneys during the period of
acclimatization to the rarefied air.
Other observers aver that the acidosis does not really exist, but that
the excitability of the respiratory center itself becomes raised (its
threshold lowered), so that it responds more readily to the normal CH
of the blood. It has been stated that the increase in excitability of the
center is dependent upon the action of the intense light rays at high
altitudes — the erythema of the skin, etc., being evidence of this excit-
ing action of light. The constant irritation of the skin, these authors
say, serves by stimulation of afferent nerves to maintain a hyperexcit-
ability of the respiratory center. Others believe that the hyperexcit-
ability of the center is a direct result of the maintained 02 deficiency.
The balance of evidence, however, stands in favor of the view that the
phenomena of mountain sickness depend on changes occurring in the in-
organic nonvolatile acids of the blood. The other phenomena of this
interesting condition will be discussed elsewhere (page 399).
APNEA
If a man breathes forcibly and quickly for about two minutes, he
will experience no desire to breathe for a further period of about the
THE CONTROL OF THE RESPIRATION 363
same duration — he becomes apneic. "When the desire to breathe re-
turns, the breathing is at first very shallow, but gradually becomes more
marked, until at last normal respiration'is reestablished. If a sample of
alveolar air is removed at the time when the desire to breathe returns,
it will be found to contain a very small percentage of 02 indicating
that for some time previous to the onset of breathing there had been in
the alveolar air, and therefore in the blood, so low a percentage of 02
that if 02 deficiency could stimulate breathing, this would have started
much earlier than it actually did. A curve showing the results of such
an experiment by Haldane is given in Fig. 131. The person may begin
to show symptoms of 0, want, such as cyanosis, before the desire to
breathe returns, which furnishes strong proof that 0, want itself can
not serve as a stimulus to the respiratory center. The failure of the
center to act must rather be due to the lowering of the CH consequent
upon the removal of CO, from the blood by the forced respiration which
preceded the apnea — washing out of the CO,, as it is called. That this
has really occurred can readily be shown by estimating the C02 con-
tent of a sample of alveolar air collected by having the subject make a
forced expiration early in apnea. Extremely low values along with a
respiratory quotient (page 547) of about 0.2 are often found, whereas,
during the preceding forced breathing while the C02 is being washed
out, the quotient is often ten times as great — viz., 2.0.
As would be expected, the low 02 percentage present in the alveolar
air toward the end of the apneic pause is not without some effect, indi-
rect though it may be, 011 the excitability of the respiratory center.
This accounts for the fact that the alveolar air, at the moment the de-
sire to breathe returns, usually- contains a lower percentage of C02 than
the normal, indicating that some nonvolatile acid must have accumulated
in the organism so as to raise the CH of the blood, and thus require a
lower tension of CO, to overstep the threshold of excitability of the re-
spiratory center. In agreement with this explanation it has been found
that, if the last two or three forced respirations preceding the apnea
are made in an atmosphere of 0, instead of air, so as to fill the alveoli
with 02, the apnea can be maintained for a very much longer period;
and when the natural desire to breathe returns, the C02 tension of the
alveolar air, instead of being below the normal, is above it. The effect
of 02 in prolonging apnea must, therefore, be dependent on the fact that
it prevents the accumulation in the organism of the unoxidized acids,
leaving to C02 alone the function of raising the CH in the blood to the
level required to excite the respiratory center. By this means the period
during which the breath can be held after breathing 02 is sometimes
364
THE RESPIRATION
tFi&txvuA <m, m/m3ta
Fig. 131. — Curves showing variations in alveolar gas tensions after forced breathing for two
minutes. Thin line = O2 tension; thick line — CO2 tension. Double line = normal alveolar
CO2 tension. Dotted line shows the alveolar CO2 tension at which breathing would recommence
at the end of apnea with the alveolar Oz pressures shown by the thin line. The actual breathing
is indicated at the lower part of the figure. It is periodic to start with. (From Douglas and
Ilaklane.)
THE CONTROL OF THE RESPIRATION 365
phenomenal; in one individual, for example, after breathing forcibly for
a few minutes and then filling the lungs with 02, apnea lasted for eight
minutes and seventeen seconds.
The Supposed Nervous Element in Apnea
It is necessary to point out that; prior to the elaboration of accurate
methods for the investigation of the chemistry of respiration, many
physiologists interpreted the apnea following forced breathing as the
result of a sort of inhibition, of the respiratory center brought about by
its repeated stimulation by afferent- nervous impulses transmitted to it
along the vagus nerves, these impulses being set up by the frequent col-
lapse and distention of the alveoli acting on the terminations of the
nerve. In justification of the nervous interpretation of apnea, it was
claimed by the earlier observers that it could not readily be produced
in animals after severing both vagus nerves. More recent work has
shown tha* this is not an accurate observation, for if the severing of
the vagi is accomplished not by cutting but by freezing, then apnea is
as readily produced as in an intact animal (Milroy).28
That chemical and not nervous factors cause the apnea is further
demonstrated by the well-known experiment of Fredericq, who, after
ligating the vertebral and one of the carotid arteries in two dogs, anas-
tomosed the central end of the remaining carotid of the one to the
peripheral end of the carotid of the other animal, thus establishing a
crossed circulation. He then found that by applying forced artificial
respiration to the one animal, the apnea which supervened affected the
other animal and not that to which the artificial respiration had
actually been applied. Another proof of the chemical theory of
apnea is furnished by the observation that if forced breathing is per-
formed in an atmosphere containing C02 in about the same partial pres-
sure as in the alveolar air, no apnea supervenes, and if the experiment
is repeated several times with progressively declining percentage of
C02 in the air each time, the length of the apneic pause proportionally
increases as the C02 pressure in the inspired air diminishes.
Although in the foregoing account we have adopted Haldane's view
that oxygen deficiency per se can act as an excitant of the respiratory
center only when it is of extreme degree, it should nevertheless be pointed
out that studies by A. S. Loevenhart on the action of cyanides on the
respiratory center have led him to conclude that interference with oxida-
tive processes may be a more potent factor in its stimulation than the
experiments in which oxygen-poor atmospheres are respired would lead
us to expect. We must await further evidence before a final verdict is
pronounced on this most perplexing problem of modern physiology.
CHAPTER XLII
THE CONTROL OF RESPIRATION (Cont'd)
THE EFFECT OF MUSCULAR EXERCISE ON THE
RESPIRATION
During muscular exercise the pulmonic ventilation increases to an
extraordinary extent. At rest an average man respires 6 to 8 liters of
air per minute, but during walking on the level at the rate of 5 kilometers
an hour, this figure may increase to about 20 liters.
The first investigations as to the cause of the relationship between
muscular activity and pulmonic ventilation were made by animal ex-
periments in which tetanus of the muscles of the hind limbs was pro-
duced by electric stimulation of the spinal cord. The problem was to
find out what serves as the means of correlation* (nerve reflex or hormone
control) between the muscular activity and the respiratory activity.
By cutting the spinal cord above the point of stimulation, it was found
that the tetanus was still accompanied by as marked a hyperpnea as
before. On the other hand, Avhen the spinal cord was left intact but the
blood vessels of the limb were ligated, no hyperpnea followed the teta-
nus. Evidently therefore the pathway of communication is the blood.
The next step was to seek in the blood for the substance or hormone that
acted as the respiratory excitant, and naturally the first possibility con-
sidered was a change in the. gases of the blood, either a deficiency
of 02 or an increase in C02. Direct examination of the blood for the
quantity of these gases, however, yielded results \vhich were quite con-
trary to such an hypothesis. It was found that the percentage of 02,
if anything, was slightly increased, and that of the C02, if anything,
diminished. Moreover, when the expired air was analyzed during the
hyperpnea, the percentage of CO, contained in it was distinctly below
the normal average, and the percentage of 02 above it. Evidently, there-
fore, the amount of gases in the blood has nothing to do with the excita-
tion of the respiratory center, and the conclusion drawn by the earlier
investigators was to the effect that the exciting substance carried from
the active muscles to the respiratory center must be some unusual meta-
bolic product, possibly the lactic acid produced by contraction.
It was further found, by examination of the respiratory quotient, that
366
THE CONTROL OF THE RESPIRATION
367
an excess of C02 was being expired during the work and immediately
after it, but that this was subsequently followed by a much lower quo-
tient, indicating that C02 was being retained. Such a result would be
in conformity with the view that an acid such as lactic is discharged
into the blood, on. the carbonates of which it would act as explained on
page 355. Breathing in and out of a small rubber ba,g causes the same
alterations in the respiratory quotient (see page 358).
That lactic acid is actually produced by contracting muscle could not,
however, be shown by all investigators, and it was not until some years
later that Fletcher and Hopkins29 clearly demonstrated the conditions
under which it may appear in active isolated muscle. These observers
found that lactic acid is produced in excised muscles only -when the
muscular contraction occurs in a deficiency of 02. When it occurs in an
adequate supply of 02, C02 instead of lactic acid is produced.
Taking these facts together with what we already know concerning
the conditions under which the respiratory center reacts to conditions
which presumably cause a change in the CH of the blood, we may formu-
late the hypothesis that respiratory activity during muscular exercise
is due to a slight increase in ^ie CH of the blood, and that this increase
is OAving partly to an actual increase in C02 production by the acting
muscles and partly to the production of lactic acid. Such an hypothesis
would satisfactorily explain Avhy the actual amount of C02 in the blood
might be below the normal during muscular exercise, for the CO, would
be "washed out" from the blood by the hyperpnea induced by the in-
crease in CH.
The obvious method of putting this hypothesis to the test is to ex-
amine the alveolar CO, tension and the respiratory quotient under various
conditions of muscular activity. The results of such observations are
given in the accompanying table.
(1)
(2)
(3)
(4)
(5)
O2 used
CO2 pro-
R. Q.
CO2in
Total alveolar
in c.c.
duced in c.c.
vol. CO2
alveolar
ventilation in
per min.
per min.
vol. 02
air
liters per min.
I.
During rest, standing 328
264
. 0.804
5.70
5.80
2.
Walking at the rate of
3 kilometers per hour 780
662
0.849
6.04
13.6
3.
Walking at the rate of
5 kilometers per hour 1065
922
0.866
6.10
18.8
4.
Walking at the rate of
6 kilometers per hour 1595
1398
0.876
6.36
27.6
5.
Walking at the rate of
7 kilometers per hour 2005
1788
0.891
6.20
35.6
6.
Walking at the rate of
8 kilometers per hour 2543
2386
0.938
6.10
48.2
THE RESPIRATION
In the first column is given the 02 used in c.e. per minute. Among other
things these figures indicate the actual amount of work done. In the
second column is given the C02 production in c.c. per minute. By divid-
ing the figures of the second column by those of the first, we obtain the
figures of the third column, representing the respiratory quotient. The
fourth column gives the C02 content of the alveolar air, and the last
column the total alveolar ventilation in liters per minute.
Taking for the present the figures in the first and fourth columns and
postponing a consideration of the respiratory quotient, it will be noted
that, as the muscular work increases up to a total consumption of about
1600 c.c. of 02 per minute, the C02 percentage in the alveolar air
steadily increases. The question arises, does the alveolar ventilation
increase in proportion to the increase in C02 tension? If it does so,
increase in C02 tension in the blood can be held solely responsible for
the hyperpnea (i. e., a pure C02 acidosis) ; whereas if the hyperpnea is
greater than can be accounted for by the increase in C02 tension, other
acids must be partly responsible for the acidosis. By making this same
individual breathe atmospheres containing different percentages of C02
it was found that to produce a doubling of the alveolar ventilation it
required an increase amounting to 0.33 per cent of an atmosphere of C(X
in the alveolar air (see also page 357). When we examine the above
figures during muscular exercise, however, we find that a rise in alveolar
C02 from 5.70 to 6.36 (i. e., 0.66 per cent) multiplied the normal alveolar
ventilation by considerably more than four times, whereas had it been
entirely due to an increase in C02, it should not have been more than
twice as much. Evidently therefore^ some other factor than C02 tension
must have been responsible for the increased respiratory activity. This
conclusion is further confirmed by examination of the alveolar C02
during very strenuous muscular effort, when a relative decrease in the
C02 percentage becomes apparent.
If it is true that the exciting agency has been dependent partly on an
increase in the C02 tension of the blood, and partly on the production of
nonvolatile organic acids (lactic acid), we should expect that imme-
diately after discontinuing the muscular exercise the C02 tension of the
alveolar air would fall to a level distinctly below normal, that it would
only slowly recover thereafter, and that further exercise before the re-
covery had occurred would produce only a slight increase in alveolar
C02. These results we should expect because of the much slower rate at
which the nonvolatile organic acid is got rid of from the organism, com-
pared with the volatile C02. By actual experiment these suppositions
have been found to be correct, as is shown in the following table.
THE CONTROL OF THE RESPIRATION 369
TIME AFTER DISCONTINUING ALVEOLAE CO., TENSION
A BRIEF PERIOD OF IN MM. HG
MUSCULAR EXERCISE
1st Period:
10"
49.2
3' 0"
35.4
6' 30"
35.3
12' 30"
35.8
2nd Period:
10"
38.9
3' 0"
33.7
6' 30"
34.4
3rd Period:
10"
36.9
3' 0"
34.4
8' 30"
32.4
18' 30"
33.7
24' 0"
36.2
Normal resting:
39.0
(Douglas.)
In this table the figures of Period 1 represent the alveolar C02
tension in mm. Hg immediately following a period of strenuous work.
The figures in Period 2 are for the same individual again performing
the same amount of work with, however, only a short period of rest in-
tervening, and the figures of the third period are a repetition of the same
conditions. It will be observed that the muscular exercise at first raised
the alveolar tension of C02 from the normal of 39 mm. to 49.2 mm., but
that in three minutes after the work had been discontinued the tension
was considerably below the normal. During the second period of mus-
cular exercise the C02 in the alveolar air collected immediately after the
effort did not increase above the normal level, and in the third period
the increase was still less — results which are entirely in conformity with the
view that as a consequence of the first period of muscular exercise non-
volatile organic acids had accumulated in the blood, so that to produce
the required respiratory activity in the second and third periods a
much less increase in C02 tension was required.
We may sum up the conclusions which these observations justify by
stating that during muscular exercise the CH of the blood becomes slightly
increased because of the liberation into it of C02 and of lactic acid from
the acting muscles. The respiratory center is, however, so sensitive to
the slightest increase in CH that it immediately responds and produces
hyperpnea, with the result that the volatile C02 is so washed out of the
blood that the CH is held down in spite of the continued production of
acid substances by the muscles. The more strenuous the exercise, the
less able is the 02 content of the blood to keep pace with the metabolic
activity of the muscles, so that relatively more and more lactic acid is
produced, necessitating therefore a greater and greater washing out
of C02.
370 THE RESPIRATION
The readiness with which C02 can be got rid of prevents the hormone
which excites the respiratory activity from continuing to act after it is
no longer required. Provision for the removal of a hormone after its
activity has been displayed is of course essential to efficient correlation
of function, and is seen in the case of other hormones, such as epinephrine
and secretin, whose discontinuance of action is effected by their de-
struction in the blood (see page 745).
Direct evidence that lactic acid is formed during strenuous muscular
exercise in man has been furnished by Ryffel.30 Blood removed from a
person immediatey after running at full speed for about three minutes
contained 70.8 milligrams of lactic acid per 100 c.c. of blood, the normal
amount being 12.5 milligrams. Much of the lactic acid accumulating in
the blood is no doubt got rid of by oxidation, but a large part of it is
also excreted by the urine, in which it was found by Ryffel in consider-
able amount after strenuous muscular exertion.
Finally, let us consider for a moment the behavior of the respiratory
quotient. This ratio rises early in the muscle work (Table on page 367),
indicating that more C02 is being excreted than O2 absorbed. After the
work is discontinued, it usually falls below the normal because of retention
of C02 to take the place of the lactic acid that is being gradually used up
or excreted. A similar fall may sometimes occur in the respiratory
quotient during muscular exercise, if this is continued for a long time.
It probably indicates that a balance has been struck between the produc-
tion of lactic acid in the muscles and the loss of this substance by oxida-
tion. In any case it is a significant occurrence, for it coincides with the
great improvement in the subjective sensations accompanying muscular
exercise. It occurs, for example, at the same time as the appearance of
the "second wind," when the circulatory and respiratory distress expe-
rienced during the earlier stages of strenuous muscular exertion disap-
pear. The stages prior to the second wind correspond to the period when
considerable quantities of free C02 are being got rid of from the blood
and are probably creating a temporary maladjustment of the CH which
acts on the various medullary centers. If by forced breathing much of
this C02 is discharged before the muscular exercise is undertaken, the
initial hyperpnea is not nearly so marked.
THE CONTROL OF RESPIRATION (Cont'd)
PERIODIC BREATHING
Types of Periodic Breathing
In the best known. of these, called Cheyne-Stokes respiration, a period
of hyperpnea supervenes upon one of apnea, each period following in
regular sequence. After an apneic period, the breathing begins at first
faintly, gradually becomes more pronounced until it is markedly exag-
gerated, and then fades off again to the apneic pause. Sometimes the
apneic period is immediately followed by one of intense hyperpnea, there
being no gradual 'increase 'In the respiratory movements. Between these
two types all varieties of the condition are met (Fig. 132).
The conditions in which periodic breathing occurs may be divided into
physiologic and pathologic groups. Of the physiologic conditions the
following may be taken as examples: (1) Breathing in an atmosphere
containing a deficiency of 02; thus, periodic breathing is very readily
produced in persons living in rarefied air. (2) The initial breathing fol-
lowing an apnea induced by forced ventilation of the lungs. In this post-
apneic periodicity, the apneic periods may at first be quite marked, but
as breathing returns they become gradually shorter and the breathing
intervals gradually longer, until normal respiration is restored (Fig.
131). (3) Breathing through a long tube having a small vessel contain-
ing soda lime inserted between the tube and the mouth, the Avhole capacity
of this vessel and tubing being about a liter. This will cause periodic
breathing in persons that are susceptible to oxygen deficiency. Even
breathing through the tube without soda lime will sometimes cause a
periodic type of breathing in such individuals.
The pathologic conditions in which periodic breathing becomes devel-
oped are particularly those associated with renal disease and cerebral
hemorrhage. In many of these cases, the periodic breathing does not
appear to depend on the same factors as are concerned in the experi-
mental types. The symptoms would rather appear to depend on some
influence of the higher cerebral (supranuclear) centers on the respiratory
center. At least some other evidence of disturbance of the cerebral func-
tions is always forthcoming, such as a slight paralytic shock, and the
371
372
THE RESPIRATION
periodic breathing is nearly always aggravated during sleep. Many of
these cases are greatly benefited by administration of caffeine.
In both the physiologic and the pathologic groups, the breathing may
develop a periodic character only when the person is asleep, and even
normal people, particularly infants or very old people, may exhibit it to
a certain degree.
Fig. 132. — Various types of periodic breathing. (From Mosso's "Life of Man in the High Alps.")
Causes of Periodic Breathing
Great interest attaches' to an investigation of the causes of periodic
breathing, but it can not be claimed that any perfectly satisfactory ex-
planation has as yet been offered. Pembrey31 attributes it to a diminished
excitability (a raised threshold) of the respiratory center due to faulty
blood supply, the supposition being that, when thus suppressed, the
normal CH of the blood is unable to excite the center, so that breathing
stops. During the resulting apnea, C02 again accumulates until it has
THE CONTROL OF THE RESPIRATION 373
raised the CH sufficiently to excite the depressed center. Hyperpnea
follows, causing a washing out of the C02 and a resulting diminution of
the effective stimulus, so that again the center fails to be stimulated and
apnea supervenes, and so on. Support for this explanation would appear
to be furnished by the fact that, when patients exhibiting periodic breath-
ing are made to breathe an atmosphere containing a high percentage of
C02, the periodicity of the breathing may give place to regular breath-
ing; a result which may also be obtained by making such patients
breathe in atmospheres rich in oxygen. In the former case, the stimulus is
raised to meet the depressed excitability of the center; in the latter, the
excitability of the center is increased because of- better blood supply
so that it is enabled to react to the diminished stimulus. But even
granted that the excitability of the center is depressed, it is difficult to
see why this should occasion a periodic type of breathing unless we as-
sume that it is only when stimulus (i. e., CH of blood) and threshold of
excitability of the center are adjusted at a certain physiologic level that
smooth and continuous action can go on.
Haldane and his school aver that there is np permanent alteration in
the excitability of the center, but that the periodicity is due to several
causes, which do not always operate to the same degree in the different
conditions in which such periodicity exists. To study these causes the
exact conditions existing in the various types of periodic breathing that
can be produced experimentally in man have been investigated.
The most simple to consider first is the periodic breathing that is
produced in a person susceptible to 02 want, by breathing through a tube
and bottle (of a total capacity of 1 liter), containing soda lime.
In such a case no outside air enters the lungs, for what we have really
done, besides providing for the absorption of C02, is greatly to prolong
the dead space. The oxygen tension of the rebreathed air, therefore,
quickly falls, until at last a point is reached at which the respiratory cen-
ter is. directly stimulated by 02 deprivation, as we have seen it to be
when this falls to a sufficiently low level (see page 350). The deep
breaths (hyperpnea) which follow, being of greater volume than 1000
c.c., cause outside air to be inspired so that the 02 want is made good
and the hyperpnea again disappears, possibly to the extent of apnea, for
now, in consequence of a coincident "washing out" of C02, there has
been a lowering of the CH of the blood below the threshold value. During
the apnea the 0, is rapidly used up, till a point is reached at which the
center again becomes excited. In such an experiment the effect of 02
want becomes very marked, as shown by the intense cyanosis which
develops.
That breathing under these conditions should be periodic and not
THE RESPIRATION
merely shoAv a steadily increasing hyperpnea is probably due to the un-
equal rates at which the 02 and C02 tensions change in the blood. Be-
cause of a "buffer action" the latter fluctuates much less than the for-
mer. Another cause for the periodicity is no doubt the delay between
the gas exchange in the lungs and the arrival of the blood in the brain.
When the 02 tension of the blood supplying the respiratory center falls
to so low a level that excitation of the center occurs, the resulting in-
creased breathing aspirates outside 02 into the alveoli. After a moment
or so, the 02 is carried by the blood to the center, so that its stimula-
tion by 02 deficiency is removed, and it is left in a condition in which
it fails to discharge any impulses, since there is a subnormal CH of the
blood as a consequence of the lowering of the C02 tension produced by
the hyperpnea. A little time must noAv elapse before the C02 again
rises or the 0, falls sufficiently to excite the center.
Fig. 133.— Quantitative record of breathing air through a tube 260 cm. long and 2 cm. in diameter.
(From Douglas and Haldane.)
A similar although less marked degree of periodic breathing can
sometimes be obtained by merely respiring through a long tube without
any provision for the absorption of C02. In this case it is more difficult
to explain the cause of the periodic breathing, but that the main factor
concerned is one of 02 deprivation is evidenced by the fact that in this
as in the previous experiment, the periodic nature of the respiration is
immediately changed to the regular breathing if 02 is introduced into
the tube. The interest of the experiment lies in the fact that a similar
relative elongation of the dead space is probably accountable for the
periodic breathing seen in the winter sleep of hibernating animals. Dur-
ing this condition, on account of the depression of metabolism less 02
is required and less C02 is produced, so that the exchange of gases
through the pulmonary endothelium is greatly diminished. The dead
space, however, remains of the same capacity, which amounts to the
same thing as if the latter had been prolonged under unchanged con-
ditions of pulmonary gas exchange.
THE CONTROL OF THE RESPIRATION 375
The explanation for other types of experimental periodic breathing is
much less satisfactory. Important evidence that changes occurring in
the tensions of 02 and C02 in the alveolar air and therefore in the
arterial blood of the respiratory center are largely responsible for periodic
breathing has been secured by studying the condition that develops after
a period of apnea produced by voluntary forced breathing. The results
of such observations are given in the curve shown in Fig. 131.
The thin line represents the 02 tension of the alveolar air, the thick
line the CO, tension. The double line running across the chart repre-
sents the average tension of C02 during quiet normal breathing. The
respiratory movements are represented by the tracing at the foot of
the curve along the abscissa. It will be observed that the oxygen ten-
sion falls very rapidly during the apneic period, until just before breath-
ing recommences it may be as low as 30-35 mm. Hg instead of the nor-
mal of about 95. Meanwhile the C02 tension rises from the very low
level of 12 mm., at first very rapidly, then more gradually, although,
M'hen breathing recommences, it has not yet gained the normal level.
As a result of the first periods of breathing, the 02 tension suddenly
shoots up, but the C02 falls only slightly. During the next apneic stage
the 02 quickly comes down again, and the CO, rises so as almost to at-
tain normal tension before breathing again supervenes. As the apneic
periods subsequently become less pronounced, the C02 tension comes to
stand almost at its normal level, whereas considerable variations in the
0, tension continue to occur.
Several interesting features of these results demand attention. In
the first place, it is plain that the body is possessed of some mechanism
by which it can prevent great fluctuations in the C02 tension of the
blood, whereas towards 02 no such "buffer action" is displayed. It will
further be observed that the C02 tension of the alveolar air rises very
rapidly during the first part of the apneic period, and then more grad-
ually, the explanation being that during the forced breathing the C02
has been washed out from the blood but not from the body as a whole.
At first sight one might attribute the periodicity to the same cause
as that operating during breathing through a long tube with soda lime —
namely, to oxygen deficiency. But this explanation is untenable, be-
cause the periodicity remains evident for some time after all possibility
of direct stimulation of the center of 02 deficiency is over. A possible
clue is furnished by the fact that breathing returns while the C02 ten-
sion is still considerably below its normal level. The return, as we have
seen, is accounted for by the appearance of lactic acid, and if we assume
that this has occurred particularly in the respiratory center itself, a
slight degree of hyperpnea will be excited, which by supplying 02 will
376 THE RESPIRATION
quickly oxidize the lactic acid, so that the still slightly subnormal CH
of the blood is unable to excite the center. Apnea therefore supervenes
and lasts until lactic acid has again accumulated in the center. To ex-
plain why local accumulation of lactic acid in the center should produce
a periodic type of breathing, we must further assume that there is con-
siderable delay between the moment at which equilibrium of the gases
in the blood and alveolar air becomes established and that at which
the blood arrives at the respiratory center. This delay is caused by
the slowing of the bloodflow on account of the absence of respiratory
movements.
Emphasis is placed on the fact that it is in the center itself and not
in the blood that the lactic acid becomes oxidized by the excess of 02,
because lactic acid is known to disappear slowly under these conditions
from isolated blood, but to do so very quickly from tissues such as muscle,
and presumably therefore also from nervous tissue.
In support of the above explanation it has been found that, if toward
the end of the forced breathing the lungs are filled with sufficient 02
so that the tension of this gas in the alveoli is not lower than 120 mm.
Hg, breathing is regular in type when it returns, and the C02 tension
of the alveolar air is several millimeters above instead of below the nor-
mal stimulating level.
To sum up, the periodic character of the breathing supervening on
a period of apnea may be explained as follows: Under ordinary condi-
tions of breathing and barometric pressure the 02 tension of the blood
is sufficient between normal respirations to prevent any accumulation of
lactic acid in the respiratory center, so that the stimulus afforded by the
CH of the blood produces a constant effect. During the apnea which
supervenes upon forced breathing, lactic acid accumulates in the center,
causing this to respond to the gradually rising CH of the blood before, the
latter has reached its physiologic level. The hyperpnea thus excited
does not, however, bring about a prompt oxidation of the lactic acid
in the center or a lowering of the CH of the blood circulating through it,
because more time than usual is taken for the blood to get from the
lungs to the brain on account of the absence of respiratory movements.
When the aerated blood does reach the respiratory center, the excess of
02 which it contains oxidizes the lactic acid so that apnea supervenes,
and the lactic acid again accumulates, although not now so much as
before because of the gradually rising CH of the blood itself. The essen-
tial factor in the causation of periodic breathing is therefore a delayed
mass movement of the blood from the pulmonary capillaries to the re-
spiratory center. The delay may be caused by cessation of the respira-
THE CONTROL OF THE RESPIRATION 377
tory movement, as in postapneic periodicity, or by some pathologic cir-
culatory disturbance.
Periodic breathing is produced by forced respiration mor.e readily in
rarefied air than at sea level. It was found by Douglas,26 after breath-
ing forcibly for one minute at sea level, that the breathing when it
returned showed 8 to 10 different periods of apnea and hyperpnea. On
repetition of the experiment at an altitude giving a barometric pres-
sure of 600 mm., 25 such periods followed the apnea ; at a height cor-
responding to 520 mm., 40 periods. Indeed, at high altitudes periodic
breathing may be brought about by the slightest alteration in normal
respiration ; even taking a deep breath may be sufficient to cause distinct
periodicity in the succeeding respirations, and in many persons living
at high altitudes periodic breathing is very apt to occur during sleep.
As in pathologic cases exhibiting Cheyne-Stokes respiration, the peri-
odic breathing at high altitudes can be immediately removed by inspir-
ing oxygen.
We have devoted considerable space to a discussion of these extremely
difficult problems in the hope that clinical observers, by becoming ac-
quainted with the purely experimental work, may be in a position to
conduct more searching investigations as to the cause of Cheyne-Stokes
and other pathologic forms of periodic breathing.
CHAPTER XLIV
RESPIRATION BEYOND THE LUNGS
Up to the present our studies in respiration have concerned the various
mechanisms involved in bringing about a constant change in the com-
position of the alveolar air. We must now consider the nature of the
means by which the oxygen is conveyed to the tissues and the C02 re-
moved from them.
In the first place, it is important to note that it is not for purposes
of oxidation in the blood itself that the 02 is required. In its respiratory
function this fluid serves as a transporting agency between the lungs
and the tissues, in which reside the furnaces of the body that con-
sume the 02 and produce the C02. This does not imply that there is no
oxidation in the blood itself; indeed, we should expect a certain degree
of oxidation because of the fact that the blood contains some living
cells — the leucocytes. It is scarcely necessary nowadays to offer evi-
dence for the foregoing conclusion. One well-known experimental proof
consists in replacing the Mood in a frog with physiologic saline solution
and then subjecting the frog with the saline in its blood vessels to an
atmosphere of pure 02, Avhen it will be found that the animal continues
to absorb the normal amount of 02 and exhale the normal amount of
C02. It respires normally without any blood in the blood vessels.
In order that this transportation of gases between the lungs and the
tissues may be efficiently performed, the blood must be provided with
means for carrying adequate amounts of gases to supply the requirements
of the tissues, both during rest and during their varying degrees of
activity. Not only, therefore, must the 02 and C02 capacity of the
blood be very considerable, but it must be capable of very rapid adjust-
ment from time to time.
Our problem naturally resolves itself into three parts: (1) the call
of the tissues for oxygen (Barcroft) ; or, as it is styled, tissue or internal
respiration ; (2) the mechanism by which the blood transports the proper
amounts of gases to meet the requirements of the tissues; and (3) the
mechanism by Avhich the blood gases are exchanged in the lungs — ex-
ternal respiration. For convenience, however, we shall change this nat-
ural order and consider the transportation of the gases first.
378
RESPIRATION BEYOND THE LUNGS 379
THE TRANSPORTATION OF GASES BY THE BLOOD
The Transportation of Oxygen
It is plainly not by mere solution in the plasma of the blood that the
transportation of 02 occurs, for at the partial pressure of this gas ex-
isting in the alveolar air at the temperature of the body the amount that
could be dissolved in the blood would be only one-fortieth of that which
is actually found to be present. If there were only plasma in the blood
vessels, it would require a volume of fluid amounting to 150 kilograms
or more in order to convey the necessary amount of 02 from the lungs
to the tissues; that is, the contents of the vascular system would weigh
twice as much as the average weight of a man.
The substance that carries the 02 in the blood is the hemoglobin, which
may be described as a highly complex iron compound of protein espe-
cially evolved for the purpose of transporting 02. In some of the lower
animals other compounds exist in the blood for this purpose,- but none
of them is to be compared in its efficiency with hemoglobin. They are
merely poor imitations of it.
Regarding the conditions under which hemoglobin combines with or
delivers up 02, the first question that presents itself is whether or not
the reaction is a strictly chemical one. If so, a definite amount of 0,
must be capable of combining with a definite amount of hemoglobin. It
is impossible to secure hemoglobin of sufficient purity to test this rela-
tionship directly on hemoglobin itself, so that we must test it indirectly
by examining the combining equivalent between 02 and that portion of
the hemoglobin molecule upon which the combining power depends. This
is the part of the molecule containing iron. • Now, if we compare the
amount of O2 which hemoglobin can take up with the amount of iron
present in the hemoglobin, we shall find that one atom of iron becomes
combined with two atoms of 02. Evidently, then, we are here dealing
with a definite chemical reaction occurring between the 02 and the iron
of the hematin portion of the hemoglobin. This relationship is known
as "the specific oxygen capacity of hemoglobin. "
In showing that the union of 02 and hemoglobin occurs according to
chemical laAvs, we throw into prominence consideration of the mechanism
by which the 02 combined with hemoglobin in the blood is rapidly de-
livered up in the capillaries so as to supply the tissues with their require-
ment, and is then as rapidly recombined again in the lungs. Moreover,
we must reconcile facts implied by the idea of a specific 02 capacity with
the well-known observation that the hemoglobin in the circulation is
usually united with considerably less 02 than the total amount possible.
380 THE RESPIRATION
In other words, we must recognize that, although it is essentially a
chemical reaction, the combination of 02 with hemoglobin is greatly in-
fluenced by other factors, and that it is these that are likely to be of
physiologic importance.
In order to understand the conditions under which hemoglobin will
take up and give off 02 in the animal body, we must study the combining
power of hemoglobin when it is exposed to different partial pressures
of 02 (for laws governing this, see page 336). In the blood, the ex-
tremes of the partial pressure of 02 are represented, at the one end, by
that in the alveolar air, which we have seen to be about 10Q mm. Hg,
and at the other, by that existing in the tissues, such as muscle, which
has been shown to be not more than 19 or 20 mm. Hg. We must further
bear in mind that the 02 in its passage from the alveolar air to the hemo-
globin and from the hemoglobin to the tissues, is transmitted in solution
through the plasma; that is, so far as the supply of 02 to the tissue cells
is concerned, the plasma serves as the immediate source. Since the tis-
sues are using up 02 at a very great speed, especially when active, and
are thus tending to lower the tension of 02 in the plasma, favorable con-
ditions have to be created whereby the hemoglobin liberates 02 at the
same rate as that at which it is leaving the plasma. In brief, it is the
02 tension of the plasma in the tissue capillaries that is the important
factor, the hemoglobin merely serving as a storehouse, which delivers
its 02 at just such a rate as to maintain the plasma-oxygen tension at
a _ constant level. It is obviously of the greatest importance that we
should understand how this mechanism of an adequate plasma-oxygen
tension is maintained.
Methods of Investigation. — We must remember that the combination
of 02 and hemoglobin, being a definite chemical reaction, will be re-
versible, and must, therefore, obey the laws of mass action (see page
23) according to the equation: Hb -j- 02?±Hb02. In order to ascertain
the position of the balance of this equation at different partial pressures
of 02, — that is, the relative quantities of oxy- and reduced hemoglobin
formed in a solution of hemoglobin when this is shaken with 02 at differ-
ent pressures,— we may proceed as folloAvs: A few c.c. of the hemoglobin
solution are placed in each of a series of vessels called tonometers, like
those shown in Fig. 134. In addition to the hemoglobin solution, each
tonometer contains a mixture of nitrogen and 02 in different propor-
tions. Suppose we use six vessels and in No. 1 have pure nitrogen; in
No. 2, nitrogen containing 5 mm. partial pressure of 02; in No. 3, 10
mm. ; in No. 4, 20 ; in No. 5, 50 ; and in No. 6, 100. We now rotate the
tonometers in a water-bath at body temperature for about twenty min-
utes, so that, by the formation of a thin film of hemoglobin solution over
RESPIRATION BEYOND THE LUNGS
381
the walls of the vessel, perfect equilibrium between the atmosphere and
the fluid may- be attained (s,ee page 338). A measured quantity of hemo-
globin solution (0.1 or 1.0 c.c.) is then removed from each tonometer
Fig. 134. — Bancroft's tonometer for determining the curve of absorption of oxygen by hemoglobin
or blood. (From Starling's Physiology.')
and placed, together with some very dilute ammonia to lake the blood,
in one of the small bottles of the differential manometer, shown in Fig.
135.* This manometer consists in principle of a graduated U-shaped
tube of narrow bore, containing clove oil, the free end of the U-tube
Fig. 135. — Barcroft's differential blood gas manometer. The capillary U-tube contains clove oil.
The pockets on the sides of the blood bottles should be. deeper. For manipulation see context.
being connected with small bottles provided with some device so that
two fluids can be placed in each of them but kept unmixed until the
bottle is violently shaken. The three-way stopcock between the small
*The blood-gas manometers are made in two sizes for use with 1 c.c. and 0.1 c.c. quantities of
blood, respectively. The results with these small quantities are as accurate as with larger amounts.
382
THE RESPIRATION
bottles and the manometer serves to permit communication of the
manometer with the outside air.
An equal quantity of hemoglobin solution that has been saturated
with oxygen — i. e., oxyhemoglobin — is placed in the bottle on the other
end of the manometer tube from that containing the bottle with the un-
saturated hemoglobin solution. The bottles having been attached to
the manometer Avith the stopcocks open to the outside, the apparatus
is placed in a water-bath until the temperature conditions are constant.
The manometers are then closed to the outside air and the bottles are
shaken in order that the hemoglobin solution that is unsaturated Avith
0, may take up 02 from the atmosphere in the bottle until it becomes
saturated. The resulting shrinkage in the volume of the atmosphere
on the side of the unknown hemoglobin solution causes the clove oil
meniscus to move towards that side, the degree of movement being pro-
portional to the initial unsaturation of the hemoglobin. The manometer
tubes are then again brought into communication with the atmosphere
so that the meniscus of clove oil may move back to its old level, and the
bottle with saturated hemoglobin is removed from the manometer and a
drop or two of a saturated solution of potassium ferricyanide placed
in the separate compartment of the bottle without allowing it to mix
with the hemoglobin. The bottle is then reattached, the temperature
r.
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Percentage saturation
with oxygen
0 10 20 30 40 50 QO 70 80 90 100
Oxygen pressure
Fig. 137. — Upper left hand, percentage saturation of hemoglobin with oxygen at 37" C. cor-
responding to oxygen pressures of 0, 10, 20, 40 and 100 mm. of oxygen, respectively.
Upper right hand, the same spaced with the oxygen pressure as the abscissae.
Lower figure, dissociation curve representing the equilibrium between oxygen, oxyhemoglobin
(red) and reduced hemoglobin (purple). (From Joseph Barcroft.)
RESPIRATION BEYOND THE LUNGS 383
conditions readjusted, the manometer closed off from the ouside air,
and the apparatus again shaken so that the ferricyanide mixes with the
hemoglobin solution. This drives off all the 02 from the oxyhemoglobin
solution, and, therefore, raises the pressure in the atmosphere of that
bottle so that the clove oil moves to the opposite side of the manom-
eter, the degree of displacement being proportional to the amount of
oxyhemoglobin.
We have now all the necessary data for estimating the relative amounts
of reduced hemoglobin in the hemoglobin solution as removed from the
tonometers, for it is plain that the second estimation, as described above,
tells us how much oxyhemoglobin might have been formed had all the
hemoglobin been saturated and the first one, how much 02 had yet to be
taken up by the original hemoglobin solution to produce saturation.
The Dissociation Curve. — The next step is to plot the results obtained
from the various hemoglobin solutions in the form of a curve. This is
known as the dissociation curve of hemoglobin. It is plotted with the
relative percentages of reduced and oxyhemoglobin in each of the solu-
tions along the ordinates, and the partial pressures of 0, in millimeters
of mercury to which they were exposed along the abscissae. The -curve
thus drawn is exactly of the same shape as that Avhich would be pro-
duced if we were to place the tonometers in a row at distances from one
another corresponding to the partial pressure of 02 which each con-
tained, and then to mark on each tonometer the. relative amounts of
reduced and oxyhemoglobin found in the solutions after shaking. A
line joining these marks on the tonometers would then exactly corre-
spond to the curve drawn by the method described above. This will be
clear from the accompanying figure from Barcroft's book (Fig. 137).
In such a chart the space below the curve can be taken to represent
the percentage of oxyhemoglobin (red in chart), and that above it of
reduced hemoglobin (blue in chart), at the varying partial pressures of
02 which are indicated along the abscissas as being contained in the at-
mosphere of the tonometers, and which must be proportional to the
partial pressure of 02 in the solution in which the hemoglobin is dis-
solved.
Difference between Curves of Blood and Hemoglobin Solutions. — The
curve obtained from pure hemoglobin solutions is very far, however,
from clearing up the problem as to how the blood absorbs and
discharges 02. On the contrary, it makes this problem appear
all the more difficult, for, according to the curve (Fig. 137) the hemo-
globin is already more than half combined with 02 at a partial pressure
of this gas of no more than 10 mm. Hg, which means that in the low
partial pressure of 02 existing in the capillaries the oxyhemoglobin, in-
384
THE RESPIRATION
stead of readily yielding up its load of 02, would greedily retain prac-
tically the whole of it. The curve, in other words, would satisfactorily
explain why hemoglobin should readily absorb 02 from the alveolar air,
but would fall far short of explaining how this 02 is readily released
when it is required in the tissues. Obviously there is some artificial con-
dition present in the above experiment which can not obtain in the nat-
ural environment of the blood.
10 $.0 30 W 50 60 70 90 <iO 100
Fig. 138. — Average dissociation curves.
Ordinates — Percentage saturation of hemoglobin with oxygen.
Abscissas — Tension of oxygen in mm. of mercury.
Curve A — Degree of saturation of pure hemoglobin solutions^ at varying pressures.
Curve B — Disregard this curve.
Curve C — Effect of 20 mm. COa pressure on above solution.
Curve 1) — The saturation curve in normal blood at 40 mm. carbon dioxide pressure.
Since hemoglobin takes up 02 in proportion to its iron, it can not be
because of changes in the 02 combining part of the hemoglobin itself
that blood and pure hemoglobin solutions have dissimilar dissociation
curves, but rather because of differences in the environment in which the
hemoglobin acts. That this is so can be readily shown by plotting the
dissociation curve, not for a hemoglobin solution, but for blood itself
RESPIRATION BEYOND THE LUNGS
385
(J> in Fig. 138). The results are very different. At a partial pressure
of 02 of about 60 mm. Hg— that is, a lower pressure than exists in the
lung alveoli (100 mm.)— the blood becomes nearly saturated with 02,
whereas at pressures below 50 mm. it -readily loses 02, so that at 10 mm!
there is nearly complete reduction.
The question is: What are the environmental conditions under which
the hemoglobin in the blood so alters its combining power for 02 as to
'00
90
80
70
60
40
30
20
20 30 40 50 60 70 80
Fig. 139. — Dissociation ' curves of hemoglobin.
90
TOO
Ordinates — Percentage saturation of hemoglobin.
Abscissa— Tension of oxygen in mm. of mercury.
I. Dissociation curve of hemoglobin dissolved in water.
II. Dissociation curve of hemoglobin dissolved in 7% NaCl
III. D^sociation curve of hemoglobin dissolved in 9% KC1
Temperature 37-38° C. (From Joseph Barcroft.)
produce such a difference in the dissociation curve? By experimenting
with hemoglobin solutions, three such factors have been found to come
into play: (1) the presence of inorganic salts, (2) the hydrogen-ion con-
centration (C02 tension) of the solution, and (3) the temperature. If
hemoglobin is dissolved in water containing the various salts of plasma
in the same proportion as in blood (artificial plasma), the dissociation
curve will be found to change so as to resemble that of blood (Fig. 139)'.
386
THE RESPIRATION
Since the plasmas of different animals contain different proportions of
salts, the artificial plasma required to secure the result is not always the
same. It differs, for example, for the dog and man. Potassium salts
are particularly efficient in causing hemoglobin to absorb 02. The in-
fluence of varying hydrogen-ion concentrations of the solution may
be conveniently studied by adding varying percentages of C02 to the
gas mixture in the tonometers, when it will be found that the curve be-
comes lowered in proportion to the amount. of C02 present. This is shown
in Fig. 140.
The effect of temperature on the dissociation curve is twofold: (1) on
the rate with which equilibrium is established at the given partial pres-
0 10 20 30 40 00 CO 70 80 90 100
Fig. 140 — Dissociation curves of human blood, exposed to 0, 3, 20, 40 and 90 mm. CO2. Ordinate,
•percentage saturation. Abscissa, oxygen pressure. (From Joseph Barcroft.)
sure of 02, and (2) on the position of the curve; the lower the tempera-
ture, the higher the curve.
The Rate of Dissociation. — Though it is now clear that the three con-
ditions— namely, saline content, CH, and temperature — are capable of
altering the dissociation curve of a pure hemoglobin solution so as to
make it correspond with that of blood, this does not entirely solve our
problem, for we have yet to show how the cooperation of these forces
renders it possible for the rate at which hemoglobin takes up 02 in
the lungs to correspond exactly with that at which it gives up its 02
to the tissues. To study this problem a somewhat different kind of
experiment must be undertaken. The hemoglobin solution is placed in
a tube and the gas mixture slowly bubbled through it, samples of the
solution being removed at intervals for analysis in the differential blood-
KESPIRATION BEYOND THE LUNGS
387
gas apparatus. To obtain the rate of oxidation, a mixture of N2 or H2
and 02 is bubbled through the blood with the partial pressure of the
02 the same as that which obtains in alveolar air — namely, about 95-100
mm. Hg; and to obtain the rate of reduction pure N2 or H2 gas is bub-
bled through.
The rates of reduction or of oxidation as thus determined are then
plotted in curves constructed with the percentage saturation of the
100,
1?° Oxidation
17-5° C. no CO,
Reduction
100
Oxidation
37-5° C. no C02
Reduction
80
60
20
V
Oxidation
37-5° C.
+ 40 mm. pressure
ofC02
Reduction
Fig. 141. — Curves showing relative rates of oxidation and reduction of blood as influenced by
temperature and tension of CO2.
Ordinates — Percentage saturation.
Abscissae. — Time in minutes.
Reducing gas, hydrogen.
Oxidizing gas, oxygen.
A, temperature 17.5° C., with no CO2.
B, temperature 37.5° C., with no CO2.
C, temperature 37.5° C., but the O2 and H contained 40 mm. Hg pressure of CO2. (From
Joseph Barcroft.)
388 THE RESPIRATION
hemoglobin on the ordinates and the time in minutes along the abscissas
(Fig. 141). Even if we use blood in this experiment and therefore make
certain that the hemoglobin is acting in the presence of the proper pro-
portion of salts, we shall find, as Fig. A shows, that at room temperature
the rate of oxidation is very much greater than the rate of reduction.
If now we repeat the observation at a temperature of 37° C., the two
curves come more nearly to correspond, but still the rate of reduction is
slower than that of oxidation. If in a third experiment, besides having
proper temperature and chemical conditions, we produce the oxidation
and reduction in the presence of a partial pressure of C02 of 40 mm.,
which corresponds to that of the arterial blood, we shall find that oxida-
tion becomes a little slower, whereas reduction is further quickened.
Indeed the two curves, as seen in C in the figure, come practically to
correspond, indicating that the environmental conditions under which
hemoglobin combines and gives off 02 in the blood are exactly adjusted.
One word more with regard to the influence of CH. Its effect in flat-
tening out the curve, especially at the lower partial pressures of 02,
indicates that when a high CH is present, the blood will very readily part
writh its 02 supply. Now, the most significant application of this fact
is that high concentrations of H ion will occur just exactly where it
will be of benefit — namely, in the capillaries (because of the C02 and
lactic acid produced by the tissues) . Some doubt has, however, recently
been thrown on the importance of this factor.
Since, as we have seen, hemoglobin absorbs 02 according to chemical
laws, it will naturally be asked not only why the dissociation curve flat-
tens out while yet maintaining the shape of a right-angled hyperbola,
as by the action of acids or an increase in temperature, but also why it
should change its shape when salts are also present. The explanation
offered by Barcroft and his pupils is that the changes depend on the
fact that hemoglobin being a colloidal substance, its molecules undergo
processes of aggregation under the conditions referred to above, and
therefore cause the reaction to become of a different type from that
represented by the equation Hb02 ^± Hb + 02. As has been pointed out
by Bayliss, although such an explanation might suffice to explain the
flattening out of the curve, it fails to explain the change in its shape;
for, according to the laws of mass action, such a change could occur
only if molecules of a different type came to take part in the reaction.
Dissociation Constant. — Notwithstanding these criticisms, it is of con-
siderable practical importance to know that an equation exists from
which the entire dissociation curve can be plotted by making only one
determination of the relative amounts of oxy- and reduced hemoglobin
at a particular tension or partial pressure of oxygen. This equation is as
RESPIRATION BEYOND THE LUNGS 389
y Kxn
follows: = -TJ — -_ n , where y equals the percentage saturation of
hemoglobin with 02, x the 02 tension, and K and n are constants, K
being the equilibrium constant and n the average number of molecules
of hemoglobin supposed to exist in each aggregate.
When this equation is applied to human blood, the value of n remains
unchanged and is given as 2.5, so that by transposition we are enabled
y
to find the value of K as follows: K = — -r^ r . If we find the value
xn(100-y)
of K by measuring the relative saturation of the blood with 02 at one pres-
sure of this gas, then by changing the value of x to correspond to other
02 pressures, we can find all positions of the curve for a given sample of
blood.
An important practical application of this method is found in the
determination of the (7H of blood, for, as we have seen, the dissociation
curve becomes lowered in proportion to the concentration of hydrogen
ions. The acidity of a sample of blood can therefore be found by com-
parison of its dissociation curve, as plotted from the values found for
K, with that of normal blood to which known quantities of acid have
been added. When the curves correspond, the bloods must contain the
same amounts of acid, other things being equal. In brief, then, the re-
action of the blood is proportional to the value of K. When this is low,
it indicates that the blood is taking up an abnormally low percentage
of its possible load of 02 at a given pressure of 02, and that the acidity
is greater than normal ; when K is high, for the same reason the acidity
must be low.
In determining K for the blood as it exists in the body, it is necessary
that it should be subjected to the same tension of C02 as obtains in the
blood vessels. K will then be proportional to the CH of the living blood.
This condition would be impossible to fulfil in drawn samples were it
not for the fact that we can place in the tonometer an atmosphere con-
taining the same partial pressure of C02 as is found in the alveolar air.
Since this value varies in different individuals, it must be separately
ascertained in each case (see page 344). As determined with these
modifications, K has been found to vary in healthy men between
0.000212 and 0.000363 (ten individuals). When acid substances appear
in the blood, as in acidosis, K becomes extremely low; thus, in one case
suffering from acidosis with dyspnea, it was found a few hours before
death to be only from 0.000082 to 0.00011. Similarly K becomes low
in the acidosis associated with mountain sickness, and it is said to be
raised after taking food that is rich in alkali.*
*When K is found to be normal, the blood is said to be mesectlc; where K is low, it is said to
be myonectic; and when K is high and the acidity is therefore small, it is said to be pleonectic.
CHAPTER XLV
RESPIRATION BEYOND THE LUNGS— Cont 'd
THE MEANS BY WHICH THE BLOOD CARRIES THE GASES
In the foregoing account of the physiology of the blood gases, empha-
sis is placed on the tension under which the gases exist rather than on
the total amount of each gas present in the blood. This has been done
because the exchange of gases between alveolar air and blood and be-
tween blood and tissues proceeds according to the laws of gas diffusion,
which are of course dependent upon differences in gas pressure or
tension.
Something must now be said regarding the amount of the gases. This
may be measured either by physical or by chemical methods. In the
former, a measured quantity of blood is received into an evacuated glass
vessel, which is then attached to a mercury pump, by which the gases
are -sucked out of the blood and transferred, by suitable manipulations
of stopcocks, to a graduated tube, in which they are then analyzed by
chemical means. The principle of the chemical method has already been
described in connection with the measurement of oxygen in hemoglobin
solutions (see page 382). A measured quantity of blood, kept free from
contact with the air, is transferred under some weak ammonia solution
to one of the blood-gas bottles of the blood-gas differential manometer,
and a few drops of a saturated solution of potassium ferricyanide is
placed in the pocket of the bottle. After the blood has been laked and
temperature conditions adjusted, the ferricyanide is mixed with the
blood solution, thus causing the 02 to be quantitatively displaced. From
the increased pressure produced in the manometer the amount of 02 can
readily be computed. To determine the C02 of the blood, the bottle is
now removed from the manometer and a few drops of a saturated solu-
tion of tartaric acid placed in the pocket. When this is mixed with the
deoxygenated blood mixture, after the usual adjustment for tempera-
ture, the pressure caused by the evolved C02 is recorded and the amount
present calculated.
The results of the analysis are expressed as the number of cubic centi-
meters of gas present in 100 c.c. of blood — the volume percentage, as it
is called. The following are approximate percentage values:
390
RESPIRATION BEYOND THE LUNGS 391
OXYGEN CARBON DIOXIDE TOTAL GAS
Venous blood 12 48 60
Arterial blood 20 ' 40 60
The estimation of the amounts of the gases, although of little value
in connection with the physiology of gas exchange, is very important in
supplying information regarding the respiratory activities of the various
organs and tissues. Just as we determine the total respiratory exchange
of an animal by measuring the differences in 02 and C02 in inspired and
expired air, so may we determine the degree of tissue respiratory ex-
change by analysis of the gases in blood removed from the artery and
vein of the tissue. It should be clearly understood, however, that it is
not the percentage but the total amount of the gases that must be con-
sidered, and that it is therefore necessary to know the volumes of blood-
flow as well as the percentage of the gases. Something will be said later
of the results of such investigations (see page 393).
At present we are concerned with the manner in which gases are
carried in the blood. The 02, as we have seen, is carried by the hemo-
globin, some being also in a state of simple solution in the plasma. The
C02, which it will be noted is present even in arterial blood in con-
siderably greater amount than the 02, is partly combined with alkali to
form bicarbonates. The alkali available for this purpose varies from
time to time according to the amount of other acid substances present.
Since these are stronger acids than carbonic, any increase in their
amount (acidosis) causes displacement of some of the C02, thus bring-
ing about, as we have seen, a relative increase in free C02 in the blood
and therefore raising the CH.
What particularly interests us here is the agency by which the com-
bined C02 is carried in the blood. If blood is exposed to a full atmos-
phere of C02, it will take up as much as 150 per cent of the gas — that
is, between two and three times the amount ordinarily present in it.
It has therefore a great reserve capacity for C02. A greater propor-
tion of the CO2 is carried in the plasma than in the corpuscles; but if
plasma (or serum) is exposed in a vacuum, all of the C02 present in it
will not be evolved. When blood itself is similarly exposed, on the
other hand, all the C02 is given off. To liberate all of the C02 from
plasma in vacua, some acid must be added, from which it has been in-
ferred that blood corpuscles act like weak acids. It is commonly stated
that hemoglobin or some constituent of blood is capable of freeing C02
from solutions of sodium carbonate, but the recent work of Buckmaster32
shows that this is not the case. The decomposing power of blood is
caused by the development of acidity in the shed blood and any similar
power that the corpuscles may exhibit is due to a discharge from
392
THE RESPIRATION
them into the plasma of acid radicles. At least it has been found that
the alkalinity of the plasma increases when C02 is bubbled through
blood, this increase in alkalinity being interpreted as the result of the
migration of acid radicles into the corpuscles. This would lead us to
expect that under the opposite conditions (i. e., in vacuo) acids would
leave the corpuscles.
Proteins are amphoteric substances — that is, they combine with acids
or alkalies — which would lead us to expect that they would be capable
of absorbing some C02. That this is the case, particularly for hemo-
75
70
65
60
50
40
A
30 40
60 70
vn. rtvm.
80
90
Fig. 142. — Curve of CO2 tension in blood. For description, see text. (From Christiansen, ' Doug-
las and Haldane.)
globin, has been shown by comparing the C02-combining powers of water
and a solution of pure hemoglobin.
Attempts have been made to determine the relative amounts of C02
carried by these various agencies in the blood. The following is an ex-
ample of such a table:
In simple solution in plasma and corpuscles
] a in corpuscles 6
As sodium bicarbonate } ^ in plagma 12
In combination with hemoglobin 7.5
In combination with proteins of plasma 11
1.9 c.c.
8 "
8 1
.0 } 18'
"I } 19.3 "
.8 J
(Loewy.)
40.0
RESPIRATION BEYOND THE LUNGS 393
The power of blood to absorb C02 at various tensions of this gas, as
determined in the same way as for 02 (see page 380), has shown that
saturation of the hemoglobin with 02 distinctly diminishes the C02-
carrying power of the blood. This is shown in the accompanying curves
The various tensions of C02 are given along the abscissae and the
volume per cents of C02 taken up by the blood on the ordinates. The
upper curve is drawn from results obtained when the blood was shaken
with C02 in the presence of hydrogen, and the lower, when in the
presence of air. (The dotted curve may be disregarded.) The line AB
drawn between the two curves represents the absorption of C02 by the
blood within the body. At a tension of 40 mm. C02 — that present in
alveolar air (see page 356) — A stands in arterial blood at about 52 vols.
per cent; and at a pressure of 62 mm. — possibly present in the tissues —
B stands in venous blood at about 67 vols. per cent. The C02-containing
power would be 7 per cent lower (i. e., 60 vols. per cent) in blood saturated
with 02 at the latter pressure. The oxygenation of blood in the lungs,
therefore, helps to drive out the C02; and conversely, its deoxygenation
in the tissues enhances its power of absorbing this gas.
Having shown how the blood transports its charge of 02 from the
lungs to the tissues, we may now proceed to study the call for 02 by
the tissues, and in this connection we have to consider (1) the amount
of .02 which they require under varying conditions of rest and activity,
and (2) the mechanisms by which their varying demands are met.
THE OXYGEN REQUIREMENT OF THE TISSUES
In order to ascertain the average 02 requirement of the different tis-
sues of the body, it is necessary to adopt as a standard of measurement
the amount of 02 in c.c. absorbed per gram of tissue per minute. To ob-
tain it we must know: (1) the weight of the particular organ or tissue
under investigation; (2) the bloodflow through the vessels of the organ
in c.c. per minute; and (3) the different percentages of 02 in the arterial
and venous blood of the tissue. It would be beyond the scope of this
book to review in any detail the many experimental investigations which
have been undertaken in this connection. A few of the most recent
and important results are given in the accompanying table from Halli-
burton's Physiology:
In the order of their oxygen requirements, or the coefficient of oxida-
tion, as it is called, the tissues may be divided into four groups ; glandular,
muscular, connective, and nervous. The nervous tissues should possibly
stand above the connective, but very little is known regarding their
oxygen consumption, although it appears that this is quite low (Hill and
394
THE RESPIRATION
ORGAN
CONDITION OP REST
OXYGEN USED
PER MINUTE
PER GRAM
OF ORGAN
CONDITION OF ACTIVITY
OXYGEN
USED PER
MINUTE
PER GRAM
OF ORGAN
Voluntary
muscle
Nerves cut. Tone
absent
0.003 c.c.
Tone existing in rest
Gentle contraction
0.006 c.c.
0.020 c.c.
Active contraction
0.080 c.c.
Unstriped
muscle
Besting
0.004 c.c.
Contracting
0.007 c.c.
Heart
Very slow and
feeble contractions
0.007 c.c.
Normal contractions
Very active
0.05 c.c.
0.08 c.c.
Submaxillary
gland
Nerves cut
0.03 c.c.
Chorda stimulations
0.10 c.c.
Pancreas
Not secreting
0.03 c.c.
Secretion after injec-
tion of secretin
0.10 c.c.
Kidney
Scanty secretion
0.03 c.c.
After injection of
diuretic
0.10 c.c.
Intestines
Not absorbing
0.02 c.c.
Absorbing peptone
0.03 c.c.
Liver
Suprarenal
gland
In fasting animal
Normal
0.01 to
0.02 c.c.
0.045 c.c.
In fed animals
'0.03 to
0.05 c.c.
Nabarro). It is of course necessary in making these comparisons to
secure the coefficient of oxidation both when the tissue is at rest and
when it is thrown into varying degrees of activity. Special attention
has been devoted to the requirements of skeletal muscle, heart muscle
and the salivary glands.
Skeletal Muscle.^In observations on skeletal muscle, Verzar (cf. 27)
isolated the gastrocnemius muscle of the cat, and without disturbing its
blood supply collected samples of blood by introducing a 1 c.c. pipette
into a branch of the saphenous vein. Activity was produced by throw-
ing the muscle into tetanus by the application of an electrical stimulus
to the sciatic nerve. During its contraction the muscle lifted a weight,
so that it did about 70 gram-centimeters of work at the beginning of
each period of tetanus. The velocity of bloodflow was determined by
the rate at which the blood flowed along the pipette, and the 02 consump-
tion, by the difference in percentage of 02 in the venous and the arterial
blood. These measurements were made: (1) before contraction, (2) dur-
ing contraction, and (3) after contraction. It was found that during the
tetanus the 02 consumption in some cases was greater than during rest,
while in others it was actually less,, but in every instance a great increase
in 02 consumption followed the tetanus — that is, the call for 02 continues
for some time after the actual work has been performed. This result
RESPIRATION BEYOND THE LUNGS 395
shows that the contraction is not dependent upon oxidation, but that
the oxidation occurs after the contraction is over. The mechanism involved
in muscular contraction can not therefore be analogous with that by
which energy is liberated in a steam engine by the oxidation of the coal.
The mechanism must rather be like that of a spring, which becomes un-
wound during the muscular contraction and requires 02 for its rewinding.
Interesting results corroborative of these conclusions have been se-
cured by observations on the heat production of isolated muscles. It
was found that heat production occurred after a single shock to the
muscles, not only during the contraction, but for a considerable period
after it, provided 02 was present. In the absence of 02 this recovery
was either greatly delayed or entirely abolished. Such results favor
the view that 02 is used largely in the processes whereby the muscles,
"like an engine charging an accumulator, synthesize substances con-
taining a considerable amount of potential energy, which again, like the
accumulator, it discharges when appropriate stimuli are applied" — (L.
V. Hill, cf. 27). One immediately thinks of lactic acid in connection
with these interesting results, for, as has already been stated, Hopkins
and Fletcher29 have shown that this acid is produced in the absence of
02 in excised frog muscles, but when 02 is present, it is either not pro-
duced or, if so, quickly disappears.
Heart Muscle. — Another muscle that has been thoroughly investigated
in this connection is that of the heart. The gaseous exchange has been
studied both on isolated heart preparations and by examining the ex-
change in the lungs of a combined lung and heart preparation. The
most important investigations by the first of these methods are those of
Rohde (cf. 27), who arrived at the very important conclusion that the
02 taken in by the heart muscle varies directly with the maximal ten-
sion set up in the heart by the contraction. This tension was measured
by placing a rubber bag in the ventricle and distending it with water at
a known pressure. By altering the initial pressure and by observing the
pulse rate, it was found that the 02 used by the heart depends on the
product of the pulse frequency and the maximal increase in pressure
produced by each cardiac contraction; or, in the form of an equation:
Q
= a constant quantity ; where Q is the oxygen used, T the maximal
NT
increase of pressure at each beat, and N the frequency of the pulse.
It should be pointed out, however, that constancy in the product of
the above equation does not hold under abnormal conditions of the heart-
beat. For example, when the pressure in the heart is very high, the
amount of 02 required begins to go up out of proportion, indicating that
396 THE RESPIRATION •
the heart is becoming overtaxed — that it is losing its efficiency. The
same result occurs when the heart is dying, and when depressing drugs
are used, such as chloral hydrate, potassium cyanide, veratrine, etc..
Some other drugs, however, such as epinephrine, do not cause altera-
tion in the ratio, nor does vagus stimulation. Of course when the vagus
is stimulated, the 02 consumption in a given period decreases because
the heartbeats are slowed ; but the absorption of 02 is not increased rela-
tively to the slowing of the heart.
Glands. — Most work has naturally been done on the most accessible
gland — the submaxillary. By stimulating the secretory nerve of this
gland (the chorda tympani) in the dog, it has been found that, whereas
the more abundant secretion lasts only so long as the stimulus is ap-
plied to the nerve, the 02 consumption is increased to several times that
of rest, and remains increased for a considerable period after the stimulus
has been removed. Accompanying the increased functional activity in
such structures as muscles, there is a very marked increase in bloodflow
due to vasodilatation, which, in part at least, is dependent upon the
secretion into the blood of some substances resulting from the glandular
activities, and is not entirely due to the action of vasodilator nerve fibers.
Similar results have been obtained in the case of the pancreas when
excited to secrete by the injection of secretin (see page 425). Under
such conditions, the oxygen consumption has been observed to increase
about fourfold and to be accompanied by a dilatation of the gland.
The work on the kidney has been especially interesting, because it
has been found that increased activity, which of course is measured by
the rate of urine excretion, is not always accompanied by increased
consumption of oxygen. When diuresis is produced by injecting Ring-
er's solution into the circulation, a great increase in urine outflow may
occur without any change in oxygen consumption; whereas, on the other
hand, when a diuretic such as sodium sulphate or caffeine is used, the
oxygen consumption increases enormously.
Regarding the other tissues and organs, the 02 consumption of the
lungs and brain appears to be small. It is a very significant fact, how-
ever, that the higher cerebral centers are extremely sensitive to depri-
vation of 02.
The Blood. — In the blood itself, a certain amount of oxidation goes
on because of the presence of leucocytes. This oxidation becomes con-
siderable in the blood of animals rendered anemic by the injection of
phenyl hydrazin. A thorough investigation of the cause of this greater
oxidation has shown it to be owing, not to an increase in nucleated
corpuscles, but to the presence of the young unnucleated red blood
RESPIRATION BEYOND THE LUNGS 397
corpuscles, which appear in large numbers in the blood under these con-
ditions. A similar increase in blood oxidation occurs during posthemor-
rhagic anemia, the rate of oxidation running parallel with the rate of
regeneration of the red corpuscles.
The Mechanism by Which the Demands of the Tissues for
Oxygen Are Met
There are two possible methods by which this may be brought about:
(1) by a change in the CH or the saline constituents or the temperature of
the plasma, so that the hemoglobin more readily delivers up its load
of 02; and (2) by an increase in the mass movement of blood through
the vessels of the acting tissue.
Kegarding the first of these possibilities, there is no doubt that acids
are produced during metabolism of acting tissues. As we have seen,
when muscles contract in the presence of an abundance of 02, C02 is
produced in large amounts, and when they contract in a deficiency of 02,
sarcolactic acid. In the submaxillary gland, too, it has been possible to
show that the CH of the venous blood, as measured by the value of K of
the dissociation curve of hemoglobin, becomes distinctly increased dur-
ing glandular activity. That this increase in CH will dislodge 02 we have
already seen (page 386). As to the possible influence of local changes
in temperature and in saline constituents of the plasma, nothing can at
present be said.
Regarding the second possibility, vasodilatation may be dependent
either upon the action on the blood vessels of nerve impulses coming
along vasomotor nerves, or upon the production by the active tissue of
vasodilating or depressor substances (see page 243). Much evidence
has been accumulating in recent years which tends to show that such
depressor substances are produced, and they may be either (1) acids, or
(2) organic bases of a similar nature to /3-imidazolylethylamine (hista-
mine). This latter substance is of considerable physiologic interest be-
cause of its close relationship to one of the main amino acids of the
protein molecule — namely, histidine (see page 604). Its effect in pro-
ducing vasodilatation is extraordinary. Thus, half a milligram of the
drug injected intravenously into a monkey will lower the mean arterial
pressure by fifty per cent.
But before such an hypothesis can be entertained, it is necessary to
show that, independently of nerve impulses, the blood vessels of an acting
organ may dilate. The best evidence has been secured by studying the
effects of stimulating with epinephrine the cervical sympathetic nerve to
the submaxillary gland of a cat. The gland cells become more active,
398 THE RESPIRATION
and dilatation of the artery occurs, although on blood vessels alone
epinephrine in similar dosage produces constriction. Of course in show-
ing that local chemical products of activity serve as the excitant of local
dilatation, we do not mean to imply that the vasodilator fibers going to
the blood vessels are of no use. Indeed we know that such fibers do be-
come active in the case of a salivary gland whose cells have been para-
lyzed by atropine, but it is a significant fact that this dilatation is of rela-
tively short duration, whereas that produced by glandular activity lasts
for some time. The suggestion seems therefore not out of place that un-
der normal conditions the initial dilatation of an acting gland may be
brought about through nervous stimuli, but the later dilatation is main-
tained by metabolic products.
CHAPTER XLVI
THE PHYSIOLOGY OF BREATHING IN COMPRESSED AIR AND
IN RAREFIED AIR
In the application of a knowledge of the physiology of respiration to
the investigation of disease, a group of conditions arises in which con-
siderable interference with physiologic mechanisms occurs, not as a result
of disease, but of changes in the atmospheric environment. The regula-
tion of the functions of respiration depends very largely on changes in
the physical and chemical properties of the alveolar air, so that it is to
be expected that similar changes in the atmosphere will have a marked
influence on the respiratory activity and on the general well-being of
the animal.
The most thoroughly investigated of these conditions are those which
develop in rarefied and compressed air. Either condition can be pro-
duced experimentally in the laboratory by the use of air-tight chambers
(pneumatic cabinets) and suitable pumps, although most of the im-
portant work on the effects of rarefied air has been conducted at high
altitudes, where the barometric pressure is low.
MOUNTAIN SICKNESS
This condition depends primarily on disturbances in the control of the
respiratory function, and it is on account of the useful information con-
cerning the nature of these functions, rather than because of the so-called
disease itself, that so much attention has been devoted to its investiga-
tion during recent years. The disturbances produced by the rarefied
atmosphere develop rather quickly, but after some time they gradually
disappear, indicating that the organism has acclimated itself — that is,
the compensatory mechanisms have come into play to bring the respira-
tory control back to normal. When animals are placed in pneumatic
cabinets from which some of the air is pumped out, most of the imme-
diate symptoms observed in mountain sickness occur, but it is usually
impracticable to continue the observations for a sufficient length of
time to allow the compensating mechanisms to develop.
Because of their great value in revealing the nature of the respiratory
hormone, many of the results of the recent investigations on mountain
399
400 THE RESPIRATION
sickness have been given elsewhere in this volume (page 360), where the
general symptoms are also described. In this place we shall consider
very briefly some of the more general aspects of the condition, and, mare
particularly, the nature of the adaptation that occurs. All of the symp-
toms are essentialy dependent upon lack of oxygen. Cyanosis is com-
mon and the symptoms are much the same as those of coal-gas poisoning.
Not only, does this deficiency of oxygen cause acid substances to appear
in the blood, thus raising the CH and stimulating the respiratory center,
but it allows other poisonous materials to accumulate. These act on the
various nerve centers, producing symptoms which vary in different in-
dividuals according to their relative susceptibilities. In some, the diges-
tive centers are affected and nausea and vomiting occur; in others, the
higher cerebral centers are affected, causing depression and general men-
tal apathy, great drowsiness, muscular weakness, or it may be mental
excitement and loss of self-control.
The susceptibility of different individuals also varies according to the
amount of previous experience in mountaineering and the type of breath-
ing. Much of the value of previous experience and training depends on
the ability to perform muscular effort economically; to adjust the effort
to the available oxygen supply without permitting unoxidized harmful
products to accumulate in the body. It often happens that no symptoms
appear so long as the person is at rest, but immediately do so whenever
any muscular effort demands a much more abundant oxygen supply.
The type of breathing that best withstands the rarefied air is slow and
deep, rather than rapid and shallow. The reason for this is of course
that much more of the outside oxygen gets into the alveoli in the former
case than in the latter, the dead space being practically constant. The
following figures taken from observations on three different individuals
will illustrate the importance of this factor.
C.C. PER
NO. OF RES-
HEIGHT IN METERS
RESPIRATION
PIRATIONS
AT WHICH SYMP-
PER MINUTE
TOMS OCCURRED
Subject 1
270
20
3300
" 2
440
14
6000
" 3
700
8
6500
(From Halliburton.)
After living for some time in the rarefied air and quite independently
of training in the efficient performance of muscular work, adaptation
occurs, so that the symptoms pass off. The essential feature of this adap-
tation is increased absorption of 02 into the blood. Three mechanisms
have been described as responsible for this effect: (1) increase in the ten-
sion of 02 in the alveolar air; (2) assumption by the pulmonary epithelium
BREATHING IN COMPRESSED AND IN RAREFIED AIR 401
of the power of secreting 02 into the blood; (3) increase in the erythrocytes
and hemoglobin of the blood. The increased alveolar 02 tension is a result
of the more rapid breathing brought about by the increased CH of the
blood. If no adaptation occurred, the 02 tension at 10,000 feet would be
59 mm. and at 15,000 feet, 33.8 mm. Actual observations on men, how-
ever, gave at 10,000 feet a tension of 65 mm. and at 15,000 feet, 52 mm.
The evidence for an increased secretory activity of the pulmonary
epithelium depends on observations made by Haldane and his cowork-
ers,33 who found that blood collected from the finger of a man living on
a high mountain is brightly arterial, whereas if this same blood is
shaken in a flask with alveolar air from the man from whom it was
taken, it will become darkly venous. To account for this difference it is
believed that the pulmonary epithelium forces 02 into the blood contrary
to the laws of diffusion.
A more exact proof was sought for by comparing the relative amounts
of O2 and CO that blood would take up (1) when exposed outside the
body and (2) while in the blood vessels. Carbon monoxide has a very
great avidity for hemoglobin, so that if blood is shaken in a flask with
air containing 0.07 per cent of this gas, colorimetric measurement will
show an equal mixture of oxy- and carboxy-hemoglobin. Since carbon
monoxide is destroyed with extreme slowness in the body, it is possible
by causing a man to breathe a mixture of it in air to determine, in a
sample of drawn blood, whether as much carboxy-hemoglobin has been
formed as in vitro. If so, the O2 tension in the blood must equal that in
the alveoli; if less carboxy-hemoglobin should be formed, it would indi-
cate that a higher tension of 02 exists in the blood. This latter is the re-
sult which Haldane states he has secured. In one experiment, for ex-
ample, when blood was shaken outside the body with 0.04 per cent CO2,
the amount of carboxy-hemoglobin formed was 31 per cent of the whole
hemoglobin. >When the same mixture was inhaled for three or four hours
the percentage of carboxy-hemoglobin in the blood rose only to 26 per
cent, which would correspond to an O2 tension of 25 per cent of an atmos-
phere, whereas even at sea level the tension of 0.2 in the alveolar
air can not be above 15 per cent of an atmosphere.
The constant low tension of 02 in the plasma stimulates the red blood
corpuscles and the percentage of hemoglobin to become markedly in-
creased after residence for some time in high altitudes. At first this is
due to a concentration of the blood by a diminution in plasma, but grad-
ually the blood-forming organs become excited and an actual increase
in the total amount of hemoglobin occurs. In the light of these facts it
is interesting to compare the average number of red corpuscles in the
blood of inhabitants living at different altitudes.
402 THE RESPIRATION
HEIGHT ABOVE SEA RED CORPUSCLES
(METERS) (PER C.MM. BLOOD)
• Christiania
Zurich
Davos
Arosa
Cordilleras
0
412
1560
1800
4392
4,970,000
5,752,000
6,551,000
7,000,000
8,000,000
(From Starling.)
COMPRESSED-AIR SICKNESS; CAISSON DISEASE;
DIVER'S PALSY
Divers and caisson workers are susceptible to peculiar symptoms.
These are frequently of sufficient severity to cause death, but may be so
mild as almost to escape notice. They first appear, not when the worker
is subjected to the high pressure, but after he has come back to atmos-
pheric pressure.*
While in the compressed air the worker as a rule suffers no discom-
fort. A stuffiness may be felt in the ears and temporary giddiness ; the
respiration and pulse rate may become slow and frequency of micturition
may be noticed, but none of the symptoms of disease appear until after
the caissonier or diver has been decompressed (after he has returned to
atmospheric pressure), the exact time of their onset being either imme-
diately after decompression or af the end of several hours. The worker
may have returned home and spent the evening feeling perfectly well
until he went to bed, when symptoms supervened which may include mus-
cular and joint pains, vertigo, embarrassed breathing, subcutaneous em-
physema and hemorrhages, pains in the ears and deafness, vomiting,
perhaps hemoptysis and epigastric pain. These symptoms usually pass
off after some hours but the arthralgia and myalgia sometimes persist
for a considerable time.
In the more severe cases the first symptom is severe pain in the mus-
cles and joints, quickly followed by motor paralysis, so that the patient
falls and is likely to become unconscious. The pulse is almost imper-
ceptible, the respiration is labored, sometimes even asphyxial, the face
cyanosed, and the surface of the body cold. Many of the cases are fatal ;
indeed, death may be almost instantaneous. Such cases are common in
careless diving when the divers, to return the more quickly, screw up the
outlet valve in their helmets so as to fill their suits with air, which car-
*A caisson is a steel or wooden chamber sunk in water and prevented from filling by means of
compressed air. For the passage of the workmen and of material, into and out of the caisson, the
latter is connected with a second smaller chamber fitted with air-locks and decompressing cocks. ' A
diver works in a waterproof suit, the head being enclosed in a copper helmet connected by hose with
air pumps. Every 10 meters or 33 feet of water corresponds to one atmosphere pressure (15 pounds
to the square inch), so that at this depth the total air pressure in a caisson, or in a diver's helmet,
would amount to 30 pounds to the square inch, that is, + 1 atmosphere.
BREATHING IN COMPRESSED AND IN RAREFIED AIR 403
ries them to the surface, where they .decompress themselves by opening
the valve.
Autopsies of persons dead of caisson disease have shown, as a rule,
intense congestion of the viscera, hemorrhages in the spinal cord and
brain, and ecchymoses on the pleura and pericardium. In some cases
interlobar emphysema of the lungs and laceration of the spinal cord and
brain have been noted.
The Cause of the Symptoms
The cause for the symptoms is not, as was at one time supposed, that
the pressure drives the blood from the peripheral into the deep regions
of the body, including the nerve centers. Such a process is impossible,
because the fluids of the body — and all tissues, even the bones, are full
of fluid — are incompressible. Pressure applied to any part of the body
will be immediately distributed equally to every other part. If this were
not so, life would be impossible during any variation of atmospheric pres-
sure. It is now clearly established that all the symptoms of caisson disease
are due to decompression, and not, in the slightest degree, to the mechan-
ical effect of the pressure itself (Paul Bert, Leonard Hill and Macleod34).
When an animal is under pressure, its tissue fluids dissolve a large
amount of gas. They absorb it in obedience to the law of solution of a
gas in a fluid, which -states that the amount of gas dissolved in water is
directly proportional to the partial pressure of that gas in the atmos-
phere; at two atmospheric pressures twice as much gas will pass into
solution as at zero pressure (Dalton's law). So long as the gas is in
simple solution, it does not in any way change the physical condition of
the blood and tissue fluids. If, however, the animal is suddenly decom-
pressed (i. e., the pressure of air surrounding it is reduced to zero), the
dissolved gas will be so quickly thrown out of solution that bubbles of
it are set free. These bubbles act as air emboli, sticking in the pulmonic
capillaries or blocking up a terminal artery in the brain; or they may be
large and tear the capillary wall and so lead to hemorrhage. If these
bubbles are produced in the posterior spinal roots, intense pain results;
if in the anterior, motor paralysis. Frothing of the blood in the heart im-
pedes the action of the organ and death soon follows.
The following experiments furnish proof of this explanation: A frog
was placed in a small steel chamber connected with a cylinder of com-
pressed air and provided with two windows by which a strong arc light
could be passed through the chamber. The web of the foot was stretched
on a wire and fixed so that the small blood-vessels could be seen by apply-
ing a microscope to the outside of the window. After carefully observing
the circulation of the blood in the vessels at atmospheric pressure, a posi-
404 THE RESPIRATION
tive pressure, amounting in some experiments to + 50 atmospheres, was
introduced but no effect could be noted on the circulating blood. By
opening a tap in the chamber, decompression to zero pressure was quickly
effected and, immediately, large bubbles were seen to develop in the
blood, blocking the vessels and producing stasis. The bubbles were de-
rived from the gas that had gone into solution under pressure. On re-
applying the pressure the bubbles of gas again went into solution and
the blood circulated normally. When the pressure was subsequently very
gradually lowered to zero, the circulation went on undisturbed, and the
frog was removed from the chamber in normal condition.
The process involved in causing caisson disease is evidently the same as
that which can be observed in a bottle of aerated water; if the cork in
such a bottle is drawn, the dissolved gas escapes as bubbles and effer-
vescence results ; if the bottle is recorked, the gas reenters solution and
the fluid becomes quiet. If a pin hole is made in the cork, the gas will
gradually escape and no effervescence will result.
Confirmatory results have been secured by observations on mammals.
The arterial blood pressure of rabbits was not found to become altered
by exposure to compressed air, and various animals placed in a large,
strong steel chamber at pressures far in excess of those to which man
ever subjects himself did not show any symptoms like those of caisson
sickness, unless the pressure was suddenly lowered.* Many times also, if
symptoms had appeared they could be removed by again subjecting the
animals to the compressed air.
Investigations were also carried out to determine exactly how much
gas the blood of an animal subjected to high pressures contains, and how
long it takes to absorb the maximal amount of gas and to release it. It
was found that the gases that increased in amount were nitrogen and
oxygen, and that these become dissolved in the blood according to Dai-
ton's law.
The Prevention of the Symptoms
The most important practical application of these observations con-
cerns the length of time required for the saturation and desaturation to
occur, for the results serve as a basis upon which the safe regulation of
work in compressed air by man can be conducted. The most significant
outcome of the above experiments from this standpoint is that it takes
considerable time for the blood to absorb its full quota of gas at a given
atmospheric pressure and to liberate it again when the animal is decom-
pressed. The cause of delay is that the tissue fluids other than the blood
take much longer than would be expected to reach equilibrium with the
partial pressure of gas in the blood plasma.
BREATHING IN COMPRESSED AND IN RAREFIED AIR 405
To understand why this delay should occur, let us suppose that the
only gas concerned is nitrogen. As the pressure rises, the blood in the
capillaries of the lungs must dissolve nitrogen in proportion to the pres-
sure of this gas in the alveoli; the blood carries the dissolved gas to the
tissues and these dissolve it until the pressure is again equalized between
them and the blood. The blood, after giving up its excess of dissolved
nitrogen, returns to the lungs and again becomes saturated and this goes
on until blood and tissue have become saturated with gas at the external
pressure. The tissues are two-thirds water and they contain (in man)
from 15 to 20 per cent of fat. Fat, however, dissolves five times more
nitrogen than water (Vernon) ; consequently, it takes longer for a given'
volume of tissue than of blood to become saturated at a given pressure.
The blood in man constitutes one-twentieth of the body weight; so
that if the tissues were all liquid they would dissolve 20 times as much
nitrogen as the blood. On account of the fat which they contain, however,
the tissues take up more than this proportion — namely, in an average
man about 35 times more than the blood. All the blood in the body takes
about one minute to complete a round of the circulation, so that in this
time, after being suddenly subjected to an increased pressure — assuming
that the blood circulates equally throughout the body — the tissues will
be one-thirty-fifth saturated; in the next minute another thirty-fifth of
thirty-four thirty-fifths will be saturated, and so on. After five minutes
the body will be about 22 per cent, and in 25 minutes about one-half,
saturated; but it will take about two hours before saturation is complete.
These calculations assume that the blood is evenly distributed through-
out the body; but this is not the case, for its mass movement varies
considerably in different parts, being much greater in the active muscles
and in the glands than in passive structures, such as fat. These less vas-
cular parts will therefore lag behind the others in taking up their full
quota of gas, and therefore prolong the time necessary for complete
saturation of the body as a whole.
We see therefore that, after some time in compressed air, the blood
and active tissues will be saturated and contain volumes of dissolved
gas in proportion to their relative bulks ; the fat, although not saturated,
will yet contain up to five times more gas than an equal volume of
blood, and the passive tissues will be incompletely saturated.
These considerations regarding the saturation of the different parts
of the body apply also in its desaturation. Suppose, for example, that
the external pressure is suddenly lowered: the blood, on leaving the
lungs, will contain no excess of gas; when it reaches the tissues it will
remove gas until the pressure is equalized, discharge this into the alveoli
and return again for more. Other things being equal, it will take the
406 THE RESPIRATION
same number of minutes to desaturate that it took to saturate, and the
parts of the body that will lag behind the others, in being desaturated,
are those with a sluggish circulation.
When the mass movement of the blood is increased by muscular exer-
cise, the rate of saturation and desaturation with nitrogen is increased
in proportion. During active work the increase in movement of the
blood may be four or five times over the normal, so that the tissues of
the caisson worker become much more quickly desaturated during decom-
pression than the above figures would lead one to expect.
Application of Foregoing Laws in Practice
With regard to the application of these principles in the decompression
of caisson workers, it is impracticable to occupy as much time as it takes
to saturate the body even at comparatively low pressures. If the great
dangers attending work in compressed air are to be avoided, we must
either insist on very gradual decompression or we must show how the
dissolved gases may be got rid of by some modification in the decom-
pression procedure. With this object in view, we must determine what
difference of pressure may be allowed between the external air and the
body without the formation of bubbles. Actual experience shows that
there is no risk of bubble-formation, however quick the decompression,
after exposure to + 15 pounds pressure ( i. e., 2 atmospheres absolute) .
"Now, the volume of gas capable of being liberated on decompression
to any given pressure is the same, if the relative diminution of pressure
is the same" — (Haldane35). On reduction from 4 to 2 atmospheres,
the same volume of gas will tend to be liberated as on reduction from 2
to 1 atmospheres — that is to say, no bubbles will form. The practical
conclusion is "that the absolute air pressure can always be reduced to
half the absolute pressure at which the tissues are saturated without
risk." Thus, after saturation at 90 pounds absolute pressure (H- 5 atmos-
pheres), a man can be immediately decompressed to 45 pounds (+ 2
atmospheres) in a few minutes without risk, but from this point on the
decompression must be conducted slowly, so as to insure that the nitrogen
pressure in the tissues is never more than twice the air pressure. The
great advantage of this method is that it makes the greatest possible use
of difference of pressure between tissues and blood in order to get rid of
the gas that these contain.
When the decompression from the start is gradual, the desaturation
of the tissues will progressively lag behind that of the blood, and the
tendency to the liberation of free gas will become greater. In such a
case the decompression is far too slow at first and far too rapid later.
BREATHING IN COMPRESSED AND IN RAREFIED AIR 407
Theoretically, therefore, tlie decompression should be rapid at first and
very slow later.
Before recommending the adoption of this principle of stage de-
compression in caisson work, Haldane and his coworkers made numerous
observations on the incidence of decompression symptoms in laboratory
animals. They assert that the stage method is decidedly safer than the
uniform method, the advantage being particularly after short exposures.
On the other hand, Leonard Hill could make out no definite advantage
for the stage method. The two methods have also been compared in
actual caisson work at the Elbe Tunnel, where the pressure was + 2
atmospheres. Very little advantage could be demonstrated for the
stage as compared with the uniform method at this comparatively low
pressure. The general conclusion which we may draw is that the stage
method should be employed, although it is not to be expected that it
will absolutely insure absence of decompression symptoms. Of course
the great advantage of the stage method is the saving of time, making
it possible to persuade the workmen to adopt it.
There are two other factors that are to be considered in hastening the
desaturation of the tissues; these are muscular exercise, and the breath-
ing of an indifferent gas.
It is clear, from what has already been said, that the gas dissolved in
the tissues will become removed in proportion to the mass movement
of the blood, and it is probably true that muscular exercise, performed
in the decompression chamber, is of as great importance in preventing
the subsequent development of symptoms as a much prolonged decom-
pression. In a man at rest, the circulation through the central nervous
system and the viscera is constantly influenced by the pumping action
of the respiratory movements, but in the capillaries of the muscles,
joints, fat, etc., this influence is not felt and the blood flows more slowly.
It is consequently in these parts that bubble formation is likely to oc-
cur, especially some time after decompression. The bubbles cause the
neuralgic pains — the ' ' bends ' ' and ' ' screws ' ' so. well known to caisson
workers. These could no doubt be entirely prevented by muscular
exercise and massage of the limbs during decompression. In illustration
of these facts the following experiment by Greenwood may be cited:
During decompression from + 75 pounds pressure in 95 minutes "Green-
wood flexed and extended all the limb joints at frequent intervals, with
the exception of the knees. Subsequently pain and stiffness were ex-
perienced in the knees and nowhere else." In another experiment the
knees also were flexed and no pain was felt.
But even in the parts with active circulation, the gas in the tissues
may lag considerably behind that in the blood, although the decompres-
408 THE RESPIRATION
sion has been properly controlled. This has been shown by Leonard
Hill in the case of the kidney. The "tissue" gas in this case can be
taken as the gas dissolved in the urine, by analyzing which, therefore,
at different stages of decompression, the excess of nitrogen over what
it should be at the external pressure, can be ascertained. On decom-
pression from + 30 pounds by two stages to zero, a considerable super-
saturation was found to exist. The excess of nitrogen can, however, be
cleared out of the kidneys rapidly and completely by breathing oxygen,
which should therefore be administered during decompression in cases
where great care has to be exercised (Leonard Hill).
When symptoms do appear, they can, in most cases, be relieved by
recompression, and all modern caisson works are provided with a special
chamber for this purpose. We need scarcely say anything about this
treatment here, as its value is so well known. Suffice it to say that,
although it is most likely to afford relief when applied as soon as pos-
sible after the appearance of the symptoms, yet it is often efficacious
when applied several days after their onset.
Quite apart from the dangers of decompression, it must of course be
remembered that the working conditions in a caisson are somewhat dif-
ferent from those at atmospheric pressure, as the air, owing to its com-
pression, is warmer and is loaded to saturation point with moisture.
This hot, wet air interferes with the heat-regulating mechanism of the
body, making hard muscular work very uncomfortable because of the
tendency of the body temperature to rise. The reaction of the body
against this tendency to hyperthermia consists in dilatation of the su-
perficial capillaries and increased heart action.
When such working conditions are repeated day by day, the appetite
is likely to fail, partly because of the tendency of the body to suppress
the activity of the metabolic processes, so as to keep down heat produc-
tion, and partly, no doubt, because the' digestive processes are working
below par on account of there being less blood circulating through the
visceral blood vessels, it having been sent to the surface of the body to
be cooled off. The worker therefore tends to take less food, his metabo-
lism becomes depressed, and his factors of safety against bacterial
infections become lessened.
The risk of the appearance of symptoms on decompression is also
greater when the air in the caisson has been moist and hot, for the heart
has been overworking to maintain the bloodflow in the dilated vessels;
it gets fatigued and is consequently unable to maintain, during decom-
pression, a rate of bloodflow that is adequate for carrying the gas-
saturated blood to the lungs, where the excess of gas becomes dissi-
pated.
BREATHING IN COMPRESSED AND IN RAREFIED AIR 409
The criterion of proper working conditions in the caisson is there-
fore the wet-bulb temperature. This should stand below 75° F. To
maintain this condition it is necessary to ventilate the caisson, pref-
erably with air that has been cooled by cold-water radiators; in any
case, the ventilation should be adequate to keep down the wet-bulb
temperature. The increased expense of ventilation with cooled air
would soon be balanced by the greater working efficiency of the men.
Constant circulation of the air in the caissons by means of fans assists
also in improving the conditions, for it helps to increase dissipation of
heat from the body.
CHAPTER XLVII
THE CIRCULATORY AND RESPIRATORY CHANGES ACCOM-
PANYING MUSCULAR EXERCISE*
During activity the muscles require many times more blood than dur-
ing rest. When the activity is widespread the greater blood supply is
provided by increased heart action accompanied by dilatation of the
muscular arterioles and constriction of those of the splanchnic area, so
that the entire available blood supply of the body is made to circulate
more rapidly. When, on the other hand, the activity is confined to a
limited group of muscles, the increased blood supply is mainly provided
by a local dilatation of the blood vessels of the active muscles accom-
panied by a reciprocal constriction of those of inactive parts. Under
these conditions there may therefore be no quickening of the bloodflow
as a whole. In order that this accurate adjustment of blood supply to
tissue demands may be promptly and adequately brought about, all
available types of coordinating mechanism are called into play; that is
to say, mechanical, nervous and hormone factors cooperate to an extent
which is dependent upon the type of work being performed.
Besides the changes in pulse rate and blood pressure which are evi-
dently designed to supply more blood to the acting muscles, changes
dependent upon a secondary effect of the muscular movements have also
to be considered. Although the various factors work together and are
more or less interdependent, the final effect can be understood only after
we have studied the relative influence of each separately.
The Mechanical Factor. — It is particularly with regard to this factor
that the circulatory changes may be an unavoidable consequence of,
rather than a useful adjustment to, the muscular effort. The effects vary
with the type of exercise performed. In repeatedly lifting and lowering
dumbbells from the floor to above the head, the contracting muscles of
the back and extremities and of the abdomen compress the veins and
cause the blood to flow more rapidly into the hear,t, so that the arterial
pressure suddenly rises. So long as this compression exists, the veins
remain relatively empty and the arteries overfilled, but whenever it
ceases and the muscles relax, the veins fill up again and the arterial pres-
*This chapter is placed here rather than following circulation because of the interdependence of
the circulatory and respiratory adjustments.
410
CHANGES ACCOMPANYING MUSCULAR EXERCISE 411
sure markedly falls, until the extra space in the veins has been occupied
by blood. It is for this reason that the arterial blood pressure is always
found to be little, if any, above normal when taken within a few seconds
after such exercise. It subsequently rises because the other factors
responsible for the increased pressure (quick heart and arteriole constric-
tion) are still in operation at the time the veins again become filled with
blood. The purely mechanical influence outlasts the exercise for a com-
paratively short time, whereas the nervous and hormone influences con-
tinue acting. This interpretation is supported by the observation that
the fall of blood pressure is greater when the subject is left standing
after a given amount of dumbbell exercise than when he is allowed to sit
with his elbows resting on his knees. In the standing position the pres-
sure on the abdominal veins is less and the hydrostatic effect of gravity
causes more blood to collect in the large veins (Cotton, Rapport and
Lewis36). Being purely mechanical in its causation, the preliminary fall
following dumbbell exercise can always be demonstrated if the observa-
tions are made at close enough intervals of time.
The mechanical response of the circulation to exercise acts therefore
through the rate of filling of the right heart with blood, and if this organ is
in a healthy condition, it will respond to the greater inflow by correspond-
ingly increased discharge. Like every other physiologic mechanism, the
heart works with a large factor of safety — a reserve power — and it is
the rate of venous filling that determines how much of this reserve must
be called upon to maintain the circulation. In isolated heart-lung prep-
arations Starling and his coworkers have very clearly demonstrated the
close dependence of cardiac output upon rate of venous filling and the
enormous range through which the systolic discharge can be made to
vary by altering this factor. As explained elsewhere, when the reserve
*power of the heart is lessened, the rise in blood pressure following exer-
cise is longer in attaining its maximum, which is set at a higher level and
persists for a longer time. Observation of the extent of these changes
furnishes a most useful functional test of cardiac efficiency.
Other mechanical factors that augment the cardiac output depend on
the increased respiratory movements. During each respiration the in-
crease in capacity in the thorax causes both an opening up of the thin-
walled veins, so that blood is aspirated towards them from the extra-
thoracic venous system, and a dilatation of the blood vessels of the lungs,
so that the blood finds its way from right to left heart more readily.
Although this dilatation will at first tend to cause more blood to collect
in the intrathoracic vessels and less to be pumped out of them, the expira-
tory act when it supervenes will, by compressing the veins, cause the
extra blood to be expelled into the left ventricle and thence into the
412 THE RESPIRATION
arteries. It is obvious that increased depth and frequency of the respira-
tory movements will accelerate the bloodflow and tend to raise the arte-
rial blood pressure.
The above factors will come into play during most kinds of muscular
exercise such as walking, running, or swinging dumbbells, etc. There
are certain types of muscular effort, however, in which the mechanical
factors produce decidedly disturbing effects on the circulation. During
a sustained effort as, for example, in pulling against a resistance or in
attempting to lift a heavy load, the respirations are suspended, often after
a deep inspiration, and the contracted abdominal muscles press the dia-
phragm up into the thoracic cavity. After a preliminary squeezing out
of blood first of all from the veins of the abdomen into the thorax and
then from those of the latter into the systemic arteries, with a consequent
rise in arterial pressure, there comes to be a damming back of blood into
the peripheral veins, causing them to swell and, if continued, marked
cyanosis may develop. When such efforts are maintained for long, the
arterial pressure begins to fall, and this fall is very pronounced indeed
at the end of the effort, because, the compression being removed from the
abdominal and thoracic veins, these open up and form a large unfilled
blood reservoir.
A similar mechanism comes into play during expulsive acts such as
defecation, parturition, etc. In these the glottis is closed, usually after
a preliminary inspiration, and a powerful expiratory movement is per-
formed, with the consequence that the intrathoracic and intraabdominal
pressures rise considerably, greatly augmenting the systolic discharge
and causing the blood pressure to rise. Because of the obstruction to
the bloodflow in the large veins of the abdomen and thorax, however,
the later effect of the effort is to diminish the systolic discharge, but the
fall in blood pressure which this would be expected to occasion is masked,'
The pressure remains high because other factors increasing the peripheral
resistance come into play. The fall in blood pressure following these acts
may be very marked indeed. Similar mechanical effects are produced
in the acts of coughing, sneezing, etc.
The capacity of the veins varies considerably with the position of the
body, and it is in order that we may cause alterations in this capacity
and therefore encourage a more rapid bloodflow that we stretch the body
after sitting for some time in a cramped position.
The Nervous Factor. — The vagus, vasoconstrictor and respiratory cen-
ters are all excited during muscular effort. In the earlier stages the
excitation depends entirely on nervous impulses transmitted to the cen-
ters, but later it depends on changes in the composition and temperature
of the blood flowing through them — the hormone factor. The initial
CHANGES ACCOMPANYING MUSCULAR EXERCISE 413
stimulation of the centers must be due to cerebral impulses independ-
ently transmitted to the above centers, since the quickening of the pulse
and respirations may be observed to begin before the actual muscular
contractions.
The Hormone Factor. — We have to consider first the nature of the
hormone, and secondly the mode of its action.
The Nature of the Hormones. — The most important hormone is car-
bonic acid, but when the exercise is strenuous and continued, or from
the very start is of such a nature that it uses up oxygen more quickly
than the blood can supply it to the muscles, lactic acid also appears.
Evidence for these statements can readily be supplied in man by analy-
sis of the expired air (for carbon dioxide) and of the urine (for lactic
acid) before and during muscular work. The real hormone in both cases
is believed to be an increase in the H-ion concentration of the blood.
There is, however, no direct proof of this assertion — that is to say, no
one has actually shown that a measurable change in the H-ion concentra-
tion of the arterial blood (for of course a change in the venous blood
would be of no significance) does occur before the changes believed to
be dependent upon acid production make their appearance. The well-
known buffer action of the blood (that is, its ability to take up con-
siderable quantities of acid or of alkali before any perceptible change
occurs in H-ion concentration) furnishes another reason why doubt
must be cast upon the H-ion hypothesis. The most delicate means for
demonstrating a change in H-ion concentration of the blood consists in
finding the dissociation constant for hemoglobin and the results have
shown that acidosis develops during exereise at least at high altitudes
(Barcroft1). So far as we are aware, however, it has not been possible by
direct measurement (page 29) to detect a rise in H-ion concentration.
Of course it may well be that the sensitiveness of the various nerve
centers and other structures towards the H-ion concentration is very
much greater than our most refined and sensitive laboratory methods
can reveal. Such is at least commonly believed to be the case for the
respiratory center (see page 351), and it may also be so for those of
vascular tone and cardiac action. It is nevertheless possible that an
increase in the free carbonic acid itself — the carbonate anion (-HC03),
in other words — is the effective hormone. In the first stages of muscular
work, this increase would be due to greater production of C02, .whereas
later, especially when the work is strenuous, lactic acid would decom-
pose the NaHC03 of the blood, liberating -HC03, which would become
added to that still being produced by the active muscles, and as the
NaHC03 (buffer substance) became gradually used up, would cause a
relatively greater and greater proportion of -HC03 to exist in a free
. 414 THE RESPIRATION
state. That the C02 tension of the alveolar air should be found to be
lowered by prolonged muscular exercise in no way detracts from this
explanation, for it is dependent upon the greatly increased rate of
movement of air into and out of the alveoli (see also page 366).
One serious difficulty in accepting the HC03 ion as the exciting hor-
mone of the nerve centers during muscular exercise depends on the ob-
servation that the alveolar C02 after some time is lower than normal.
If we accept Haldane's teaching that there is accurate correspondence
between the tensions of C02 in arterial blood and alveolar air not only
during rest but also during muscular activity, then obviously we must
discard the HC03 hypothesis. Leonard Hill and Flack,37 however, have
shown quite clearly both in experimental animals and in man that equi-
librium between the blood and alveolar tensions of C02 may fail to
occur. "When blood with excess of C02 is injected into the jugular vein
of dogs, the respiratory center is stimulated, as shown by the increased
breathing, which indicates that the C02-rich blood must have passed
through the lungs without the excess of C02 being removed from it.
Hill believes that the diffusion of C02 out of the blood into the alveolar
air may be depressed in muscular exercise, and that this rather than the
appearance of lactic acid in the blood is responsible for the low C02 ten-
sions usually found present (see page 369). He points out in support
of this view that a person after exercise can hold his breath for a much
shorter time than is usual, and the C02 meanwhile mounts in the alveolar
air very rapidly.
The only way by which progress may be made in a problem like that
under discussion is, however, to adopt some hypothesis and then to
gather evidence for or against it. At the present stage of our knowl-
edge, the hypothesis usually adopted is that a slight change in H-ion
concentration of the blood is the effectual hormone. It is an hypothe-
sis which is supported by the parallelism between the effects observed
during muscular exercise and those produced by experimental increase
in H-ion concentration.
The Effects of the Hormone. — 'These may be classified as follows: (1)
strictly local effects on the muscles themselves; (2) effects on the heart;
and (3) effects on the nerve centers. The local production of acids in
the muscles will cause dilatation of the arterioles, for it has been shown
by various observers that acids cause relaxation of vascular muscle.
Even the capillaries themselves are said to be dilated by carbonic acid
(Severini). The effects produced on the heapt by changes in H-ion con-
centration of the blood have been particularly studied by Starling and
Patterson,38 who, working on isolated heart-lung preparations, have
shown that the heart relaxes more and more and discharges less blood
CHANGES ACCOMPANYING MUSCULAR EXERCISE 415
»
as the H-ion concentration of the perfusion fluid is increased by adding
C02 to the air ventilating the lungs.
The influence of changes in H-ion concentration of the blood on the
vagus and vasomotor centers is usually believed to be stimulatory.
There is no doubt that an increase in CH stimulates the vasoconstrictor
centers, not only of the medulla, but also, although much more feebly,
of the spinal cord. But it is a question whether any part of the rise in
systolic pressure during muscular exercise can be attributed to this
cause, for the enormously increased bloodflow which is known to occur
makes it problematical whether any vasoconstriction really occurs. If
it does so, it must be confined to the splanchnic area, where it would
have the effect of bringing about a redistribution of the total available
blood by expressing it from the viscera and sending it to the active
muscles.
The effect of increased H-ion concentration on the vagus center must,
be insignificant. It is commonly believed that it would cause not what
is actually observed, a quickening, but rather a slowing of the heart rate.
But even this is doubtful. The slowing of the heart that is observed in
asphyxia, for example, is in part at least due to the increased intra-
. cranial pressure, for when the carotid artery is connected with a mer-
cury valve so- that the blood escapes as the pressure rises above the
normal level, no slowing of the heart is said to occur in asphyxia. As
Leonard Hill and Flack37 have shown, however, a part of the slowing is
due to the direct effect of C02. If increase in the H-ion concentration
does affect the heart during muscular exercise, it must act by inhibiting
the vagus tone, which is opposite to the action which it is usually be-
lieved to have. The activity of .the respiratory center is of course ex-
cited" by increase in H-ion concentration, and this, as we have seen, will
cause important changes in the circulation because of the mechanical
effects which follow.
Along with hormones we must consider the effect of change in the
temperature of the Hood. That this rises during muscular exercise is
well known, but that it should be responsible for many of the cardio-
vascular adjustments that occur is quite commonly overlooked. It is,
for example, very likely that rise in blood temperature is responsible
for the acceleration of the heart that occurs during exercise when both
vagi have been severed, and it no doubt is responsible for a part ' at
least of the vasodilatation and respiratory acceleration.
Finally, it is interesting to speculate as to the nature of the changes
that occur when the " second wind" is acquired during strenuous mus-
cular exercise. In running, for example, considerably more distress is
experienced a short time after the start than some time later. Three
416 THE RESPIRATION
very definite changes occur at the time the relief is experienced — namely,
a slowing and steadying of the previously much quickened and irregu-
lar pulse, sweating, and a marked fall in the respiratory quotient. The
last mentioned change possibly gives a clue to the cause of the others.
In the early stages R. Q. is raised, which indicates that relatively more
C02 is being expelled from the blood into the alveolar air than oxygen
is being absorbed, perhaps because of inadequate movement of blood
through the lungs. At the time of the adjustment it is possible that a
pronounced vasodilatation occurs in the muscles and coronary arteries.
The former change by lowering the arterial blood pressure will relieve
the pumping action of the heart, and the latter will improve its power of
contraction by supplying it with more oxygen.
%
RESPIRATION REFERENCES
(Monographs)
Barcroft, J.: The Respiratory Function of the Blood, University Press, Cambridge,
1914.
Borrutau, H.: Nagel's Handbueh der Physiologic, 1905, i, 29.
Douglas, C. G.: Die Regulation der Atmung beim Menschen, Ergebnisse der Physiol-
ogie, 1914, p. 338.
Hill, Leonard: Caisson Sickness, International Medical Monographs, E. Arnold,
London, 1912.
Keith, Arthur: The Mechanism of Respiration in Man, Further Advances in Physi-
ology, E. Arnold, London, 1909.
Schenck, F.: Innervation der Atmung, Ergebnisse der Physiologic, 1908, p. 65.
(Original Articles)
iKeith, Arthur: Cf. Further Advances.
sHoover, C. F.: Arch. Int. Med., 1913, xii, 214; ibid., 1917, xx, 701.
3Lee. F. S., Guenther, A. E., and Meleney, H. F.: Am. Jour. Physiol., 1916, xl, 446.
*Meltzer, S. J.: Jour. Physiol., 1892, xiii, 218.
sHaldane, J. S., and Priestley, J. G.: Jour. Physiol., 1905, xxxii, 225.
Haldane and Douglas: Ibid., 1913, xlv, 235.
6Henderson, Y., Chillingworth and Whitney: Am. Jour. Physiol., 1915, xxxviii, 1.
Henderson and Morriss: Jour. Biol. Chem., 1917, xxx, 217.
7 Krogh, A., and Lindhard: Jour. Physiol., 1913, xlvii, 30; ibid., 1917, li, 59.
sPearce, R. G.: Am. Jour. Physiol., 1917, xliii, 73; ibid., 1917, xliv, 369.
eSiebeck, R.: Skand. Arch. f. Physiol., 1911, xxv, 87; Carter, E. P.: Jour. Exper.
Med., 1914, xx, 21.
loPeabody, F. W., and Wentworth, J. A.: Arch. Int. Med., 1917, xx, 443.
"Lewis, T.: Jour. Physiol., 1908, xxxiv, 213, 233.
isPorter, W. T.: Jour. Physiol., 1895, xvii, 455.
^Christiansen and Haldane, J.: Jour. Physiol., 1914, xlviii, 272.
"Boothby, W. M., and Berry, F. B.: Am. Jour. Physio!., 1915, xxxvii, 433; also
Boothby, W. M., and Shamoff, V. K: Ibid., p. 418.
isAlcock, N. H., and Seemann, J. : Jour. Physiol., 1905, xxxii, 30.
leScott, F. H.: Jour. Physiol., 1908, xxxvii, 301.
"Stewart, G. N., and Pike, F. H.: Jour. Physiol., 1907, xx, 61.
i7aCoombs, H. C., and Pike, F. H. : Proc. Soc. Exper. Biol. Med., 1918, xv, 55.
isKrogh, A.: Skand. Arch. f. Physiol., 1910, xxiii, 248; and A. Krogh with Marie
Krogh, ibid., 179.
CHANGES ACCOMPANYING MUSCULAR EXERCISE 417
isHaldane, J. S., and Priestley, J. G.: Jour. Physiol., 1905, xxxii, 225.
zoScott, E. W.: Am. Jour. Physiol., 1917, xliv, 196.
2iNewburg, Means, and Porter, W. T. : Jour. Exper. Med., 1916, xxiv, 583.
22Hasselbalch, K. A., and Lundsgaard, Chr.: Biochem. Ztschr., 1912, xxxviii, 77, and
Skand. Arch. f. Physiol., 1912, xxvii, 13.
23Hooker, D. E., Wilson, D. W., and Connett, H.: Am. Jour. Physiol., 1917, xliii, 357.
24Campbell, J. M. H., Douglas, C. G., and Hobson, F. G.: Jour. Physiol., 1914, xlviii,
303.
25Lindhard, J.: Jour. Physiol., 1911, xxxviii, 337; Haldane, J. S., and Douglas, C. G.:
Ibid., 1913, xlvi.
26Douglas, C. G. : Art, Ergebnisse der Physiologic, see Monographs.
2?Barcroft, J. : see Respiratory Function of Blood.
28Milroy, T. H.: Quart. Jour. Physiol., 1913, vi, 373.
29Fletcher, W. M., and Hopkins, F. G.: Jour. Physiol., 1907, xxxv, 247; also Fletcher,
W. M.: Jour. Physiol., 1913, xlvii, 361.
soEyffel, J. H.: Proc. Physiol. Soc. in Jour. Physiol., 1909, xxxix, 29.
siPembrey, M. S., and Allen, E. W.: Jour. Physiol., 1909, xxxii, 18.
32Buckmaster, G. A.: Jour. Physiol., 1917, li, 105.
33Douglas, C. G., Haldane, J. S., Henderson, Y., and Schneider, E. C. : Phil. Trans.
Eoy. Soc., 1913, 203, B, 185. -
3*Hill, Leonard, Macleod, J. J. E.: Jour. Physiol., 1903, xxix, 507; Hill, Leonard,
Greenwood, M., Flack, M., etc.: see Hill's Caisson Sickness.
ssHaldane, J. S. : Deep Water Diving, Committee of the Admiralty (British), see
Hill's Caisson Sickness.
seCotton, T. F., Eapport, and Lewis, T.: Heart, 1918.
3'Hill, Leonard, and Macleod, J. J. E. : Jour. Physiol., 1908, xxxvii, 77.
3spatterson, S. W., Piper, H., and Starling, E. H.: Jour. Physiol., 1914, xlviii, 465.
PART V
DIGESTION
CHAPTEK XLVIII
GENERAL PHYSIOLOGY OF THE DIGESTIVE GLANDS
The function of digestion is to bring the food into such a condition
that it can be absorbed through the intestinal epithelium into the blood
and lymph. Carbohydrates are broken down as far as monosaccharides ;
neutral fats are split into fatty acids and glycerine; and proteins are
broken down into the amino acids. The agencies which effect these
decompositions are the digestive enzymes, or ferments, contained in the
various digestive fluids or juices. The digestive juices are produced by
glands, which are most numerous in the upper levels of the gastro-
intestinal tract, the lower levels having as their main function that of
absorption of the digested products. In order that the masses of food
may be kept in a state of proper consistency, and that they may move
readily along the digestive canal, numerous mucous glands are also
scattered along the whole extent of the canal. Some of the digestive
glands, such as the main salivary glands, the pancreas, and the liver,
discharge their secretions into the digestive canal by special ducts,
whereas others, such as the isolated salivary gland follicles in the mouth,
the gastric glands and the crypts of Lieberkiihn in the intestine, do not
have an anatomically distinct duct, but discharge their secretions directly
into the digestive tube.
It will be convenient to consider, first of all, certain properties that are
common to the digestive glands, and then, the conditions under which
each gland functionates during digestion.
MICROSCOPIC CHANGES DURING ACTIVITY
Structurally the active part of the glands, represented by the acinus
or tubule, is composed of a basement membrane lined internally with the
secreting epithelium. Outside the basal membrane are the lymph spaces
and blood capillaries. After the gland has been at rest, the cells become
418
PHYSIOLOGY OF THE DIGESTIVE GLANDS
419
filled with granules or small globules, which are often so numerous as
almost entirely to obliterate the nucleus. When the gland becomes active,
on the other hand, the granules or globules leave the cells, except for a
few which remain toward the lumen border. (Figs. 143 and 144.)
A.
Fig. 143.— Cells of parotid gland showing zymogen granules: A, after prolonged rest; B, after a
moderate secretion; C, after prolonged secretion. (From Langley.)
These observations indicate that the granular or globular material must
represent part at least of the secretion of the glands. Sometimes, even
before they are extruded, the granules become changed into some differ-
ent material, as is indicated by the fact that they stain differently from
D.
Fig. 144. — Parotid gland of rabbit in varying states of activity examined in fresh state. The
upper left-hand acini are resting. The upper right-hand acini are from a gland stimulated to
activity by injecting pilocarpine, and the two lower acini from one after stimulation of its sym-
pathetic nerve. (After Langley.)
those of the resting gland. It must not be thought, however, that an
extrusion of granules necessarily accompanies secretory activity, for
under certain conditions a copious secretion of water and inorganic salts,
as well as a certain amount of organic material, may be produced with-
420 DIGESTION
out any change in the arrangement of the granules. In such cases it has
been observed, as in the pancreas, that fine channels develop in the
protoplasm of the cell (see page 429).
From this histologic evidence it would appear that the gland cell
during rest is endowed with the property of building up out of the pro-
toplasm, as granules or globules, the material which is to serve as one of
the main organic constituents of the secretion. It is commonly believed
that this is the precursor of the active ferment of the secretion ; hence its
name, zymogen. It has been shown that the process of separation of the
zymogen granules starts around the nucleus with the production of a
basophile substance, which in hardened specimens sometimes takes the
form of filaments. From this basophilic ergastoplasm, as it is called, the
granules are gradually formed, and then for some time continue to
undergo slight further changes, as is evidenced by the fact that the
staining reaction of those near the base of the cells differs from that of
those at the free margin. When the gland cell is excited to secrete,
the granules before being extruded, as noted above, often undergo a
definite change, becoming swollen and more globular in shape.
MECHANISM OF SECRETION
These histologic studies merely tell us that active changes, associated
with the production and liberation of certain of the constituents of its
secretion, are occurring in the gland cell, but they throw no light on the
mechanism whereby the gland cells secrete water and inorganic salts.
This may be dependent, to a certain extent at least, on differences in
osmotic pressure (see page 11). A possible explanation of the flow of
water is as follows: If a watery solution of some osmotically active sub-
stance is put in a tube, which is closed at one end" by a membrane
impermeable to this substance and at the other by one permeable to it,
and the tube immersed in water, a continuous current will be
found to issue from the permeable end so long as there remains any
osmotically active substance in the tube. If we assume, then, that the
membranes at the two ends of the secreting cell are of such a nature that
the one next the basement membrane is impermeable to some osmotically
active substance manufactured by the cell, and the other toward the
lumen is permeable, it will be clear that, so long as this substance
exists in the cell, it will attract water from the blood, and the water
together with the osmotically active substance will be discharged into
the lumen.
It is possible that when anything excites the cell to secretory activity,
such as a nerve impulse or' hormone, it does so by causing a change in
PHYSIOLOGY OF THE DIGESTIVE GLANDS 421
the permeability of the lumen border of the cell. This change in permea-
bility may be dependent upon alterations in surface tension brought
about by the migration of electrolytes to the border. That such a migra-
tion of electrolytes does actually occur has been demonstrated by A. B.
Macallum8 who developed a microchemical test for potassium, and was
able to show that this electrolyte accumulates at the lumen border of the
cell during secretory activity. Potassium may be taken as a prototype
of electrolytes in general. Support is given to this view by the fact
that potassium always accumulates at the border of the cell through
which the secretion takes place. In the epithelium of the small intes-
tine, for instance, where the current goes in the opposite direction to that
in gland cells, the accumulation of potassium occurs at the portion of
the cell next the basement membrane.
Another possibility is that, when the gland becomes more active, the
molecules present in the cell become broken down into smaller molecules
and so raise the osmotic pressure of the cell content, with the result that
water is attracted from the blood.
When the gland is excited so that the zymogen granules, as well as
water and salts, are secreted, the primary change appears to involve the
granules only. Those near the lumen swell up by absorbing water, and
become converted into spheres in which salts are dissolved in smaller
proportions than exist in the lymph bathing the cells. These swollen
structures are then ruptured at the periphery of the cell and discharged
into the lumen. This discharge of a fluid containing fewer saline con-
stituents than the cell or surrounding blood plasma brings about in-
creased concentration in the remaining parts of the cell, a process which
possibly is assisted by a breaking up of molecules in the protoplasm itself,
and which causes an increase in osmotic pressure with a consequent
flow of water from the lymph to the cells and therefore from the blood
to the lymph.
OTHER CHANGES DURING ACTIVITY
Whatever may be the nature of the physiological changes that
are responsible for the secretory activity of the cell, the fact stands out
prominently that a considerable expenditure of energy is entailed. This
is indicated by the fact that considerably larger quantities of oxygen
are taken up by the gland when it is in an active state than when at
rest. Thus, the oxygen consumption of the resting submaxillary gland
of the cat may be increased five times during active secretion. On
account of this increased oxygen consumption it is not surprising that
it should be found that the secretory activity of the cell is greatly im-
paired by a deficiency in oxygen.
422 DIGESTION
These active processes occurring in the gland when it is excited to
secrete are associated with changes in electric reaction and in the
volume of the gland. The electric changes have been most extensively
studied in connection with the salivary gland. Cannon and Cattel,6 by
connecting a galvanometer with nonpolarizable electrodes, one placed
on the gland and the other on neighboring connective tissue, were able
to show that with each period of active secretion a current of action was
set up. This was first discovered by Rose Bradford and Bayliss, and
has been carefully studied by Gesell.Ga That the electric current is
definitely associated with the secretion of saliva and is not caused by
the vascular changes which usually accompany this act was shown by
its occurrence when the blood supply was shut off from the gland,
and by its absence when there was no secretion even though the vascular
changes were brought about; neither is the electric change due to the
movement of fluid along the duct, as evidenced by its persistence after
ligation of the duct.
With regard to change in volume, it might be expected, on account of
the greater vascularity of the gland accompanying activity, that this
would increase. On the contrary, however, it has been shown to de-
crease, because of the large quantity of fluid secreted from the gland cells.
The action of two drugs on the gland cells is of considerable physio-
logic importance: that of atropine, which paralyzes the secretion, and
that of pilocarpine, which stimulates it. We shall see later how this
information may be used in working out the exact mechanism of the
different glands.
Important observations concerning the relationship of glandular activ-
ity to the blood supply have been made by experiments in which glands
were artificially perfused outside the body. When the submaxillary
gland of the dog is perfused with oxygenated Ringer's solution, stimula-
tion of its nerve supply does not produce the usual secretion, but if the
Ringer's solution is mixed with blood plasma, the nerve stimulation has its
usual effect for a short time. Although no secretion occurs when
oxygenated Ringer's solution is perfused alone, the usual vascular
changes still occur in the gland. The results seem to indicate that the
presence of some constituent of the blood plasma is essential for the
change in the permeability of the cell wall necessary for the process of
secretion. Similar results have been obtained during artificial perfusion
of the pancreas when secretin was used as the stimulus.
CONTROL OF GLANDULAR ACTIVITY
Having outlined the general nature of the changes occurring in gland
cells during their activity, we may now proceed to study the nature of
Center for
for
Facial nerveJNML
Cerebellui
(N.
Medulla oblo'ngataj
Parotid gland-
Cord
Thoracic
nerves
it
fympani nerve
wall superficial ^^
petrosal nerve'^Parotid duct
Inf. max. div. N.V^ (Sten son's)
•rS>-— -* , ^^, /Submaxillary
I U^^^ff^PJl5 / duct(Wharron's)
'ublingual
ducf
(Bartholin's)
Lingual nerve
Chordo-linguaJ
triangle
nerve
/,)Chorda
tympani
branches.
/Superior
cervical gang.
Electrodes
fiffeetf
.
(Small amount of thkk saliva
vaso-constnction )
, Vaso constrictor fibers
sympathetic secretory fibers
Outgoing sympathetic
rami communicantes
Electrodes
( Large amount of thin
'Siibmaxillary \ vase-dilatation
gland
\Sublingual gland
Post- qanglionic fibers are
dotted thus — -
Fig. 145. — Diagrammatic representation of the innervation of the salivary glands in the dog. (From
Jackson.)
PHYSIOLOGY OP THE DIGESTIVE GLANDS 423
the process by wh'ich this glandular activity is controlled. Two mechan-
isms of control are known: (1) by the nervous system, and (2) by means
of hormones.
Nervous Control. — Control through the nervous system is most marked
— indeed it may be the only means of control — in glands which have to
produce their secretion promptly, whereas hormone control pre-
dominates in those in which prompt changes in secretory activity are not
required. Thus, nervous control alone is present in the salivary glands,
whereas hormone control is predominant in the pancreas, intestinal
glands and liver. The gastric glands are partly under nervous control,
and partly under hormone control. It should be pointed out here that
the glands of the body other than the digestive glands are also subject to
nervous or hormone control according to the promptness with which they
are required to secrete. The lachrymal and sweat glands, and the venom
glands of reptiles, for example, are practically entirely under nervous
control, whereas most of the ductless glands, with the exception of the
adrenals, are mainly under the influence of hormones.
The exact nature of the nervous control of glandular function has,
therefore, been most extensively studied in the salivary glands, and that
of the hormonic in the pancreas. With regard to the salivary glands,
the following points are of importance: Their nerve supply comes from
two sources: the cerebral autonomic, and the sympathetic autonomic
(see page 877). These two nerve supplies have usually an opposite influ-
ence on the secretory activity of the glands, and very frequently also on
the vascular changes that accompany secretory activity.
On account of its ready accessibility, the submaxillary gland in the
dog and cat has been most thoroughly investigated. The cerebral auto-
nomic nerve in this case is represented by the chorda tympani, and the
sympathetic autonomic by postganglionic fibers that run from the
superior cervical ganglion to the gland along its blood vessels (Fig. 145).
After tying a cannula into the duct of the gland, it will be found in the
dog that stimulation of the chorda tympani produces an immediate and
abundant secretion of thin watery saliva accompanied by a marked
dilatation of the blood vessels of the gland.
That this secretion is not dependent on the vasodilatation is easily
shown by repeating the experiment after administering a sufficient dose
of atropine to paralyze the secreting cells. Stimulation of the nerve then
produces a vasodilatation but no secretion. " The same conclusion is
arrived at by an experiment of an entirely different nature ; namely, by
observing the pressure produced in the duct when the chorda tympani is
stimulated. This pressure rises considerably above that in the arteries,
so that no such physical process as mere diffusion can be held accountable
424 DIGESTION
for the secretion, and therefore vasodilatation alone can not be respon-
sible for it. If the sympathetic nerve supply is stimulated, a very scanty,
thick secretion takes place accompanied by vasoconstriction.
Repetition of these experiments in the cat yields different results,
particularly with regard to the influence of the sympathetic, a copious
secretion being produced by such stimulation. The histologic changes
produced in the gland cells are marked after sympathetic stimulation,
but very slight, if present at all, after chorda stimulation.
The outstanding conclusion which may be drawn from these results
is that two kinds of secretory activity are mediated through the nerves;
one causing a thin watery secretion, containing only a small percentage
of organic matter, and the other, a thick viscid secretion with a large
amount of organic material. To explain these differences the hypothe-
sis has been advanced that! there are really two kinds of secretory
fibers, called secretory and trophic, the former having to do with the
secretion of water and inorganic salts, and the latter with the secretion
of organic matter ; i. e., with the extrusion of the zymogen granules.
Certain authors (Langley) believe that such an hypothesis is unneces-
sary, and that the different results are dependent upon the concomitant
changes in the blood supply produced by stimulating one or other nerve.
That there are really different kinds of true secretory fibers is, however,
evident from the following experiment. If the duct of the gland is
made to open through a fistula in the cheek, secretion of saliva through
the fistula can be induced by placing various substances in the mouth, such
as meat powder or hydrochloric! acid. If the experiment is performed in
such a way that the bloodflow through the gland can be observed, it will
be found that the saliva produced by the stimulation with the meat powder
contains a very much higher percentage of organic material than does that
produced when hydrochloric acid is the stimulant, whereas the vascular
changes in the gland and the inorganic constituents of the saliva are the
same in both cases. Since stimulation of the chorda tympani causes the
secretion of a watery saliva, while that caused by stimulation of the
sympathetic is thick, it might be thought that the secretory fibers were
contained in the former and the trophic fibers in the latter nerve ; that
this is not the case can be shown by a repetition of the above experiment
in animals from which the superior cervical ganglion has been removed.
The same results are obtained, indicating that the chorda tympani con-
tains both secretory and trophic fibers.
CHAPTER XLlX
PHYSIOLOGY OF THE DIGESTIVE GLANDS (Cont'd)
THE HORMONE CONTROL
This is exhibited best in the case of the pancreas. The crucial experi-
ment demonstrating that this gland is not primarily dependent upon
nervous impulses for the control of its activity was performed by Bay-
liss and Starling.2 Starting with the well-known fact that the application
of weak acid to the duodenal mucous membrane excites secretion of pan-
creatic juice, these workers carefully severed all the nerve connections of
a portion of the duodenum, and found on again applying acid to the mucous
membrane that the secretion persisted. To explain this result they postu-
lated that the acid must cause some substance to be liberated into the
blood stream, which carries it to the pancreas, the cells of which it then
excites to activity. To test this hypothesis they scraped off the mucous
membrane of the duodenum and ground it in a mortar with weak hydro-
chloric acid (0.6 per cent), and, after boiling the solution so as to remove
the protein and nearly neutralizing it, they obtained a fluid which, when
injected intravenously, immediately caused a copious secretion of pan-
creatic juice.
Accompanying the secretion, however, a marked fall in arterial blood
pressure was observed, making it possible that the secretion might have
been due to a vasodilatation occurring in the pancreatic blood vessels. To
eliminate this possibility they prepared an extract that was free of the
depressor substances by extracting intestinal epithelium without any of the
submucous tissue. The resulting extract had merely the secretory effect
and produced no fall in blood pressure. This secretagoguary substance
they named secretin.
Further evidence that the action of secretin is independent of the
depressor substances has been obtained by taking advantage of the fact
that the depressor substance is more soluble in alcohol than the secretin.
If an acid decoction of duodenal mucous membrane is poured into abso-
lute alcohol, a precipitate is formed. If this precipitate is redissolved
in water and reprecipitated several times by absolute alcohol, then after
drying a white powder is obtained, which is easily soluble in water. The
resulting solution injected intravenously has a powerful secretory action,
but produces no effect on blood pressure. The concentrated alcoholic
425
426 DIGESTION
liquor, on the other hand, when similarly injected produces a marked fall
in blood pressure. It is believed that this effect is due to the action of
/3-imidazolylethylamine. A very strong preparation of secretin can also
be prepared by the method of Dale and Laidlaw,7 which depends on pre-
cipitation by mercuric chloride.
Secretin does not exist preformed in the epithelial cells, as is shown by
the fact that an extract, made with neutral saline solution, does not as a
rule, have any secretory action when injected intravenously. Sometimes
a slight secretion may be produced, but this is probably to be explained
by the fact that some secretin remains behind in the cells as a result of a
preceding phase of activity. If, on the other hand, the above neutral or
slightly alkaline opalescent solution of the mucous membrane is boiled
with acid, secretin may become developed in it. The interpretation put
upon these results is that a substance, called prosecretin, exists in the
epithelial cells, and that this becomes converted into secretin by the action
of acid on the cells. The secretin thus produced is then taken up by the
blood, none of it passing into the intestinal canal, because the free borders
of the cells are impervious to secretin. That this is actually the case has
been shown by finding that the introduction of neutralized secretin solu-
tion into the duodenum, or other parts of the small intestine, does not
cause a secretion of pancreatic juice.
We know practically nothing concerning the chemical nature of secretin.
Being soluble in about 90 per cent alcohol and in fairly weak acids, it can
not belong to any of the better known groups of proteins. As it is
readily diffusible through parchment membrane, it can not be of very
complex structure, and as it withstands heat, it can not be an enzyme.
It rapidly deteriorates in strength in the presence of alkalies.
Any acid when applied to the mucous membrane is capable of producing
secretin, and so are certain other substances, such as mustard oil. Watery
solutions of saccharose or urea, when rubbed up with the duodenal mucosa
in a mortar, produce secretin solutions of varying activity, but they do
not in the living animal excite pancreatic secretion when applied to the
duodenum. Secretin is very susceptible to destruction by such digestive
enzymes as those present in the pancreatic, gastric, and intestinal juices.
That secretin is present in the blood when acid is in contact with the
duodenal mucosa has been shown by the fact that injection into a normal
dog of blood from one in which secretin formation is going on (as a
result of acid in the duodenum), excites pancreatic secretion.
The pancreatic juice produced by the injection of secretin, like that
which is produced under normal conditions, does not contain any active
trypsin, but instead contains its precursor, trypsinogen. This becomes
converted into trypsin in the intestine, being activated by contact with
PHYSIOLOGY OF THE DIGESTIVE GLANDS
427
enterokinase, an enzyme present in the intestinal juice. By such a mechan-
ism the mucosa of the pancreatic duct is protected against autodigestion
by trypsin.
NERVOUS CONTROL OF PANCREAS
Prior to the discovery of secretin, Pavlov1 and his pupils had published
numerous experiments purporting to show that the secretion of pancreatic
Fig. 146. — Pancreatic acini stained with hematoxylin. The acini at the top and to the left
of the figure are from a resting- gland, those to the right being from one that had been secreting
for over three hours as a result of acid in the duodenum. The lowermost figure is from a gland
the vagus nerve supply of which had been stimulated off and on for several hours. Note that
the zymogen granules are extruded only after vagus activity but not after secretin activity. (From
Babkin, Rubaschkin and Ssawitsch.)
juice is controlled through the vagus nerve. The amount of secretion
produced by nervous stimulation was, however, never found to be so large
as that produced by secretin, and for several years after the discovery of
428
DIGESTION
the latter hormone, much doubt existed as to the correctness of Pavlov's
claim. As in many other fields of physiologic science,, investigators at-
tempted to show that one or the other mechanism obtained, and they were
not inclined to consider the possibility that both mechanisms might exist
side by side. That such is the case, however, is clear from the most recent
work, in which it has been found that if proper precautions are taken,
repeated stimulation of the vagus nerve does call forth a secretion of
pancreatic juice which, besides being less copious than that following
II. III.
'Fig. 147. — Three preparations of pancreatic acini stained by eosin orange toluidin blue. The
acini of Fig. I were from a gland after vagus stimulation, and it is noted that besides free ex-
trusion of the granules, globules staining with orange (and appearing in deep black in the photo-
graph) have formed and may be present in the ductules. Some of the globules, however, change
in their staining properties, becoming light red (dark gray in photograph). The acini in II and III
were from glands excited by secretin. No globules appear; the granules remain, and fine canaliculi
appear in the clear protoplasm. (From Babkin, Rubaschkin and Ssawitsch.)
secretin injection, differs from it in the important fact that it contains
not trypsinogen but active trypsin. Since the normal pancreatic juice
contains trypsinogen, this last mentioned fact would appear to indicate
that vagus control of the normal secretion can not be an important affair.
The vagus secretion of pancreatic juice is, moreover, paralyzed by atro-
pine, which has no action on the secretin mechanism (cf. Bayliss).
PHYSIOLOGY OF THE DIGESTIVE GLANDS 429
The copious secretion of pancreatic juice produced by secretin, on the
one hand, and the scanty, thick secretion produced by vagus stimula-
tion, on the other, calls to mind similar differences observed in the secre-
tion of saliva as the result of chorda-tympani or sympathetic stimulation.
It will be remembered that from these latter results it was concluded
that there must be secretory and trophic fibers concerned in the control
of the activities of gland cells. Interesting corroboration of this conclusion
has recently been obtained by histologic examination of the pancreas fol-
lowing secretin or vagus activity. After the repeated injection of secre-
tin, it is difficult to observe any signs of fatigue in the cells ; the zymogen
granules remain practically as numerous as in a resting gland, but in the
clear protopfasm of the outer third of the cell, it is said that fine channels
of fluid can be seen. Through these channels water is believed to pass
from the blood towards the lumen and in its course to carry with it some
of the zymogen granules, without, however, changing them. Thus, when
the gland cells are stained with eosin and orange, after secretin activity
some of the zymogen granules can occasionally be seen! in the lumen of
the acini stained with eosin like those in the cell itself. After vagus
stimulation the appearances are different ; not only are the granules more
freely extruded from the cells, but they undergo a preliminary change;
they lose the property of staining with eosin and become stained with
the orange, at the same time increasing in size so as to form vacuoles.
These vacuoles may wander into the ductules, and when they are present
here they are stained by orange (Figs. 146 and 147) (Babkin, etc.7a).
Why there should be both a nervous and a hormone control of the pan-
creatic secretion is not clear. This- gland, unlike the gastric and salivary
glands, is not called upon to become active all of a sudden, and it is dif-
ficult to see what could serve as the normal stimulus operating through
the nervous pathway. Taking it all in all, it is probably safe to con-
clude that the nervous mechanism is relatively unimportant, and that
under normal conditions it seldom if ever is called into operation. Cor-
roboration for this view is afforded by the fact, above mentioned, that
the pancreatic juice produced by vagus stimulation contains active tryp-
sin, which is not the case with normal pancreatic juice.
CHAPTER L
PHYSIOLOGY OF THE DIGESTIVE GLANDS (Cont'd)
Up to the present we have been concerned with the physiologic activi-
ties of digestive glands in general, but now we must study each of them
separately in order to find out the conditions under which .they become
stimulated to activity in the normal process of digestion. The secretion
of each gland has a definite role assigned to it in the complex and lengthy
process of digestion. It takes up its work where the preceding secre-
tion left off ; e. g., the pepsin of gastric juice digests protein so far as
proteoses and peptone; the trypsin of pancreatic juice then attacks the
proteoses and peptone, and the resulting lower degradation products
are finally attacked by the erepsin of the intestinal juice. The secre-
tions of the various glands are, therefore, required in a certain definite
order — they are correlated; and we must now give some attention to the
precise condition upon which the activity and correlation depend.
To make possible such observations on the normal activities of the
glands, a preliminary operation has to be performed so as to bring the
duct of the gland to the surface of the body and permit of the observa-
tion of its secretory activity after the animal has recovered from the
immediate effects of the operation. We owe to Pavlov1 the surgical
technic by which these conditions can be fulfilled. The general principle
of the operation, in the case of glands provided with ducts, consists in
making a circular cut through the mucous membrane surrounding the
opening of the duct and then, after dissecting the duct free, stitching
the edges of the cut to the skin wound. Healing then takes place without
the formation in the duct of any stricture due to the cicatricial tissue. After
the wound has healed, the secretion can readily be collected in a receiver
attached over the duct fistula, the animal being in every other way in a
perfectly normal condition. In the case of glands not provided with a
duct, other methods must be adopted to collect the secretions. These
will be described elsewhere.
430
PHYSIOLOGY OP THE DIGESTIVE GLANDS 431
THE NORMAL SECRETION OF SALIVA
The duct fistula can in this case be made either for the submaxillary
gland, representing a mucous gland, or for the parotid, representing a
serous gland. Under ordinary conditions there is very little secretion
from either duct. When secretion occurs, it is, of course, caused by
influences acting on a nerve center or centers in the medulla oblongata,
the exact location of which for the different glands has* been worked out
in recent years by Miller.9 The impulses acting on these centers may be
transmitted along afferent nerves coming from the mucous membrane of
the mouth, nares, etc., or by impulses which we may call psychic, trans-
mitted from the higher nerve centers. The reflex secretions caused by
impulses traveling by the afferent nerve from the mouth, etc., have been
called unconditioned, and those from the higher nerve centers, condi-
tioned. With regard to the former, there is considerable discrimination
in the type of stimulus that will be effective. Thus, if the dog — for most
of the experiments have been performed on this animal — is given meat,
a secretion of thick, mucous saliva will be observed to occur (submaxil-
lary gland). On the other hand, if the meat is dried and pulverized,
the secretion which it calls forth will be very copious and watery (par-
otid gland). There is, then, an obvious association between the nature
of the secretion and the function it will be called upon to perform when
it becomes mixed with the food. The mucous secretion called forth by
meat will serve to lubricate the bolus of food and thus facilitate its
swallowing, whereas the thin watery secretion produced by the dry
powder will have the effect of washing the powder from the mouth.
It is evident that the mechanical condition of the food partly deter-
mines its exciting quality. Mechanical stimulation of the mucosa alone is,
however, not an adequate stimulus, for if pebbles are placed in the mouth,
little secretion occurs, but if sand is placed in the mouth, secretion immedi-
ately becomes copious. The nerve endings also respond to chemical stimuli.
Thus, weak acid causes a copious secretion, while alkali has no effect;
disagreeable, nauseous substances also excite secretion. The above dif-
ferences in the response of the glands according to the mechanical condi-
tion of the food has been observed only in the case of the parotid gland,
increase in the submaxillary secretion being obtained only when actual
foodstuffs are placed in the mouth.
The investigations that have been made on the conditions of psychic
secretion of saliva are still more interesting and important. Their im-
portance depends not so much on the information they give us concern-
ing the secretion of saliva as such, as on the methods they furnish us for
investigating the various conditions that affect the psychic processes
432 DIGESTION
associated with the taking of food. It is from the psychic rather than
from the physiologic standpoint, therefore, that these observations are
of importance, for they permit us, by objective methods, to study on
dumb animals problems that would otherwise be beyond our powers of
investigation. Many of the results, with their bearing on the functions
of the higher nerve centers, have been discussed elsewhere. Meanwhile,
however, even at the risk of repetition it may not be out of place to cite
a few of the most interesting experiments.
If we tease a hungry animal with food for which he has a great appe-
tite, a copious secretion of saliva immediately occurs. If we go on teas-
ing him without giving him food, and repeat this procedure on several
succeeding days, it will be found that gradually he no longer responds
to the teasing by increased salivation. Evidently, therefore, the reflex
is conditioned upon the animal's afterward receiving the food.
The experiment may be performed in another way. If, for example,
we offer the animal some food for which he has no appetite, no secre-
tion of saliva will occur; but, if at the end of the process we give him
appetizing food, it will be found after repeating this procedure on
several successive days that the presentation of the unappetizing food
calls forth a secretion. He has learned to associate the presentation of
unappetizing food with the subsequent gratification of his appetite. The
experiment can even be performed so that a definite interval of time
elapses between the application of the stimulus and the salivation: if
the animal is teased on successive days with food for which he has an
appetite but is not given the food until after ten or twenty minutes,
presentation of this food will come to be followed by salivation — not
immediately, but after the exact interval of time that had been allowed
to intervene in the training process. During this interval there must be
an inhibition of psychic stimulation of the salivary centers by other nerve
centers. It is of great interest that this inhibition may itself be inhib-
ited by various forms of stimulation of the nervous system (see page 858).
THE SECRETION OF GASTRIC JUICE
Methods of Investigation
There being no common duct, the secretion of the gastric glands is a
much more difficult problem to investigate than is that of glands which,
like the salivary, are supplied with ducts. One of the most interesting
chapters in the history of physiology concerns the methods which from
time to time have been evolved for the collection of this juice and for
studying the digestive processes in the stomach. Prominent among the
problems confronting the earlier investigators was the question whether
PHYSIOLOGY OF THE DIGESTIVE GLANDS 433
the main function of the stomach is to crush or triturate the food or to
act on it chemically. The great French scientist Reaumur and a little
later the Italian Abbe Spallanzani (1729-1799) attacked this problem by
methods that anticipate those of Rehfuss and Einhorn. Spallanzani ulti-
mately devised the method of swallowing small perforated wooden tubes
containing foodstuffs and covered by small linen bags. After the bags
were passed per rectum, he found that considerable erosion or digestion
of the food had occurred, but that the wooden tubes, however thin-
walled they might be, were not crushed. In order to secure samples of
the gastric juice free from food, the only method available to the older
investigators consisted in swallowing sponges attached to threads, which
after being for some time in the stomach were withdrawn and squeezed
dry of juice.
The next great contribution came from this country, where, in 1833,
Dr. Beaumont, while a surgeon in the service of the American troops
located at Mackinaw, made observations on a Canadian voyageur by the
name of Alexis St. Martin, who by the premature discharge of his gun
had wounded himself in the stomach, the wound never healing but leav-
ing a permanent gastric fistula. Beaumont arranged to keep Alexis St.
Martin in his service for several years, during which time he made
numerous observations on the process of digestion in the stomach —
observations many of which are of great value even at the' present day.
By none of these methods, however, could a sample of pure gastric
juice be secured while the digestive process was actually in progress.
To make the collection of such a sample possible, Heidenhain devised a
method of isolating portions of the stomach wall as pouches opening
through fistulae on the abdominal wall. The results of Heidenhain 's
experiments are, however open to the objection that the secretion in
the isolated pouches may not really correspond to that occurring in the
main stomach, since the connections of the pouches with the central
nervous system must have been severed. In order that these connec-
tions might remain as nearly intact as possible, the Russian physiologist,
Pavlov,1 devised an ingenious operation in which the pouch, or "minia-
ture stomach," remains connected with the main stomach through a con-
siderable width of mucous and submucous tissue. The essential nature
of this operation will be evident from the accompanying diagram.
(Fig. 148).
The most recent investigations have been made by Cannon3 and by
Carlson.4 The former fed animals food impregnated with bismuth sub-
nitrate, and then exposed the animal to the x-rays. A shadow is
produced by the food mass in the stomach, and from the changes in the
outline of this shadow facts have been collected, not only concerning the
434
DIGESTION
movements of the viscus, but also concerning the rate of discharge of
food into the intestine and therefore the duration of the gastric digestive
process. Carlson's contribution has been rendered possible by his good
fortune in having in his service a second Alexis St. Martin, a man with
complete closure of the esophagus and a gastric fistula large enough to
permit of direct inspection of the interior of the stomach. Seizing the
opportunity thus presented, Carlson during the last four or five years
has devoted his attention exclusively to a thorough investigation, not
only of the movements of the stomach, but also of the rate of secretion
of the gastric juice under different conditions. He has also, with praise-
worthy enthusiasm and keen scientific spirit, extended his observations
both on laboratory animals and on himself and his coworkers, so as not
Fig. 148. — Diagram of stomach showing miniature stomach (S) separated from the main stomach
{V) by a double layer of mucous membrane. A. A., is the opening of the pouch on the abdominal
wall. (Pavlov.)
to incur the error, which is all too frequently made, of confining the
observations to one animal.
The Nervous Element in Gastric Secretion
The first stimulus to the secretion of gastric juice is nervous in origin,
and is dependent on the gratification of the appetite and the pleasure of
taking food. This fact, after having been suggested by observations
made in the clinic, was first thoroughly investigated by Pavlov, who for
this purpose observed the gastric secretion flowing either from a fistula
of the stomach itself, or from a "miniature stomach," in dogs in which
also an esophageal fistula had been established. When food was given
by mouth to these animals, it was chewed and swallowed in the usual
manner, but before reaching the stomach, it escaped through the esopha-
PHYSIOLOGY OF THE DIGESTIVE GLANDS 435
geal fistula. This experiment is known as that of "sham feeding."
Within a few minutes after giving food the gastric juice was found to
be secreted actively, and if the feeding process was kept up, which could
be done almost indefinitely since the animal never became satisfied, the
secretion continued to flow. Thus, in one instance Pavlov succeeded in
collecting about 700 c.c. of gastric juice after sham feeding an animal
for five or six hours in the manner above described.
After the stomach has emptied itself of the food taken with the pre-
vious meal, it is said by Pavlov to contain only a little alkaline mucus.
The more recent work of Carlson, however, shows that this is not strictly
the case, there being more or less of a continuous secretion of gastric juice
in the entire absence of food. The amount varies from a few c.c. up to
60 c.c. per hour, more secretion being produced when it is collected every
five or ten minutes than if it is collected every thirty or sixty, thus
indicating that, ordinarily, some escapes through the pylorus into the
duodenum. The secretion contains both pepsin and hydrochloric acid.
As to the cause of this continuous secretion, little is known. It may be
an example of the periodic activities of the digestive glands described by
Boldyreff, or it may in part be due to a psychic stimulation dependent
upon the thought of food. That the latter is probably not the cause, is
indicated by the fact that, at least in Carlson's patient, the psychic juice
could not be made to flow short of giving food.
The sham feeding causes stimulation of the gastric secretion through
impulses transmitted to the stomach along the vagus nerves; for it has
been found, in animals in which the vagus nerve has been cut, that the
sham feeding no longer induces a secretion of gastric juice. The ques-
tion therefore arises as to how the nerve center is stimulated. Three
possible causes may be considered: (1) mechanical stimulation of the
sensory nerves of the mouth; (2) chemical stimulation of the nerves;
(3) the agreeable stimulation of the taste buds and olfactory endings
concerned in the tasting of food. In investigating these possibilities,
mechanical stimulation was readily ruled out by showing that mere
taking of solid matter in the mouth did not excite any secretion, although
it might cause a flow of saliva. Mere chemical stimulation could not be
the cause, for no secretion was induced by placing substances such as
acetic acid or mustard oil in the mouth. By exclusion, then, it would
appear that the adequate stimulus must consist in the agreeable stimula-
tion of the taste buds, etc. — that is to say, in the gratification of appetite.
Further justification for this conclusion was readily secured by noting
that foodstuffs for which the animal had no particular desire or appe-
tite failed to excite the secretion. Most dogs, for example, although
they may take it, are not particularly fond of bread, and when fed with
436 DIGESTION
it, these animals did not produce any appetite juice. In one animal that
showed considerable liking for bread, active secretion occurred when he
was fed with this foodstuff.
Pavlov further noted that usually it was not necessary actually to
allow the animal to take the food into his mouth, but that mere teasing
with savory food was sufficient to cause the secretion, and that in
highly sensitive animals even the noises and other events usually asso-
ciated with feeding time were sufficient to excite the secretion. In the
case of a hungry animal, the mere approach of the attendant with food,
or some other noise or action definitely associated with feeding time,
was sufficient to excite the secretion. The appetite juice when started
was found to persist for some time after the stimulus causing it had
been removed.
Carlson has succeeded in confirming in man most of these observa-
tions. He noted, however, that the secretion produced by seeing or
smelling or thinking of food is much less than would be expected from
Pavlov's observations on dogs. Even when his subject was hungry,
Carlson did not observe that the bringing of a tray of savory food into
the room caused any secretion of gastric juice. It is, of course, to be
expected that the quantity of the psychic secretion will not be the same
in different individuals. It has been observed, for example, by Pavlov
to vary considerably in the case of dogs, and it is very likely that it will
vary still more in man, with his more highly complicated nervous system.
In no case could Carlson observe any secretion of gastric juice produced
by having his patient chew on indifferent substances, or by stimulating
the nerve endings in the mouth by substances other than those directly
related to food.
In man the rate of secretion is proportional to the palatability of the
food, the smallest amount, during twenty minutes' mastication of pal-
atable food, being 30 c.c. and the largest 150 c.c., in a series of 156 obser-
vations. A typical curve showing the amount of the secretion is given
in Fig. 149. To construct this curve the gastric juice was collected dur-
ing five-minute intervals while the man was chewing a meal of average
composition and of his own choice. An interesting feature depicted on
this curve is that the secretion rate was highest in the last five-minute
period, this being the time during which the dessert was being taken,
for which this man had a great relish. Quite clearly there was a direct
relation between the rate of the secretion of the appetite juice and the
palatability of the food. It will further be observed that it took only
from fifteen to twenty minutes after discontinuing the chewing before
the juice returned to its original level.
The practical application of these facts in connection with the hygiene
PHYSIOLOGY OF THE DIGESTIVE GLANDS
437
of diet and the feeding of patients during convalescence, is obviously
very great. However perfect in other regards a diet may be, it will
probably fail to be digested at the proper rate unless it is taken with
relish. Frequent feeding with favorite morsels is more likely to be fol-
lowed by thorough digestion and assimilation than occasional stuffing
with larger amounts. We see too in these experiments an explanation
of the well-established practice of starting a meal with something
savory. A hors d'oeuvre is nothing more than a physiologic stimulant
to appetite. It is also interesting from a practical standpoint to observe
that with those who have a keen relish for sweetmeats the taking of des-
sert has a real physiologic significance, for, as in Carlson's patient, it
stimulates toward the end of a meal a further secretion of the gastric
25'
SO'
Chewing food
Fig. 149. — Typical curve of secretion of gastric juice collected at 5-minute intervals on mas-
tication of palatable food for 20 minutes. The rise in secretion during the last 5 minutes of
mastication is due to chewing the dessert (fruit) for which the person had great relish. (From
Carlson.)
juice, and thus insures a more rapid digestion of the food. Good cooking,
it should be remembered, is really the first stage in digestion, and it is
the only stage over which we can exercise voluntary control.
The Hormone Element in Gastric Secretion
Although gastric digestion is initiated by the appetite juice, it is
clear that this alone can not account for all the secretion that occurs
during normal gastric digestion. After an ordinary meal gastric diges-
tion lasts usually about four hours, whereas we have seen, particularly
from Carlson's observations, that the appetite juice lasts only for some
fifteen or twenty minutes after the exciting stimulus has been removed.
The appetite juice, in other words, serves only to initiate the process of
secretion, and the question arises, What keeps up the secretion during
the rest of gastric digestion? The answer was furnished by Pavlov, who
438 DIGESTION
observed animals in which not only a miniature stomach had been made,
but a fistula into the main stomach as well. The behavior of the secre-
tion of gastric juice as a whole could be followed by collecting that
which was secreted in the miniature stomach, for it was shown, in con-
trol experiments, that this secretion runs strictly parallel with that in
the main stomach, being quantitatively a definite fraction of it — accord-
ing to the relative size of the miniature stomach — and qualitatively
identical. The miniature stomach, in other words, mirrors the events
of secretion in the main stomach.
It was observed that when the animal was allowed to take the food
into the main stomach by the mouth and esophagus, the secretion from
the miniature stomach continued to flow until the process of gastric
digestion had been completed, a result which was quite different from
that obtained after sham feeding. The only possible explanation for this
result is that the food in the stomach sets up secretion as a result of
local stimulation. To investigate the nature of this local stimulation,
whether mechanical or chemical, food and other substances were placed
in the main stomach through the gastric fistula without the animal's
knowledge so as to avoid possible psychic stimulation, and the secretion
observed from the miniature stomach. When' the mucous membrane of
the main stomach was stimulated mechanically, as by placing inert
objects such as a piece of sponge or sand in the stomach, no secretion
occurred. Evidently, therefore, the stimulus is dependent upon some
chemical quality of the food.
By introducing various foods it was found that there is considerable
difference in the degree to which they can excite the secretion. "Water,
egg white, bread and starch, were all found to have very little if any
effect. On the other hand, when protein that had been partly digested
by means of pepsin and hydrochloric acid was introduced into the
stomach, it immediately called forth a secretion. The conclusion is that
the partly digested products, even of insipid food, are capable of directly
exciting the secretion. These include proteoses and peptones, and it
was, therefore, of great interest to find that a solution of commercial
peptone is also an effective stimulus. This is a result of deep significance,
for it indicates that the food which has been partially digested by the
appetite juice will serve as a stimulus to continued secretion.
The psychic juice has been aptly called the "ignition juice," because
by producing partial digestion it serves to ignite the process of gastric
secretion. Experimental evidence of its great importance in gastric
digestion was secured by Pavlov in experiments in which he placed
weighed quantities of meat attached to threads in the stomach through
a gastric fistula, and after some time removed them and determined by
PHYSIOLOGY OF THE DIGESTIVE GLANDS 439
the difference in weights the extent to which they had become digested.
It was found that when the appetite juice was excited by sham feeding
at the same time that food was placed directly in the stomach, its diges-
tion was much more rapid than in cases in which it was placed in the
stomach without the animal's knowing, as when he was asleep.
Other foods having a direct stimulating effect on the gastric secre-
tion are meat extracts and, to a certain extent, milk. This effect of meat
extract is interesting in connection with the practice of taking soup as
a first or early stage in dining. It not only excites the appetite juice,
but also serves as a direct stimulus to the gastric secretion.
As to the nature of the -mechanism by which this direct secretion takes-
place, it was shown by Popielski10a that the secretion still occurs after all
the nerves proceeding to the stomach are cut. Evidently, therefore, it
is independent of the extrinsic nerve supply of the viscus. As a result
of his experiments Popielski concluded that the secretion must depend
on a local reflex mediated through the nerve structures present in the
walls of the stomach itself. Another explanation of the result has,
however, in recent years been given more credence by the experiments of
Bayliss and Starling on the influence of hormones on the secretion of
pancreatic juice (cf. page 425). Edkins10 suggested that a similar
process in the stomach might account for the continued secretion of
gastric juice. To test the possibility this investigator, after ligating the
cardiac sphincter in anesthetized animals, inserted a tube into the
pyloric end of the stomach, through which he placed in the stomach
about 50 c.c. of physiologic saline. After this had, been in the stomach
for an hour, he found that no water was absorbed, and that if the fluid
was removed after this time, it contained neither hydrochloric acid nor
pepsin. On the other hand, if during the time the saline was in the
stomach a decoction of the mucous membrane of the pyloric end, made
either with peptone solution or with a solution of dextrine, was injected
intravenously in small quantities every few minutes, it was found that
the saline contained distinct quantities of hydrochloric acid and pepsin.
Furthermore, it was found that, if the peptone solution or the dextrine
solution alone was injected intravenously, there was no such evidence
of gastric secretion. The conclusion which Edkins drew from his experi-
ments is to the effect that the half-digested products of the earlier stages
of gastric digestion act on the mucous membrane of the stomach so as to
produce a hormone, which is then carried by the blood to the cells of
the gastric glands, upon which, like secretin, it directly develops an
exciting effect. This hormone has been called gastrin. These observa-
tions of Edkins have been confirmed, and they explain very simply how
gastric secretion is maintained after the cessation of the secretion of the
440
DIGESTION
appetite juice.10 By such a mechanism gastric juice would continue to be
secreted so long as any half-digested food remains in the stomach.
The action of gastrin is the first instance of a hormone control of the
digestive glands. In the earlier stages of digestion, the secretion of saliva
and appetite juice is mediated through the nervous system, because these
juices must be produced promptly. In the later stages of gastric diges-
tion, such promptitude in response on the part of the gland is no longer
necessary, so that the slower, more continuous process of hormone con-
trol is sufficient.
Quantity of Gastric Juice Secreted
According to Carlson, the total amount of gastric juice secreted in
man on an average meal composed of meat, bread, vegetables, coffee or
Hours 12345678123456789 10 123456
Flesh. 200 gm.
Bread, 200 gm.
Milk, 600 c.c.
Fig. 150. — Cubic centimeters of gastric juice secreted after diets of meat, bread, and milk. (From
Pavlov.)
milk, and dessert, amounts to about 700 c.c., being divided into 200 c.c.
in the first hour, 150 in the second, and 350 c.c. during the third, fourth
and fifth hours. These figures were estimated partly on the basis of
observations made on the man with the gastric fistula, and partly from
the data supplied by Pavlov's observations on dogs. Carlson believes
that Pavlov overestimated the relative- importance of the appetite juice
in gastric digestion. He found, for example, that after division of both
vagus nerves in dogs normal gastric digestion might be regained a few
days after the operation, although, of course, under such circumstances no
appetite juice could have been secreted. Moreover, he observed that cats
when forcibly fed with unpalatable food may digest that food as rapidly
as when they eat voluntarily. In support of his contention, Carlson
states that he has frequently, removed all of the appetite juice from his
patient's stomach before the masticated meal was put into it. without
any evident interference with the digestive process.
Fat has a distinct inhibiting influence on the direct secretion of gas-
PHYSIOLOGY OF THE DIGESTIVE GLANDS
441
trie juice; cream takes considerably longer to be be digested than milk,
and the presence of oil in the stomach delays the secretion of juice poured
out on a subsequent meal of otherwise readily digestible food. By col-
lecting all of the gastric juice from the miniature stomach after feeding
by mouth with quantities of different protein-rich foods containing the
same quantities of nitrogen, interesting observations have been recorded
concerning the amount of juice secreted and its proteolytic power. The
results of some of the experiments are shown in the accompanying
curves (Figs. 150 and 151).
It will be seen that the most abundant secretion occurs with meat, that
of milk being not only smaller but also slower in starting. The digestive
power is greatest in the case of bread.
Hours 123456782345678923*56
10.0
e An
Mm of-Protei
Column
o .M -•**• -0* J
0 0 0 0 <
/
\
i
/
\
\
^
~~~^
^~~
/
\
t
*
?
^
\
/
\
/
\
/
\
~^-
'^ j >. ^
Flesh, 200 gm.
Bread, 200 gm.
Milk, 600 c.c.
Fig. 151. — Digestive power of the juice, as measured by the length of the protein column digested
in Mett's tubes, with diets of flesh, bread, and milk. (From Pavlov.)
THE INTESTINAL SECRETIONS
Pancreatic Juice
Eegarding the natural secretion of pancreatic juice, little need be added
to what has already been said (see page 425) . The secretion begins when the
chyme enters the duodenum, and attains its maximum when the outflow
of this is greatest. By collecting the juice from, a permanent fistula of the
pancreatic duct, it has been found that the amount varies with different
foods. When quantities of food containing equivalent amounts of nitro-
gen are fed, the greatest secretion is said to occur with bread and the least
with milk. Such differences are probably dependent upon the amount of
acid secreted in the stomach and passed on into the duodenum. It was
thought at one time that, besides variation in quantity, the nature of the
enzymes in the pancreatic juice might vary according to the kind of
food. This, however, has been shown not to be the case.
442
DIGESTION
Bile
The secretion of bile runs practically parallel with that of pancreatic
juice. The liver is producing bile more or less continuously, since besides
being a digestive fluid it is also an excretory product. The bile produced
between the periods of digestion is mainly stored in the gall bladder.
When the acid chyme comes in contact with the duodenal mucous mem-
brane, it excites afferent nerve endings that cause a reflex contraction of
the gall bladder, and this expresses some of the bile into the duodenum.
The secretin, which the acid at the same time produces, besides affecting
the pancreas, acts on the liver cells, stimulating them to the increased
secretion of bile. Thus, by a nervous reflex operating on the gall bladder
and later by a hormone mechanism operating on the liver cell, the increased
secretion of bile is insured throughout digestion. Of the bile discharged
into the intestine, a certain proportion of the bile salts is reabsorbed into
the portal blood. When these arrive at the liver they also excite secre-
tion of bile, thus assisting secretin in maintaining the secretion through-
out the process of intestinal digestion.
Fig. 152. — Loop of intestine after tying off the portions, cutting the nerves running to the middle
portion, and returning the loop to the abdomen for some time. (From Jackson.)
Intestinal Juice
The secretion of intestinal juice, or succus entericus, can obviously be
studied only after isolating portions of the intestine and connecting them
with fistulse of the abdominal walls. It appears here again that both a
nervous and a hormone mechanism exist. Mechanical stimulation of the
intestinal mucous membrane causes an immediate outflow of intestinal
juice, the purpose of which under normal conditions is evidently to assist
in moving forward the bowel contents. This mechanically excited juice
does not contain any enterokinase and only small amounts of the other
enzymes. Further evidence for nervous control of the secretion of intes-
tinal juice has. been obtained by isolating three pouches of intestine be-
PHYSIOLOGY OF THE DIGESTIVE GLANDS 443
tween ligatures, and then denervating the central pouch by carefully
cutting all the nerves without wounding the blood vessels. On returning
the pouches to the abdomen and leaving them several hours, it has been
found that the middle pouch becomes distended with secretion, whereas
the two end pouches remain empty (Fig. 152). If the pouches are left for
several days in the abdomen, however, the secretion from the denervated
portion disappears again. The explanation of the result is possibly that
the nerves under ordinary conditions convey impulses to the intestinal
glands, which tonically inhibit their activity.
The existence of hormone control is evidenced by the fact that no
enteroldnase is present in the intestinal juice unless pancreatic juice is
placed in contact with the mucous membrane. Injection of pancreatic
juice into the blood, however, does not cause any secretion of intestinal
juice ; whereas the injection of secretin has such an effect.
CHAPTER LI
THE MECHANISMS OF DIGESTION
MASTICATION, DEGLUTITION, VOMITING
Mastication
By the movements of the lower jaw on the upper, the two rows of
teeth come together so as to serve for biting or crushing the food. The
resulting comminution of the food forms the first step in digestion. The
up and down motion of the lower jaw results in biting by the incisors,
and after the mouthful has been taken, the side to side movements enable
the grinding teeth to crush and break it up into fragments of the proper
size for swallowing. The most suitable size of the mouthful is about
5 c.c., but this varies greatly with habit. After mastication, the mass
weighs from 3.2 to 6.5 gm., about one-fourth of this weight being due to
saliva. The food is now a semifluid mush containing particles which
are usually less than 2 mm. in diameter. Some, however, may measure
7 or even 12 mm.
Determination of the proper degree of fineness of the food is a func-
tion of the tongue, gums, and cheeks, for which purpose the mucous
membrane covering them is supplied with very sensitive touch nerve
endings (see page 794). The sensitiveness of the tongue, etc., in this
regard explains why an object which can scarcely be felt by the fingers
seems to be quite large in the mouth. If some particles of food that are
too large for swallowing happen to be carried backward in the mouth,
the tongue returns them for further mastication.
The saliva assists in mastication in several ways: (1) by dissolving
some of the food constituents; (2) by partly digesting some of the
starch; (3) by softening the mass of food so that it is more readily
crushed; (4) by covering the bolus with mucus so as to make it more
readily transferable from place to place. The secretion of saliva is
therefore stimulated by the chewing movements, and its composition
varies according to the nature of the food (page 431). In some animals,
such as the cat and dog, mastication is unimportant, coating of the food with
saliva being the only change which it undergoes in the mouth. In man
the ability thus to bolt the food can readily be acquired, not, however,
without some detriment to the efficiency of digestion as a whole. Soft
444
THE MECHANISMS OF DIGESTION 445
starchy food is little chewed, the length of time required for the mastica-
tion of other foods depending mainly on their nature, but also to a
certain degree on the appetite and on the size of the mouthful.
It can not be too strongly insisted upon that the act of mastication is
of far more importance than merely to break up and prepare the food
for swallowing. It causes the food to be moved about in the mouth so as
to develop its full effect on the taste buds; the crushing also releases
odors which stimulate the olfactory epithelium. On these stimuli depend
the satisfaction and pleasure of eating, which in turn initiate the process
of gastric digestion (see page 435).
The benefit to digestion as a whole of a large secretion of saliva, brought
about by persistent chewing, has been assumed by some to be much
greater than it really is, and there has existed, and indeed may still
exist, a school of faddists who, by deliberately chewing far beyond
the necessary time, imagine themselves to thrive better on less food than
those who occupy their time with more profitable pursuits.
Deglutition or Swallowing
After being masticated the food is rolled up into a bolus by the action
of the tongue against the palate, and after being lubricated by saliva is
moved, by elevation of the front of the tongue, towards the back of the
mouth. This constitutes the first stage of swallowing, and is, so far, a
voluntary act. About this time a slight inspiratory contraction of the
diaphragm occurs — the so-called respiration) of swallowing — and the
mylohyoid quickly contracts, with the consequence that the bolus passes
between the pillars of the fauces. This marks the beginning of the
second stage, the first event of which is that the bolus, by stimulating
sensory nerve endings, acts on nerve centers situated in the medulla
oblongata so as to cause a coordinated series of movements of the
muscles of the pharynx and larynx and an inhibition for a moment of
the respiratory center (page 332).
The movements alter the shape of the pharynx and of the various
openings into it in such a manner as to compel the bolus of food to pass
into the esophagus (see Fig. 153) : thus, (1) the soft palate becomes
elevated and the posterior wall of the pharynx bulges forward so as to
shut off the posterior nares, (2) the posterior pillars of the fauces ap-
proximate so as to shut off the mouth cavity, and (3) in about a tenth of
a second after the mylohyoid has contracted, the larynx is pulled up-
wards and forwards under the root of the tongue, which by being
drawn backwards becomes banked up over the laryngeal opening. This
pulling up of the larynx brings the opening into it near to the lower
half of the dorsal side of the epiglottis, but the upper half of this struc-
446
DIGESTION
ture projects beyond and serves as a ledge to guide the bolus safely past
this critical part of its course. (4) As a further safeguard against any
entry of food into the air passages, the laryngeal opening is narrowed by
approximation of the true and the false vocal cords.
So far the force which propels the bolus is mainly the contraction of
the mylohyoid, assisted by the movements of the root of the tongue.
When it has reached the lower end of the pharynx, however, the bolus
readily falls into the esophagus, which has become dilated on account
of a reflex inhibition of the constrictor muscles of its upper end. This so-
called second stage of swallowing is, therefore, a complex coordinated
movement initiated by afferent stimuli and involving reciprocal action
of various groups of muscles: inhibition of the respiratory muscles and
Fig. 153. — The changes which take place in the position of the root of the tongue, the soft
palate, the. epiglottis and the larynx during the second stage of swallowing. The thick dotted line
indicates the position during swallowing.
of those that constrict the esophagus, and stimulation of those that
elevate the palate, the root of the tongue, and the larynx. It is purely
an involuntary process.
The third stage of deglutition consists in the passage of the swallowed
food along the esophagus. The mechanism by which this is done de-
pends very much on the physical consistence of the food. A solid bolus
that more or less fills the esophagus excites a typical peristaltic wave,
which is characterized by a dilatation of the esophagus immediately in
front of and a constriction over and behind the bolus. This wave travels
down' the esophagus in man at such a rate that it reaches the cardiac
sphincter in about five or six seconds. On arriving here the cardiac
THE MECHANISMS OF DIGESTION 447
sphincter, ordinarily contracted, relaxes for a moment so that the bolus
passes into the stomach. In many animals, including man and the cat,
the peristaltic wave travels much more rapidly in the upper part of the
esophagus than lower down because of differences in the nature of the
muscular coat, this being of the striated variety above, and of the non-
striated below. The purpose of more rapid movement in the upper part
is no doubt that the bolus may be hurried past the regions where, by
distending the esophagus, it might interfere with the function of neigh-
boring structures, such as the heart. In other animals, as the dog, the
muscular fiber is striated all along the esophagus, and the bolus of food
correspondingly travels at a uniform, quick rate all the way. It takes
only about four seconds for the bolus to reach the stomach in the dog.
The peristaltic wave of the upper part of the esophagus in the cat and
presumably in man, unlike that of the intestines (see page 466), is trans-
mitted by the esophageal branches of the vagus nerves. If these are
severed, but the muscular coats left intact, the esophagus becomes dilated
above the level of the section and contracted below, and no peristaltic
wave can pass along it ; on the other hand, the muscular coat ,may be
severed (by crushing, etc.) but the peristaltic wave will continue to
travel, provided no damage has been done to the nerves.
In the lower part of the esophagus, however, the wave of peristalsis,
like that of the intestines, travels independently of extrinsic nerves.
This has been observed in animals in which all of the extrinsic nerves
have been cut some time previously. This difference between the upper
and the lower portions is associated with the difference in the nature of
the muscular fibers above noted (Meltzer).11
The propagation of the wave by the nerves in, the upper part of the
esophagus indicates that the second stage and the first part of the third
stage of deglutition must be rehearsed, as it were, in the medullary
centers from which arise the nerve fibers to the pharynx and the upper
levels of the esophagus. It is thought that the discharges from these
local centers are controlled by a higher swallowing center situated in the
medulla just above that of respiration, the afferent stimuli to which
proceed from. the pharynx by the fifth, superior laryngeal, and vagus
nerves. The exact location of the sensory areas whose stimulation is
most effective in initiating the swallowing reflex varies considerably
in different animals. In man it is probably at the entrance to the
pharynx; in the dog it is on the posterior wall. A foreign body placed
directly in the upper portion of the. esophagus of man has been observed
to remain stationary until the individual made a swallowing movement.
The afferent fibers in the glossopharyngeal nerve exercise a powerful
inhibitory influence on the deglutition center as well as on that of respira-
448 DIGESTION
tion. Thus, if swallowing movements are excited by stimulating the cen-
tral end of the superior laryngeal nerve, they can be instantly inhibited
by simultaneously stimulating the glossopharyngeal, and the respiratory
movements stop in whatever position they may have been at the time.
When the glossopharyngeal nerves are cut, the esophagus enters into a
condition of tonic contraction, which may last a day or so. This shows
that the inhibitory impulses are tonic in nature.
This inhibition of the esophagus is indeed a most important part of
the process when liquid or semiliquid food is swallowed. 'By the contrac-
tion of the mylohyoid muscle, fluids are quickly shot down the distended
esophagus, at the lower end of which, on account of the closure of the
cardiac sphincter, they accumulate until the arrival of the peristaltic
wave which has meanwhile been set up by stimulation of the pharynx.
If the swallowing is immediately repeated, as is usually the case in
drinking, the esophagus remains dilated because peristalsis is inhibited,
and the fluid lies outside the cardiac orifice until the last mouthful has
been taken.
The Cardiac Sphincter
The passage between the esophagus and the stomach is guarded by
the cardiac sphincter or cardia. This exists in a permanently con-
tracted state, or tonus, superimposed on which from time to time are
rhythmic alternations of contraction and relaxation. This tonus is never
very pronounced. In man it is said that a water pressure of from 2 to 7
cm. applied to the esophageal side of the sphincter will drive air or
water into the stomach, this pressure being less than that of a column
of fluid filling the thoracic esophagus in the erect position. During
repeated deglutition the tonus becomes less and less marked, and after
a number of swallows the sphincter may become completely relaxed.
When this relaxation disappears, however, the sphincter becomes more
contracted than usual and remains so for a longer time.
The tonic condition of the sphincter is controlled by the vagus nerve,
stimulation of which causes relaxation with an after-effect of strong
contraction. Mechanical or chemical stimulation of the lower end of the
esophagus increases the tonus of the sphincter. Forcing of the sphincter
from the stomach side requires a higher pressure than from the esopha-
geal. Eructation of gas, for example, does not take place until intra-
gastric pressure has risen to about 25 cm. of water. In deep anesthesia,
however, intragastric pressure may rise considerably higher without
forcing the sphincter.
In animals fed with starch paste impregnated with subnitrate of bis-
muth and then examined by means of the x-rays, the variation in degree
THE MECHANISMS OF DIGESTION 449
of tone of the sphincter has been observed to be responsible for occasional
regurgitation of some of the gastric contents into the esophagus up to the
level of the heart or even to the base of the neck. The presence of the
gastric contents in the esophagus starts a peristaltic wave, which pushes
the material back again into the stomach. This peristaltic wave starts
in the absence of any other phases of the deglutition process, indicating
that it has been excited by the presence of the material in the esophagus
itself, and belongs, therefore, to the lower order of peristaltic wave, as
seen in the intestines but not in the upper half of the esophagus. Regur-
gitation of food into the esophagus occurs only when the intragastric
pressure is fairly high. It may last for a period of from twenty to thirty
minutes after the meal is taken, and disappears when the tonus of the
sphincter becomes increased as a result of the presence in the gastric
contents of free hydrochloric acid.
Much information has been secured by listening with a stethoscope to
the sounds caused by swallowing and 'by observing with the x-ray the
shadows produced along the course of the esophagus when food impreg-
nated with bismuth subnitrate is taken. When a solid bolus is swal-
lowed only one sound is usually heard, but with liquid food there are
two, one at the upper end, due to the rush of the fluid and air, and
the other at the lower end (heard over the epigastrium), four or six
seconds later, due to the arrival here of the peristaltic wave with the
accompanying opening of the cardiac sphincter and the escape of the
fluid and air into the stomach. Sometimes, when the person is in the
horizontal position, this second sound may be broken up into several,
indicating that, unassisted by gravity, the fluid does not so readily pass
through the sphincter. The x-ray shadows yield results in conformity
with the above. After swallowing milk and bismuth, for example, the
shadow falls quickly to the lower end of the esophagus and then passes
sloAvly into the stomach. When the passage of a solid bolus is watched
by the x-ray method, its rate of descent will be found to depend on
whether or not it is well lubricated Avith saliva; if not so, it may take as
long as fifteen minutes to reach the stomach; if moist, but from eight
to eighteen seconds.
Vomiting
, Vomiting is usually preceded by a feeling of sickness or nausea, and
is initiated by a very active secretion of saliva. The saliva, mixed with
air, accumulates to a considerable extent at the lower end of the esopha-
gus, which it distends. A forced inspiration is now made, during the
first stage of which the glottis is open so that the air enters the lungs,
but later the glottis closes so that the inspired air is sucked into the
450 DIGESTION
esophagus, Avhich, already somewhat distended by saliva, now becomes
markedly so. The abdominal muscles then contract so as to compress
the stomach against the diaphragm and, simultaneously, the cardiac
sphincter relaxes, the head is held forward and the contents of the
stomach are ejected through the previously distended esophagus. The
compression of the stomach by the contracting abdominal muscles is
assisted by an actual contraction of the stomach itself, as has been clearly
demonstrated by the x-ray method. After the contents of the stomach
itself have been evacuated, the pyloric sphincter may also relax and
permit the contents (bile, etc.) of the duodenum to be vomited.
The act of vomiting is controlled by a center located in the medulla,
and the afferent, fibers to this center may come from many different
regions of the body. Perhaps the most potent of them come from the
sensory nerve endings of the fauces and pharynx. This explains the
tendency to vomit when the mucosa of this region is mechanically stimu-
lated. Other afferent impulses come from the mucosa of the stomach
itself, and these are stimulated by emetics, important among which are
strong salt solution, mustard water and zinc sulphate. Certain other
emetics, particularly tartar emetic and apomorphine, act on the vomit-
ing center itself, and can therefore operate when given subcutaneously.
Afferent vomiting impulses also arise from the abdominal viscera, thus
explaining the vomiting wrhich occurs in strangulated hernia, and in
other irritative lesions, involving this region. X-ray observations have
been made on the movements of the stomach of cats after the admin-
istration of apomorphine (Cannon). The first change observed is an
inhibition of the cardiac end of the stomach, which becomes a perfectly
flaccid bag. About the midregion of the organ, deeper contractions then
start up, which sweep from the pylorus, each contraction stopping as a
deep ring at the beginning of the vestibule, while a slighter wave con-
tinues. A very strong contraction at the incisura angularis finally
develops and completely divides the gastric cavity into two parts. On
the left of this constriction the stomach remains completely relaxed, but
at the right of it waves continue running over the vestibule. It is while
the stomach is in this condition that the sudden contraction of the dia-
phragm and abdominal muscles shoots the cardiac contents into the
relaxed esophagus. As these jerky contractions are continued, the gastric
walls seem to reacquire their tone.
CHAPTER LII
THE MECHANISMS OF DIGESTION (Cont'd)
THE MOVEMENTS OF THE STOMACH
The Character of the Movements
Even from the earliest days it has been recognized that the stomach
performs two important functions: (1) receiving the swallowed food
and then discharging it slowly into the intestine, and (2) initiating the
chemical processes of digestion. In order to understand the mechanism
by which the stomach collects and then discharges the food, it is neces-
sary first of all to recall certain anatomic facts concerning the organ,
and for this purpose it is most convenient to accept the description
given by Cannon, which is illustrated in the accompanying figure. The
organ is divided into a cardiac and a pyloric portion by a deep notch in
the lesser curvature, called the incisura angularis. The cardiac portion
is further subdivided into two by the cardiac orifice. The part which
lies, in man, above a line drawn horizontally through the cardia is the
fundus. The part lying between the fundus and the incisura angularis
is known as the body of the stomach, which, when full, has a tapering
shape. The pyloric portion lying on the right of the incisura angularis
is further divided into two parts: the pyloric vestibule and the pyloric
canal, the latter of which lies next the pyloric sphincter and in man
measures about 3 cm. in length (see Fig. 154).
The filled stomach of a person standing erect is so disposed that the
greatest curvature forms its lowest point, which may be considerably
below the umbilicus. As digestion proceeds and the stomach empties,
the greater curvature becomes gradually raised, so that ultimately the
pylorus comes to be the most dependent part of the stomach. From
these and many other observations it is certain that the emptying of the
stomach does not at all depend on the operation of the force of gravity.
Indeed, that this can not be the case is perfectly clear, when we con-
sider the disposition of the stomach in quadrupeds.
Exact observation on the movements which the stomach performs from
the time it is filled with food till it empties, have been made by the
x-ray method, first introduced by Cannon.12 The method consists in feed-
451
452
DIGESTION
ing the animal with food that has been impregnated with bismuth sub-
nitrate, then exposing him to the x-ray and either taking instantaneous
photographs of the shadoAvs or observing them by means of a fluorescent
screen. The descriptions of the original observations made by Cannon
Fig. 154. — Schematic outline of the stomach. At C is the cardia; F, fundus; I A, incisura an-
gularis; B, body; PC, pyloric canal; P, pylorus. (From Cannon.)
on the stomach of the cat have been so little modified by observations
on man that we may take them as a convenient type. In the accompany-
ing figure (Fig. 156) the outline of the shadow cast by the stomach is
shown at intervals of an hour each during digestion. Soon after the
Fig. 155. — Diagrams of outline and position of stomach as indicated by skiagrams taken on
man in the erect position at intervals after swallowing food impregnated with bismuth subnitrate.
A, moderately full; B, practically empty. The clear space at the upper end of the stomach is due
to gas, and it will be noticed that this "stomach bladder" lies close to the heart. (From T. Win-
gate Todd.)
THE MECHANISMS OP DIGESTION
453
stomach has become filled, peristaltic waves are seen to take their
origin about the middle of the body of the stomach, and to course
towards the pylorus. Above the region at which these waves originate —
that is, the cardiac half of the body of the stomach and all of the
fundus — there are no waves, but as digestion proceeds the walls slowly
and steadily contract on the mass of food. This so-called cardiac pouch
does not, however, diminish in size so rapidly as the part of the body of
the stomach over which the peristaltic waves are passing. The circular
fibers of the walls of this part of the stomach — sometimes called the
gastric tube — contract tonically, so that it becomes tubular in form,
with the full cardiac pouch at the left and above and the pyloric por-
Fig. 156. — Outlines of the shadows cast by the stomach at intervals of an hour each after feeding
a cat with food impregnated with bismuth subnitrate. (From Cannon.)
tion at the right. The latter portion meanwhile does not diminish much
in size, although the peristaltic waves traveling over it are very pro-
nounced. As will be clear from the figure, these changes in outline go
on until the cardiac pouch has become practically empty and the food
has been all moved along the now tubular portion of the body into the
pyloric vestibule.
From this description it is evident that the function of the cardiac
end is to serve as a reservoir for the food, which, by a slow contraction
of the walls, is gradually delivered into the gastric tube, where by
peristalsis it is carried towards the pyloric vestibule.
The time required for the peristaltic waves to travel from their place
of origin to the pylorus is considerably longer than the interval between
454 DIGESTION
the waves, so that several of these are always seen on the stomach at
the same time. They sometimes become so pronounced in the pyloric
region, especially in a half-empty stomach, that they appear almost to
obliterate the cavity. They always stop at the pylorus, never going on
to the duodenum. The rate of recurrence of the waves varies somewhat
in different animals, being about six per minute in the cat and about
three in man. Their initiation does not seem to depend on the presence
of acid in the gastric contents, for, when food Is introduced into the
stomach, they do not wait for the gastric contents to become acid in
reaction (see page 482). Nevertheless1, acid does seem somewhat to stim-
ulate the depth and frequency of the waves, and they recur oftener with
carbohydrate than with fatty food.
The pressure in the stomach contents — the intragastric pressure — is
low and constant at the cardiac end and fairly high and variable in the
pyloric end (in the former from 6 to 8 cm. of water, and in the latter
from 20 to 30). Constancy of pressure in the cardiac end indicates
that the stomach wall must adapt itself very promptly to the amount of
food in the organ. The higher and more variable pressure in the pyloric
end is, of course, due to the peristaltic waves, and it is interesting to note
that it is sufficient to propel the gastric contents through the pylorus for
several centimeters into the duodenum.
The Effect of the Stomach Movements on the Food
This has been studied: (1) by dividing the food into portions that
are differently colored and, after some time, killing the animal, freezing
the stomach and making sections of it (see Fig. 157) ; (2) by mak-
ing little pellets of bismuth subnitrate with starch and observing their
behavior under the x-rays; or (3) by removing samples of the stomach
contents by means of a stomach tube (Rehfuss tube) inserted so that
its free end lies in either the cardiac or the pyloric region. By the
first of the above methods it has been found that the first mouthfuls
of food lie along the greater curvature, where they form a layer over
which that subsequently swallowed accumulates, with the last por-
tions next the cardia. The pepsin and hydrochloric acid of the car-
diac end, therefore, act soonest on the first swallowed portion of a
meal, and the more recently swallowed central masses are not affected
by the secretions for some time, so that opportunity is given for the
saliva mixed with the food to develop its digestive action.
As has been shown by removing the stomach contents with a tube at
various periods after feeding with starchy food, considerable amylolysis
may occur for some time. When separate samples are removed in this
way from the cardiac and pyloric parts, it has been found that after
THE MECHANISMS OF DIGESTION 455
half an hour the contents of both have about the same percentage of
sugar, but that for some time after this interval the cardiac contents
contain considerably more sugar than the pyloric. Later the percentages
of sugar again become about equal, no doubt on account of diffusion.
The diastatic action in the fundus is finally brought to an end when
the contents become completely permeated by the hydrochloric acid.
In this connection it is worthy of note that the addition of hydrochloric
acid up to the point of neutrality greatly accelerates the rate of diastatic
digestion.
As the outer layers of food in the stomach become partly digested on
account of the action of the pepsin and hydrochloric acid, the food is
slowly pressed into the active right half of the stomach, where by the
action of the peristaltic waves it is moved on to the pyloric vestibule.
By observing the x-ray shadows cast by two pellets of bismuth subni-
trate it has been noted by Cannon that, as the peristaltic wave approaches
Fig. 157. — Section of the frozen stomach (rat) some time after feeding with food given in three
differently colored portions. (From Howell's Physiology.)
a pellet, it causes it to move forward more rapidly for a short distance,
but soon overtakes it and in doing so causes the pellet to move back a
little towards the fundus. This backward movement is less than the
forward movement, so that after the wave has passed, the position of
the pellet is a little forward of that which it would have occupied had
there been no wave. The behavior of the pellet, and, therefore, of the
stomach contents, is very like that of a cork floating at the edge of the
sea; as each wave approaches, it hurries the cork on a little, but after
its passage the cork recedes again until the second wave carries it still
a little farther forAvard. As the peristaltic wave approaches the pyloric
vestibule and becomes more powerful its effect on the pellets becomes
more marked. They are carried rapidly along this part of the stomach,
until the pylorus is reached. If this remains closed, they are shot back
into the vestibule. From nine to twelve minutes may elapse before they
are transferred to the pylorus from the place where they are first affected
by the peristaltic wave.
456 DIGESTION
These observations made on cats and other laboratory animals no
doubt also apply in the case of man. Removal of the contents of the
cardiac and pyloric regions separately with a stomach tube after feeding
with a test meal part of which was colored with carmine or charcoal,
has shown that none of the coloring material was present in the contents
of the pyloric end up to twenty minutes or so after the food had been
taken. It then appeared but at first only in traces. Another important
distinction between the food in the two portions of the stomach relates
to its consistency. In the pyloric end it is semifluid and homogeneous
in character; in the cardiac end, on the other hand, it is a lumpy, rather
incoherent mass.
The gastric movements must greatly facilitate the digestive processes
in the stomach. In the cardiac part the undisturbed condition of the
food will, as we have seen, facilitate the digestive action of ptyalin,
whereas in the body of the stomach the peristaltic waves, besides mov-
ing the food onward, will tend to bring fresh portions of mucous mem-
brane and food in contact, so that the latter becomes more thoroughly
mixed with the pepsin and hydrochloric acid. In the pyloric part, where
no hydrochloric acid is secreted, the contents, already sufficiently acid
in reaction, become more thoroughly churned up with the local pepsin
secretion, so that proteolytic action progresses very rapidly.
The peristaltic waves also facilitate absorption from the stomach of such
substances as glucose in concentrated solution and, probably, of hydro-
lyzed protein ; water, however, is not absorbed. One effect of such
absorption is the production of gastrin, which we have seen is the hor-
mone concerned in maintaining the gastric secretion after the psychic
flow. The fact that the mucosa of the vestibule has, relatively to the
cardiac end, feAv secreting glands is in harmony with the view that
absorption is an important function of this part of the stomach.
THE EMPTYING OF THE STOMACH
The Control of the Pyloric Sphincter
When digestion has proceeded far enough in the stomach to bring the
food into a homogeneous, soup] ike fluid (chyme), portions of this, as they
are driven against the pyloric sphincter by -the peristaltic waves, instead of
being returned as an axial stream into the stomach, are ejected into the
duodenum.
We must now consider the mechanism by which the pyloric sphincter
opens to permit the passage of the chyme. Bombardment by the peri-
staltic waves is evidently not the cause of its opening, for, as we have
THE MECHANISMS OF DIGESTION 457
seen, many such waves may arrive at it without this result. Since it is
evidently in order that the intestine may not suddenly become over-
whelmed with large masses of food that the pylorus only occasionally
opens, it might be thought that its opening depends upon the disten-
tion of the upper part of the intestine. It is true that excessive disten-
tion of the upper part of the intestine does hold the pyloric sphincter
closed, but this can not be the physiologic stimulus, because considerable
quantities of chyme are never found here.
The first clue to the real nature of the mechanism was afforded by
observing the behavior of the sphincter Avhen solutions are introduced
into the duodenum through a fistula. Acid solutions were found to
cause a complete inhibition of gastric evacuation, whereas alkaline solu-
tions had no effect. This difference indicates that acids in contact with
the duodenal mucous membrane reflexly excite contraction of the sphinc-
ter, and that it relaxes only after the acid has become neutralized
by mixing with the pancreatic juice and bile.
On account of the great importance of the pyloric mechanism in insur-
ing that the chyme shall enter the intestine only in such quantities that
it can be properly acted upon by the intestinal digesting juices, it will
be of interest' to consider briefly some of the experimental observations
by -\vhich this mechanism has been studied. We may consider first the
evidence that acid on the stomach side of the pylorus causes a relaxation
of the sphincter: (1) When carbohydrate food is fed, it ordinarily leaves
the stomach fairly rapidly, but if its acid-absorbing power is increased
by mixing it with sodium bicarbonate, exit from the stomach is greatly
delayed. (2) Proteins ordinarily leave the stomach more slowly than
carbohydrates, but if acid proteins are fed, their exit is much more
rapid. (3) If a fistula is made into the pyloric vestibule through Avhich
some of the contents can be removed, it will be found that just prior to
the opening of the pyloric sphincter, a distinctly acid reaction develops
in the food ; and furthermore if acid solutions are injected through this
fistula, they cause the pyloric sphincter to open, -\vhereas alkalies retard
its opening. (4) A similar effect of acid in opening the sphincter can
be demonstrated by applying it to the pyloric mucosa of an excised
stomach kept alive in oxygenated Ringer's solution.
The evidence that acid on the duodenal side causes closure of the
sphincter is as follows: (1) When acid is placed in the duodenum through
a fistula, the sphincter will not open; (2) when the pancreatic and bile
ducts are ligated, the stomach empties much more slowly than normally;
and (3) the discharge of protein is considerably hastened if the pylorus
is sutured to the intestine below the duodenum. After such an opera-
tion it was observed that the protein began to leave the stomach through
458 DIGESTION
the pyloric sphincter about the same time as normally, but the subse-
quent evacuation was very much accelerated, because no acid came in
contact with the duodenal mucosa. Water and egg white may leave
the stomach independently of any acid reflex control of the pylorus. By
observations made through a duodenal fistula it has been found that,
after a quantity of water has been swallowed, most if not all of it very
soon enters the duodenum in a more or less continuous stream. It is no
doubt on this account that drinking contaminated water is especially
dangerous on an empty stomach.
The nervous pathway through which these acid reflexes take place has
been shown to be the myenteric plexus. Indeed, the whole mechanism
is quite analogous with that which we shall see occurs in the intestine
during peristalsis: the stimulus, that is, the acid, causes a contraction
of the gastric tube behind it and a dilatation in front.
Fig. 158. — Outlines of shadows in abdomen obtained by exposure to x-rays 2 hours after
feeding with food containing bismuth subnitrate. The food in A was lean beef, and in B boiled
rice. The smaller size of the stomach shadow and the much greater total area of the intestinal
shadows in R than in A show that carbohydrate leaves the stomach earlier than protein. (From
Cannon.)
Rate of Emptying of Stomach
The relationship of these facts to the rate at which different foodstuffs
leave the stomach is very readily explained. The method for investigat-
ing this problem, which again we owe to Cannon, consists in feeding ani-
mals with- a strictly uniform amount of different foods made up, as
nearly as possible, of equal consistency and containing bismuth subni-
trate in the proportion of 5 gm. to each 25 c.c. By feeding such mix-
tures to cats previously starved for twenty-four hours, and examining
the abdomen by the x-ray at regular intervals, the shadows cast by the food
after passage into the intestine can be outlined on tracing paper, and
the total length* measured (Fig. 158). In taking this as an estimate of
the amount of food ine the intestine, several errors are no doubt incurred
*This is permissible since the shadows are practically all of the same width.
THE MECHANISMS OF DIGESTION
459
on account of the crossing and foreshortening of the loops, etc., but, as
their constancy testifies, there is no doubt that the results are sufficiently
close for the purpose of finding out how quickly food gains access to the
small intestine; and the method has a great advantage over all others
in that digestion is allowed to proceed practically without interruption.
The points we have to determine are: (1) when the food first leaves the
stomach; (2) the rate at which different foods are discharged; (3) the
time required for the passage through the small intestine.
Let us consider first of all the results obtained by feeding with prac-
tically pure fat or carbohydrate or protein. By plotting the length of
the shadows in centimeters along the ordinates, with hours along the
abscissae, curves such as those shown in Fig. 159 have been secured.
When fats were fed (dash line in chart), the discharge began rather
slowly, and continued at a slow rate. Even after seven hours some fat
still remained in the stomach, and at no time was any large quantity
z
o k i
34
Hours
Fig. 159. — Curves to show the average aggregate length of the food masses in the small
intestine at the designated intervals after feeding. The curve for various fat foods is in the
dash line, for protein foods in the heavy line, and for carbohydrate foods in the light line.
(From Cannon.)
present in the intestine, indicating that almost as quickly as it is dis-
charged into this part of the gastrointestinal tract fat becomes digested
and absorbed. The discharge of carbohydrates was quite different (light
line in chart) ; it began often in ten minutes, and soon became abundant,
reaching a maximum, as a rule, at the end of two hours, after which it
fell off, the stomach being empty in about three hours. Protein left at a
rate intermediate between that for fats and that for carbohydrates
(heavy line). Little left before the first half hour; the curve then
slowly rose, attaining a maximum in about four hours, and then gradu-
ally declining at about the same rate as it rose. It is interesting to note
that at the end of half an hour about eight times as much carbohydrate
had left the stomach as protein; at the end of an hour, five times as much.
These results are clearly dependent upon the rates at which the dif-
ferent foodstuffs assume an acid reaction in the stomach. Carbohydrate
460 DIGESTION
has no combining power for acids, so that the acid secreted with the
psychic juice remains uncombined and on gaining the pyloric vestibule
excites the opening of the sphincter. Protein, on the other hand, as is
well known, absorbs considerable quantities of free hydrochloric acid,
so that for some considerable time after it is taken, none of the acid exists
in a free state. Fats owe their slow discharge partly to inhibition of
gastric secretion, and partly to the longer time it takes for them to become
neutralized in the duodenum, because of the fatty acid split off by the
action of lipase.
Interesting observations have also been made on the rate of discharge
when various combinations of foodstuffs were fed. This has been done
by feeding one foodstuff before the other, or by mixing the foodstuffs.
When carbohydrates were fed first and then protein, the discharge be-
gan much earlier than with protein alone, because the carbohydrate food
first reached the pyloric vestibule (see page 454). However, at the end
of two hours, when the carbohydrate curve should begin to come down,
it remained high, indicating that the protein had by this time reached
the pylorus and was being discharged at its own rate. When the meat
was fed before the carbohydrate, the curve to start with was exactly
like that for protein, becoming, however, considerably heightened later
when the carbohydrate reached the pyloric vestibule. The presence of
protein near the pylorus, therefore, distinctly retards the evacuation of
carbohydrate from the stomach. These facts, it will be remarked, all
fit in admirably with the observations which we have already detailed
concerning the disposition of food in the stomach.
When mixtures of equal parts of different foods were fed, the results
indicated that the emptying of the stomach occurred at a rate which
Avas intermediate between those of the foods taken separately. Mixing
protein Avith carbohydrate, for example, accelerated the rate at Avhich
protein left, and mixing fats Avith protein caused the protein to leave
the stomach considerably more sloAvly than if protein alone had
been fed.
Influence of Pathologic Conditions on the Emptying
An important surgical application of these facts concerns the behavior
of food after gastroenterostomy. It has been thought that this operation
would cause the food to be drained from the stomach into the intestine
and thus leave the region of the stomach betAveen the fistula and the
pylorus inactive. This assumption is based on the idea, Avhich AVC have
seen to be erroneous, that gravity assists in the emptying of the stomach.
As a matter of fact, it has been found that, if the gastroenterostomy is
made Avhen there is no obstruction at the pylorus, the chyme takes its
THE MECHANISMS OF DIGESTION 461
normal passage through the sphincter and, almost without exception,
none leaves by the fistula. When the pylorus is partly occluded, the
food sometimes passes in the usual way, and sometimes by the stomach.
The cause for this predilection for the pyloric pathway depends on the
pressure conditions in the gastric contents. Gastroenterostomy, there-
fore, is efficient only .when gross mechanical obstruction exists at the
pylorus. The operation should never be performed in the absence of
demonstrable organic pyloric disease.
Another objection to gastroenterostomy in the presence of a patulous
pyloric sphincter rests on the fact that the food, after passing the sphinc-
ter and moving along the intestine, may again enter the stomach through
the fistula. This is most likely to occur when the stomach is full of
food, for under these conditions the stretching of its walls separates the
edges of the opening, the intestine being drawn taut between the edges,
so that the opening between the stomach and the intestine assumes the
form of two narrow slits, which act like valves permitting the food to
enter but preventing its escape from the stomach. Only seldom under
these circumstances can any food pass into the intestine beyond the
stomach opening. Repeated vomiting after gastroenterostomy has been
observed in experimental animals only when obstructive kinks or other
demonstrable obstacles were present in the gut, the obstruction being lo-
cated in that part of the intestine beyond its attachment to the stomach.
When the pyloric obstruction is complete, food must, of course, leave
by the fistula, digestion by the pancreatic juice and bile being still car-
ried on because of the fact that for a considerable distance down the
intestine, secretin, which we have seen is essential for the secretion
of these fluids, is still produced by the contact of the acid chyme with
the intestinal mucosa. Further provision for adequate digestion of
food in such cases is secured, as some of the food after leaving the
fistula passes back for some distance into the duodenum, where, however,
it soon excites peristaltic waves, which again carry it forward. This
insures thorough mixing with the digestive juices. From their experi-
mental experience Cannon and Blake13 recommend that, when the
fistula has to be made, it should be as large as possible and near the
pylorus, and that the stomach afterwards should not be allowed to
become filled with food. To avoid kinking of the gut, they also recom-
mend that several centimeters of the intestine should be attached to the
stomach distal to the anastomosis.
The effect of hyperacidity of the contents on the emptying of the
stomach has been studied by feeding animals with potatoes containing
varying percentages of hydrochloric acid. With an acidity of 0.25 per
462 DIGESTION
cent, the rate of discharge was increased, but it became slower when the
acidity rose to 1 per cent. With an acidity of 0.5 per cent, the rate of
discharge was about the normal. Hyperacidity, therefore, causes a retar-
dation of the emptying of the stomach.
The consistency of the food appears to have little influence on its rate of
discharge from the stomach — at least in the case of potatoes. Dilution
of protein food, however, increases the rate. Distinctly hard particles
in the food retard the stomach evacuation.
There is usually a considerable amount of gas in the part of the stomach
above the entrance of the cardia, on account of which this part of the
stomach has sometimes been called the stomach bladder. In the upright
position this gas forms a bright area in the x-ray plate (Fig. 155), but
when the person reclines it spreads to a new location. Its presence may
influence gastric digestion by preventing the contact of the food with
the mucous membrane, and by interfering with the efficiency of the peri-
staltic waves in moving the food. Considerable gas therefore retards the
emptying of the stomach, as has been shown experimentally by x-ray
observations on animals fed with the standard amount of food followed
by the introduction of air. It was noted that the air did not diminish
the frequency or strength of the peristaltic waves, but that these could
not efficiently act on the food. When along with gas there is also atony
of the stomach walls, the retardation in the discharge will, of course, be
still more pronounced. The temperature of the swallowed food does
not appear to have much influence on the stomach movements or on the
the rate of discharge from the organ.
CHAPTER LIII
THE MECHANISMS OF DIGESTION (Cont'd)
THE MOVEMENTS OF THE INTESTINES
The length of the small intestine and the size of the cecum of the
large intestine vary considerably in different animals. In the carnivora,
such as the cat, the small intestine is relatively short; in the herbivora,
relatively long. Thus, it is three times the length of the body in the cat,
and four to six times in the dog ; whereas in the goat and sheep, it may
be nearly thirty times the length of the body. In the carnivora the
cecum is either absent or rudimentary, whereas in those herbivora which
do not have a divided stomach the cecum is very large and sacculated,
as is also the colon. The reason for the great size in herbivora is! that
practically the whole of the digestion of cellulose takes place in this
part of the gut. This digestion, as we shall see later, does not depend
on any secretion poured forth by the animal itself, but upon the action
of bacteria and of certain enzymes (cytases) that are taken with the
vegetable food.
Movement of the Small Intestine
The movements of the small intestine have been studied (1) by the
bismuth subnitrate and x-ray method, (2) by observing them after open-
ing the abdomen of an animal submerged in a bath of physiologic saline
at body temperature, (3) by observing the changes in pressure produced
in a thin-walled rubber balloon inserted in the lumen of the gut and
connected with a recording tambour (Fig. 160), and (4) by excising
portions of the intestine and keeping them alive in a bath of saline solu-
tion at body temperature, through which oxygen is made to pass.
THE SEGMENTING MOVEMENTS
When a suitably fed animal is placed on the holder for examination
by the x-ray method, no movement in the intestinal shadows is generally
observed for some time. The first movement to appear is the breaking of
one of the columns of food into small segments of nearly equal size.
Each of these segments again quickly divides, and the neighboring
halves suddenly unite to form new segments, and so on, in a manner
463
DIGESTION
which will be made clear by consulting Fig. 161. This rhythmic seg-
mentation, as Cannon has called it, continues without cessation for more
than half an hour, and the food shadow does not meanwhile seem to change
its position in the abdomen to any extent. The splitting up of the seg-
ment and the rushing together of the neighboring halves proceed as a
rule with great rapidity; thus, if we count the number of different seg-
Fig. 160. — Apparatus for recording contractions of the intestine. (From Jackson.)
ments during a definite period, we may find the rate of division in the
cat to be as high as 28 or 30 a minute. In man the divisions occur at a
frequency of approximately 10 per minute, which corresponds to the fre-
quency with which sounds can be heard when the abdomen is auscultated.
Although half an hour is the period which this process usually oc-
cupies, it may last considerably longer. In certain animals, such as the
rabbit, segmenting movements have not been observed, but instead
THE MECHANISMS OF DIGESTION 465
of them a rhythmic to-and-fro shifting of the masses of food along the
lumen of the gut, rapidly repeated for many minutes.
When the intestines are floated out in a warm bath of saline solution,
it is seen that the rhythmic segmentation is caused by narrow rings of.
contraction. Under such conditions also it is often noted that the
loops of intestine sway "from side to side. The balloon method also re-
veals the presence of slight waves of contraction that pass rapidly along
the gut, and follow each other at the rate of twelve to thirteen per minute.
Both of the muscular coats of the intestine are involved, and it is believed
that the contractions are responsible not only for the pendular move-
ments but for the rhythmic segmentation observed by the x-ray method.
According to this view these movements are constantly passing along
the intestine, and become exaggerated by the mechanical stimulus which
is offered by the masses of food to such an extent that they divide the
masses into portions. The evidence for this belief rests on- the fact that
1. — Diagrammatic representation of the process of segmentation in the intestine. An
ihadpw is shown in / and its segmentation in 2. The dotted lines across each mass
osition of division and in 3 is shown how new masses are formed by the split portions
ether. (.From Cannon.)
Fig. 161. _
unbroken shado
show the position 01 division ana i
coming together. (.From Cannon.)
when the contraction is studied by the balloon method, it becomes marked
over the middle of the balloon, where the greatest tension exists.
Several functions can be assigned to these movements. They cause
intimate mixture of the food with the digestive juices, and by bringing
ever new portions of food in contact with the mucosa, they encourage
absorption. They also have an important massaging influence on the
blood and lymph in the vessels of the intestinal walls. Indeed, the pas-
sage of lymph from the lacteals into the mesenteric lymphatics seems to
depend very largely upon these movements.
THE PERISTALTIC MOVEMENTS
The other movement observed in the small intestine is that known as the
peristaltic wave. It occurs in two forms: (1) as a slowly advancing con-
traction (1 to 2 cm. per minute), preceded by an inhibition of the walls,
and proceeding only through a short distance in a coil (4 to 5 cm.); and
466 DIGESTION
(2) as a swift movement called the peristaltic rush, which sweeps with-
out pause for much longer distances along the canal.
Further analysis of the peristaltic -wave can readily be made by the
balloon method (Fig. 162). If the gut is pinched above the balloon, a
marked relaxation occurs over it, and this relaxation extends for about
two feet down the intestine. If, on the other hand, the gut is pinched
a little below the situation of the balloon, a long-continued contraction
occurs over the latter. The conclusion that we may draw from this result
is that the stimulation of the gut causes contraction, above the point of
the stimulus and relaxation below, this being known as ''the law of the
intestine" — (Bayliss and Starling). We have seen that it applies also in
the case of the cardiac and pyloric sphincters.
Fig. 162. — Intestinal contractions (balloon method) after excision of the abdominal ganglia and
section of both vagi. Mechanical stimulation above (/) and below (2) the balloon causes relaxa-
tion and contraction respectively. (From Starling.)
THE PHYSIOLOGIC NATURE OF THE RHYTHMIC AND PERISTALTIC MOVEMENTS
Interesting information in this connection has been gained by obser-
vation of the behavior of the movements after the application of drugs
to the gut or after cutting the nerve supply. The rhythmic movements
are not affected by the application of nicotine or cocaine. Since these
drugs paralyze nervous structures it has been concluded that the rhythmic
movements are myogenic in origin. The question is not a settled one,
however, for it has been found by Magnus that, although strips of the
longitudinal muscle, isolated in oxygenated saline solution, will continue
to beat, they do not do so if the adherent Auerbach's plexus of nerves
is stripped off from them. The nature of the peristaltic contractions is
more definite; they must clearly depend upon a local nervous struc-
ture, since they are paralyzed by the application to the gut of cocaine or
nicotine. This local nervous system no doubt also resides in Auerbach's
plexus, which must therefore be considered as complex enough to be (see
THE MECHANISMS OF DIGESTION 467
page 796) endowed with the power of directing nervous impulses so as to
bring about relaxation of the gut in front of the stimulus and contrac-
tion over it.
NERVOUS CONTROL OP MOVEMENTS
The influence of the central nervous system on the intestinal movements
has been studied by the usual methods of cutting and stimulating the
extrinsic nerve supply. Through the splanchnic nerves tonic inhibitory
impulses are conveyed to the intestine (except the ileocolic sphincter),
for after these nerves' are severed the movements become more distinct.
Indeed, in many animals after opening the abdomen no intestinal move-
ment can be observed until these nerves have been cut. Stimulation of the
peripheral end of the nerve also inhibits any movement which may mean-
while be in progress. The impulses through the vagus nerve are of an
Fig. 163. — The effect of excitation of both splanchnic nerves on the intestinal contractions. (From
Starling.)
opposite character. Section of these nerves has little effect, but stimula-
tion causes contraction. (Figs. 163 and 164.)
By observing the rhythmic contractions of an isolated strip of the small
intestine suspended in a bath of oxygenated saline solution at body tem-
perature, it can readily be shown that the presence of even a minute trace
of epinephrine is sufficient to produce complete inhibition of the movement.
The parallelism between the effects of splanchnic stimulation and those of
epinephrine injection is very significant, for in this way the marked inhi-
bition of intestinal movement which occurs during fright may possibly
be explained (see page 736).
The circular muscular coat of the last two or three centimenters of
the ileum before it joins the cecum is definitely thicker than the rest of
this coat, indicating that it has a sphincter-like action. This ileocolic
sphincter, as it is called, opens when food is pressed against it from the
ileum, but remains closed when food is pressed against it from the cecum.
468
DIGESTION
It therefore obeys the law of the intestine. That it is physiologically
distinct from the musculature of the rest of the ileum is indicated by the
fact that the splanchnic and vagus nerves do not affect it in the same
way; thus, stimulation of the splanchnic causes a strong contraction of
the sphincter, whereas it is unaffected by stimulation of the vagus.
Peristalsis is much more rapid in the duodenum than in other parts of
the small intestine. During the first stages of digestion, the food ordi-
narily lies mainly in the right half of the abdomen, and later in the left
half. There is considerable variation in the time that elapses before it
enters the colon. In the cat, carbohydrates reach this part of the gut in
about four hours.
Fig. 164. — The effect of stimulation of right vagus nerve on the intestinal contractions. (From
Starling.)
Movements of the Large Intestine
On account of the great differences which we have already seen to
exist in the size and relative importance of the colon as a digestive organ
in different classes of animals, it is not surprising that the movements
observed are very different according to the dietetic habits of the animal.
Apparently the movements are much the same in the cat as in man. As
the food passes through the ileocolic sphincter into the cecum and
accumulates there, it gradually sets up, by its pressure, a contraction of
the muscular walls of the gut somewhere about the junction between
the ascending and transverse colon. This wave of contraction then
.begins to travel slowly toward the cecum, without, however, being pre-
ceded by any relaxation of the wall of the gut, as is the case with a true
THE MECHANISMS OF DIGESTION
469
peristaltic wave. This first wave is soon followed by others, with the
result that the food is forced up into the cecum, against the blind end
of which it is crowded, being meanwhile prevented from passing into
the ileum by the operation of the ileocolic sphincter and by the oblique
manner in which the ileum opens into the cecum.
As the result of the distention of the cecum set up by these so-called
antiperistaltic waves, a true coordinated peristaltic wave is occasionally
initiated, and passes along the ascending colon preceded by the usual
wave of inhibition. These waves, however, disappear before they reach
the end of the colon, so that the food is again driven back by the so-
Fig. 165. — Diagram of time it takes for a capsule containing bismuth to reach the various parts
of the large intestine.
called antiperistaltic waves. The effect of the movements is to knead
and mix the intestinal contents, and thus encourage the absorption of
water from them. The resulting more solid portions then collect toward
the splenic flexure, and become separated from the remaining more fluid
portion by transverse waves of constriction, which develop into peri-
staltic waves carrying the harder masses into the distal portions of the
colon, where they collect chiefly in the sigmoid flexure. The descending
colon itself is never distended with contents and merely serves as a tube
for transferring the masses from the transverse colon to the sigmoid
flexure. The time taken for a capsule of bismuth to reach the various
parts of the large intestine is shown in Pig. 165.
After a certain mass has collected in the sigmoid flexure and rectum,
the increasing distention causes a reflex evacuation of this portion of the
470 DIGESTION •
gut through centers located in the spinal cord. The impulses from these
centers, besides contracting the rectum, etc., also coordinate the contrac-
tion of the abdominal muscles and the relaxation of the sphincter ani
so as to bring about the act of defecation. By the skiagraphic method it
has been found that the pelvic colon gradually becomes filled with feces
from below upward, and that the rectum remains empty until just before
defecation.
EFFECT OF CLINICAL CONDITIONS ON THE MOVEMENTS
Observations of practical value have been made on the behavior of the
peristaltic wave after various intestinal operations. After an end-to-end
anastomosis of the gut, no evidence can be obtained by the x-ray method
that any hesitation occurs in the movement of the shadows at the anas-
tomosis. On the other hand, when a lateral anastomosis is established,
stagnation of the food in the region of the junction may occur, this
having been found, on opening the gut, to be caused by the accumu-
lation of hair and undigested detritus at the opening between the op-
posed loops. Another objection to lateral anastomosis is the fact that
in performing the operation a considerable amount of the circular muscle
is cut, which interferes with peristaltic activity. Moreover, the end of
the proximal loop beyond the opening is in danger of becoming filled up
with hardened material, and the end of the distal loop may become
invaginated and induce obstruction in the region of the anastomosis.
Observations have also been made by the x-ray method on the be-
havior of the intestinal contents following intestinal obstruction. It has
been observed that, as the material collects in the gut just above the
obstruction, strong peristaltic waves are set up, which move the food
toward the obstruction so powerfully as to cause the walls of the canal
in front to become bulged, until at last the pressure causes the con-
tents to be squirted back through the advancing ring of peristaltic con-
traction. These waves were observed to succeed one another rapidly.
When a portion of gut is reversed in position, the peristaltic waves con-
tinue to travel in their old direction toward the duodenum. The effect of
this is to produce a partial obstruction at the upper end of the re-
ceptive gut.
The type of peristalsis known as the peristaltic rush can be induced
experimentally in animals by intravenous injection of ergot. It prob-
ably also occurs in conditions of abnormal irritation of the gut in man,
and is believed to be the characteristic activity of the gut after a
•
strong purge.
CHAPTER LIV
HUNGER AND APPETITE
Hunger and appetite are distinct sensations, the former being definitely
correlated with contraction of the empty stomach, and the latter being
a complex of sensations operating in the nervous system along with
memory impressions of the sight, taste, and smell of palatable food.
Appetite is therefore a highly complex nervous integration, whereas
hunger is a much simpler process. It is particularly with hunger that
we shall concern ourselves at present.
When a thin-walled rubber balloon of proper size is placed in the
stomach and connected by a rubber tube with a water, bromoform or
chloroform manometer (made of wide glass tubing 1.5 cm. in diameter
and provided with a suitable float on the free limb) a tracing may be
taken of the movements of the stomach. For use on man the capacity of
the balloon should be from 75 to 150 cubic centimeters. The record thus
obtained when the balloon is placed in the empty stomach of a normal
person shows four types of wave. Two of these may be discounted,
being due to the arterial pulse and the respiratory movements. The
third is known as the tonus rhythm, and is caused by tonic contractions
of the fundus of the stomach of varying amplitudes and occurring at a
rate which varies from 18 to 22 per second. The periods of tonus in-
crease during the powerful rhythmic contraction to be immediately
described. While these changes in tone are occurring, no subjective sen-
sation of hunger is experienced. (See Fig. 167.)
The fourth and most significant type consists of powerful rhythmic
contractions, alternating with periods of quiescence. These contrac-
tions occupy a period of about twenty seconds, and are superimposed
upon the tonus rhythm. They gradually increase in amplitude and fre-
quency.; and, in the case of young and vigorous persons, may gradually
pass into a condition of incomplete tetanus, after which they suddenly
subside, leaving only a faint tonus rhythm. These rhythmic contrac-
tions are definitely associated with the sensation of hunger, and are
more marked the more intense the sensation is. When tetanus occurs
the hunger sensation is continuous, but it instantly disappears when
the tetanus gives place to relaxation. When the contractions are com-
paratively feeble, the length of the period during which they occur is
471
472
DIGESTION
about twelve minutes, being shorter than when the contractions are power-
ful, in which case the periods are always initiated by weaker contrac-
tions with long intervening pauses. Finally, the pauses disappear and
the contractions become more and more pronounced until, as above men-
tioned, a virtual tetanus, lasting from two to five minutes, may super-
vene. The duration of the hunger period varies from one-half to one and
a half hours, with an average of from thirty to forty-five minutes, and
the number of individual contractions in a period varies from twenty to
seventy. Between the hunger periods, intervals of from one-half to
tAvo and one-half hours of quiescence may supervene. (See Fig. 168.)
Similar contractions, often passing into incomplete tetanus, have been
observed in the stomach of healthy infants, some of the observations hav-
ing been made before the first nursing. The intervals of motor quies-
Fig. 166. — Diagram of method for recording stomach movements. B, rubber balloon in stomach.
D, kymograph. F, cork float with recording flag. M, manometer. L, manometer fluid (bromo-
form, chloroform, or water). R, rubber tube connecting balloon with manometer. S, stomach.
T, side tube for inflation of stomach balloon. (From Carlson.)
cence between the hunger periods are shorter than in adults. In obser-
vations made during sleep, it was observed that, when the contractions
were very vigorous, the infant would show signs of restlessness and
might awake and cry. As in the adult, the contractions are evidently
associated with subjective sensations of hunger. Contractions , of the
empty stomach have also been recorded on a large variety of animals,
including the dog, rabbit, cat, guinea pig, bird, frog and turtle. They
vary somewhat in type in different animals.
. With regard to the time of onset of the tonus and hunger contractions,
it has been observed that the only period during which the fundus is
free of them is immediately after a large meal. After a moderate meal
the tonus rhythm begins to appear in about thirty minutes. It gradually
HUNGER AND APPETITE
473
increases in intensity, until by the time the stomach has nearly emptied
itself the tonus has become conspicuous, and the stronger hunger con-
tractions usually begin to appear. Superimposed upon those of the
tonus rhythm, hunger pangs may appear in man when the stomach still
contains traces of food.
Fig. 167. — Tracing of the tonus rhythm of the stomach (man) three hours after a meal. (From
Carlson.)
By studying the shadow of the outline of the stomach produced by
having a person or animal swallow two balloons, one inside the other
and with a paste of bismuth subnitrate between them, it has been ob-
served that the weaker type of hunger- contraction begins as a con-
Fig. 168. — Tracings from the stomach during the culmination of a period of vigorous gastric hunger
contractions. One-half original size. (From Carlson.)
striction involving the cardiac end of the stomach, and moving toward
the pyloric end as a rapid peristaltic wave. When the contractions are
very vigorous, this wave spreads so rapidly over the stomach that it is
difficult to determine whether it really occurs as a very rapid peristalsis
or as a contraction involving the fundus as a whole. These contractions
474 DIGESTION
resemble very closely the movements that have sometimes been observed
after a bismuth meal, and which have been thought by clinical observers
to indicate a hyperperistalsis of the stomach. The fundus is therefore
not entirely passive during digestion; for, although early in this act
there may be no evidence of contraction, yet the contractions of the tonus
rhythm may appear and become pronounced before the stomach is en-
tirely empty. In other words, the digestion contractions of the filled
stomach (see page 451) pass gradually over into the hunger contractions
of the empty organ.
It appears that the stomach contractions produce the hunger sensa-
tions by causing stimulation of afferent nerve endings in the muscle
layers of the viscus. Mere pressure on the mucosa itself does not originate
such a sensation; thus, sudden distention of the balloon or rubbing the
mucosa with the closed end of a test tube, inserted through a gastric
fistula, did not cause any sensation of hunger, unless the stimulus was
so strong as to cause a contraction of the musculature of the stomach.
It has been thought by some observers that, during hunger, contrac-
tions similar to those of the stomach also occur in the lower end of the
esophagus. It is believed by Carlson, however, that these contractions
are not at all responsible for the hunger sensation, although they may
give rise to a feeling that something has stuck in the esophagus. Con-
tractions of the intestine have also been observed in hunger, but it is doubt-
ful whether they have anything to do with the cause of the hunger
sensation.
REMOTE EFFECTS OF HUNGER CONTRACTIONS
It is well known that during hunger certain general subjective symp-
toms are likely to be experienced, such as a feeling of weakness and a
sense of emptiness, with a tendency to headache and sometimes even
nausea in persons who are prone to headache as a result of toxemic
conditions. Headache is likely to be more pronounced or perhaps only
present in the morning before there is any food in the stomach. These
symptoms indicate that hunger contractions are associated with hyper-
excitability of the central nervous system, and it is of considerable
interest that objective signs of this association can be elicited. If the
knee-jerk be recorded along with a record of the gastric contractions, it
will be found that it is markedly exaggerated simultaneously with the
strong hunger "contractions of the empty stomach, this augmentation
being greatest at the height of the stomach contractions, when the hun-
ger pangs are most intense, and falling off again to normal when these
disappear (Fig. 169).. Further changes occurring during the hunger
HUNGER AND APPETITE
475
period include an increase in the pulse rate and vasodilatation. By
comparing plethysmographic tracings of the arm volume (see page 230)
and stomach contractions, it has been found that the increase in volume
occurs pari passu with the increasing tonus of the stomach, but that it
begins to shrink before the stomach contraction has reached its maximum.
Occasionally, however, as in acute hunger, a somewhat different rela-
tionship obtains, vasoconstriction being more prominent. During each
hunger contraction there is also increased salivation, the degree of
which varies with different individuals. This salivation is independent
of the more copious "watering of the mouth" that accompanies the
thought or sight of appetizing food.
j L
tttttttttttttttttWtttttttttlttr'
-ttttttttttttttttttmttttttttttttttfitmtttitttnitittni
Fig. 169. — Showing augmentation of the knee-jerk (upper tracing) during the marked hunger con-
tractions (lower tracing). (From Carlson.)
HUNGER DURING STARVATION
During enforced starvation for long periods of time, it is known
that healthy individuals at first experience intense sensations of hunger
and appetite, which last however only for a few days, then become less
pronounced and finally almost disappear. It is of interest to know the
relationship between these sensations and the hunger contractions in
the stomach. This has been investigated by Carlson and Luckhardt, who
voluntarily subjected themselves to complete starvation, except for the
taking of water, for four days. During a great part of this time records
of the stomach contractions were taken by the balloon method, and it
was found that the tonus of the stomach and also the frequency and
intensity of the hunger contractions became progressively more pronounced
as starvation proceeded. Towards the end of the period it was also noted
that incomplete hunger tetanus made its appearance where ordinarily,
as in Carlson's case, this type of hunger contraction was infrequent.
Sensations of hunger were present more or less throughout the period,
being therefore probably due to the persistently increased tonus. The
onset of a period of hunger contraction could usually be foretold by an
476 DIGESTION
increase in the hunger sensation, and as these contractions became more
marked, the hunger sensations became more intense. On the last day of
starvation a burning sensation referred to the epigastrium was added to
that of hunger. The appetite ran practically parallel with the sensa-
tion of hunger, and both of these sensations became perceptibly dimin-
ished on the fourth or last day of starvation, this diminution being,
however, most marked in the sensation of appetite. Indeed, instead of an
eagerness for food, there developed on the last day a distinct repugnance
or indifference towards it. Accompanying these sensations of hunger
and appetite a distinct mental depression and a feeling of weakness were
experienced during the latter part of the starvation period.
On partaking of food again the hunger and appetite sensations very
rapidly disappeared, and also practically all of the mental depression
and a great part of the feeling of weakness. Complete recovery from
the latter, however, did not take place until the second or third day
after breaking the fast. From this time on both men felt unusually
well; indeed they state that their sense of well-being and clearness of
mind and their sense of good health and vigor were as greatly improved
as they would have been by a month's vacation in the mountains. They
further point out that, since others who have starved for longer periods
of time unanimously attest the fact that, after the first few days, the
sensations of hunger become less pronounced and finally almost dis-
appear, they must have experienced the most distressing period during
their four days of starvation. Although the hunger sensation was
strong enough to cause some discomfort, it could by no means be called
marked pain or suffering, and was at no time of sufficient intensity to
interfere seriously with work. Mere starvation can not therefore be
designated as acute suffering. It is of further interest to note that dur-
ing the starvation period a continuous flow of secretion of acid gastric
juice was found to occur in the stomach, the presence of this acid prob-
ably explaining the acid or burning sensation experienced in the epigas-
trium on the last days.
CONTROL OF THE HUNGER MECHANISM
The control of the hunger mechanism, like that of any other mechan-
ism in the animal body, may be effected through the nervous system or
it may depend on the presence of chemical substances or hormones in
the blood. As a matter of fact, it can readily be shown that both those
methods of control are operative, and we will now consider briefly some
of the facts upon which this conclusion depends.
Although many facts are now known with regard to the nervous con-
HUNGER AND APPETITE 477
trol of the hunger mechanism, it is difficult to piece these together in
such a way as to formulate a simple theory which fits in with all the
observed facts. We know that the stomach possesses in itself a local
nervous mechanism by which, like the heart or intestine, it can auto-
matically perform many of the movements which are exhibited in the
intact animal. These local movements may, however, be considerably
influenced by impulses transmitted to the stomach along the vagus and
splanchnic nerves. We have therefore to seek for evidence indicating
the relative importance of the local nervous mechanism in the stomach
itself and of the impulses transmitted to this organ by the extrinsic
nerves. We must then seek the position of the center which perceives
the sensation of hunger.
It will be simplest to consider first the effect of section of the extrinsic
nerves in observations made on lower animals. Section of the splanchnic
nerves increases gastric tonus and augments the gastric hunger contrac-
tions. Section of both vagus nerves, performed of course below the level
of the heart, leaves the stomach in a more or less hypotonic condition.
The tonus is not entirely abolished; it varies somewhat from day to day,
and may become quite pronounced even though the vagi are cut. In
this hypotonic state the hunger contractions are diminished in rate
and regularity. Section of both the splanchnic and vagus nerves throws
the stomach into a permanent hypotonus, except in prolonged starva-
tion, when hunger contractions develop that are usually of great ampli-
tude and with particularly long intervals between the contractions.
The general conclusion to be drawn from these experiments is that,
although completely isolated from the central nervous system, the
stomach still exhibits typical hunger contractions, which must therefore
be essentially dependent upon an automatic mechanism in the stomach
wall itself. Over this mechanism, extrinsic nerve impulses have merely a
regulatory control.
Variations and Inhibitions of the Hunger Contractions
The afferent stimuli that may set up impulses traveling by the extrin-
sic nerves to the stomach are conveyed by the nerves of sense or are of
psychic origin. Stimulation of the gustatory end organs in the mouth,
as by chewing palatable food, always causes an inhibition of the tonus
and a diminution or disappearance of the hunger contractions. Even the
chewing of indifferent substances, such as paraffin, suffices to produce
distinct inhibition, unless in a case in which the contraction has passed
into a tetanus. It is of interest that swallowing movements, in the ab-
sence of any food substance in the mouth, are sufficient to produce a
transitory inhibition of the gastric tonus — a receptive relaxation of the
478 DIGESTION
stomach, as it has been aptly called. The diminution in tonus and
hunger contractions in these various ways is accompanied by a diminu-
tion in the hunger pains.
Afferent nerve stimulation affecting the hunger contractions may also
originate in the stomach mucosa itself, as has been shown by Carlson on
his patient by introducing the various substances to be tested through
a tube into the stomach. A glassful of cold water introduced in this
way inhibits the tonus and the hunger contractions for from three to five
minutes unless these are severe; this inhibition being followed by no
augmentation either of the tonus or of contractions. Ice-cold water has
a greater effect than water at body temperature. This result is some-
what different from that which most men experience as the result of
drinking a glass of cold water.
Weak acids of strengths varying up to that found present in the
gastric juice itself — 0.5 per cent — cause a marked inhibition of the
hunger movements, but this inhibition does not persist until all the acid
has escaped from the stomach or been neutralized, which explains why
hunger contractions should still occur when an acid secretion is present
in the stomach, as in starvation. Normal gastric juice itself produces
an inhibition, which is no doubt dependent upon the acid which it con-
tains, and it is probable that, at the same time that it leads to inhibition
of the hunger contractions, the acid initiates peristalsis of the pyloric
region (see page 453). Weak alkaline solutions have no greater effect on
the hunger contractions than an equal volume of water. Weak solu-
tions of local anesthetics, such as phenol or chloretone, are without effect.
With regard to alcoholic beverages interesting results were obtained.
Wine, beer, brandy, and diluted pure alcohol inhibit both the tonus and
the contractions. The duration of this inhibition varies directly with the
quantity of the beverage introduced into the stomach and with its alco-
hol percentage. These observations are apparently not in harmony with
the experience of most men that the taking of alcoholic beverages serves
to awaken or increase the appetite, the difference being no doubt due to
the fact that appetite and hunger contractions of the stomach are not
dependent on each other, appetite being, as we have seen, a complex
psychic affair, whereas the hunger contractions depend upon a local
mechanism in the stomach wall itself.
As the inhibition produced in one or other of these ways passes off,
the hunger contractions are resumed at their previous intensity and not
in an augmented form. From the promptness of the inhibition, it would
appear that the stomach contractions are affected, not reflexly through
the central nervous system or by changes in the chemical composition
of the blood, but by a direct action on the neuromuscular mechanism
HUNGER AND APPETITE 479
in the stomach walls, and it is important to bear, in mind that the
inhibitory effects on the stomach contractions of the fundus may proceed
quite independently of the changes in the pyloric region that are con-
cerned with th.e mechanical processes of digestion. After one or both
of the extrinsic nerves of the stomach were severed in dogs, a certain
degree of inhibition could still be induced by the above methods, indicat-
ing that, although section of the extrinsic nerves depresses the inhibitory
reflex, it does not abolish it.
Various mitigations of the hunger contractions have been discovered.
Smoking has this effect, and compression of the abdomen by tightening
the belt also inhibits the contractions provided they are not of marked
intensity. Considerable muscular exercise, such as brisk walking or
running, causes inhibition, which usually persists until after the exer-
cise is discontinued. When the tonus and contractions return, in this
case, they seem to be somewhat more pronounced. Application of cold
to the surface of the body — as by placing an ice pack on the abdomen
or taking a cold douche, procedures which are well-known to induce
increased neuromuscular tonus, in general — causes an inhibition of the
gastric tonus and hunger contractions, the degree of which is roughly
proportional to the intensity of the stimulation. There is certainty never
an increase in the gastric tonus or hunger contractions. If such stimula-
tion is maintained, the inhibitory effects on the stomach gradually
dimmish, even though the individual be shivering intensely.
With regard to the nerve centers concerned in these phenomena, little
that is definite is known. The sensory nuclei of the vagus nerve in the
medulla must be considered as the primary hunger center, and through
this center, not only influences affecting the stomach contractions, but
also those associated with the hunger sensations, must be mediated. It
would appear from observations on the hunger behavior of decerebrate
animals that there can be no hunger center located on the cerebral cortex
itself, for such animals exhibit practically the same hunger effects as
normal animals. It is interesting to note that, at least in the case of
decerebrate pigeons, this hunger behavior entirely disappears on removal
of the optic thalami, where important nerve centers having to do with
the bodily responses of the animal to hunger impulses would therefore
appear to be located. These observations support the suggestion that
has been made by several neurologists that the sense of pain is located
somewhere in the thalamic region.
Concerning the influence of psychic states, Carlson says that in his
own case the hunger contractions became weaker and the intervals
between them greater when he was suddenly awakened during his
fast and saw two of his friends partaking at his bedside of a " feast of
480 DIGESTION
porterhouse steak with onions, potatoes, and a tomato salad." These
results are no doubt due to local inhibition dependent upon the psychic
secretion of appetite gastric juice. When no such juice is produced,
the sight and smell of good food does not appear to affect materially
the hunger contractions of the stomach. No doubt it stimulates the
appetite, but that, as we have seen, is a psychic affair.
CHAPTER LV
THE BIOCHEMICAL PROCESSES OF DIGESTION
In a book designed primarily for clinical workers, it would be out of
place to enter into details concerning the biochemical processes taking
place during the digestive process. There is, however, a certain amount
of fundamental knowledge which it is essential that we should consider.
In the first place it should be borne in mind that in the digestion of
carbohydrates and proteins, various intermediate stages are passed
through before the final absorption products are formed. The highly
complex molecule of which protein, for example, is composed, is first
of all broken down into several smaller but still highly complex mole-
cules, each of which then undergoes further disruption, until ultimately
the amino acids are set free. Certain enzymes, such as trypsin, can
carry this process from the beginning through the greater part of its
course without the assistance of other enzymes, but in the natural proc-
ess of digestion, as it occurs in the gastrointestinal tract, the different
stages of the disruption are controlled by different enzymes. One enzyme
prepares the food for action by the next. This interdependence of the
actions of the enzymes demands that some provision should be made
whereby each enzyme is secreted at the proper time; that is, when the
foodstuff has already been prepared for its action by that of its prede-
cessor. Thus, it would be useless after food is taken for the gastric and
pancreatic juices to be secreted at the same time. Instead, the gastric
juice is secreted first, and the pancreatic only after the food has been
prepared for its action. This correlation in function we have already
seen to be dependent largely on the action of hormones.
DIGESTION IN THE STOMACH
The gastric juice contains two important digestive agencies: (1) the
enzyme, pepsin, and (2) hydrochloric acid. It is particularly in juices
secreted in the cardiac end of the stomach that these two substances are
found present; towards the pyloric end the hydrochloric acid entirely
disappears, and the pepsin content becomes distinctly less.
481
482 DIGESTION
The Functions of Hydrochloric Acid
The functions of hydrochloric acid may be conveniently divided into
physiological and biochemical. The former functions have to do with
the control of the movements of the stomach, including the opening
of the pyloric sphincter, and, after the chyme has entered the duodenum,
with the secretion of pancreatic juice and bile. The biochemical functions
are concerned: (1) in assisting the pepsin in the digestion of proteins,
(2) in bringing about a certain amount of inversion of disaccharides,
and (3) in having an antiseptic action on the stomach contents. Re-
garding the last mentioned of these functions, it may be said that the
chyme, as it is ejected from the stomach, is usually sterile, although it
may contain spores and certain bacteria that are protected against the
digestive agencies of the stomach. This protection is afforded by an
outer covering of a chitinous nature (spores), or, as in the case of the
tubercle bacillus, by a covering of waxlike material. It is believed that
persons with strictly normal digestion are much less liable to infection
by such bacteria, as those of typhoid and cholera, than persons with less
active gastric secretion. When the acid of the gastric juice falls below
the level at which it develops an antiseptic action, various bacteria and
yeasts grow in the stomach contents, producing by the resulting fermen-
tation irritating organic acids and gases. It is under these conditions
that yeasts, sarcinae, and lactic and butyric acid bacilli find in the gastric
contents a suitable nidus on which to grow.
THE AMOUNT OF ACID
It has long been known that considerable variations in the amount of
hydrochloric acid in the gastric juice are associated with symptoms of
indigestion. On this account a more or less elaborate technic has been
developed for the purpose of determining the amount of hydrochloric
acid in the gastric contents.* There are three things in connection with
this activity that we may measure: (1) the total titrable hydrochloric
acid; (2) the free hydrochloric acid; and (3) the actual hydrogen-ion
concentration. The determination of the total available acids is made
by titrating a measured quantity of gastric juice against a standard
alkali, using phenolphthalein as an indicator. By this method about
75 c.c. of decinormal alkali solution are required to neutralize 100 c.c.
of normal gastric juice. The determination of the free hydrochloric acid
is made by using special indicators, such as those of Giinzberg and
Topfer, which change color at a hydrogen-ion concentration of about
10'5 (see page 27). To produce this hydrogen-ion concentration, a con-
*The methods can be found in any volume on clinical diagnosis.
THE BIOCHEMICAL PROCESSES OF DIGESTION 483
siderable quantity — 0.05 per cent or more — of an organic acid is neces-
sary, whereas it requires only a trace of hydrochloric acid. Normal
human gastric juice, when titrated with one of these indicators, gives
a figure which corresponds to about 0.03 N. hydrochloric acid (see page
22). For the accurate determination of the hydrogen-ion concentration,
it is necessary to use the gas-chain method (see page 29).
When gastric juice is collected through a fistula from an empty
stomach, very little difference will be found between the free hydro-
chloric acid and the total acid; that is, between th'e results obtained by
the second and the first of the methods described above. This is because
in such juice there is no organic matter capable of, combining with the
hydrochloric acid, and there are no other acids, such as lactic or butyric,
which might be produced by fermentative processes. The difference
between the two titrations, however, becomes quite marked when pro-
tein food is undergoing digestion in the stomach, because at its different
stages of digestion protein combines with increasing quantities of the
hydrochloric acid. The pathologic condition in which there is most
definitely a diminution of the hydrochloric acid is cancer, either of the
stomach itself or occasionally of some other part of the body. An in-
crease is particularly marked in ulcer of the stomach. A considerable
variation in hydrochloric acid may however be the result merely of func-
tional (neurotic) conditions.
THE SOURCE OF THE ACID
A question that has puzzled physiologists for many years concerns the
mechanism by which hydrochloric acid is secreted. The percentage of
hydrochloric acid in the gastric juice is considerably above that at which
any animal cells can live, and yet this acid is secreted by the lining
membrane of the stomach, its source being, of course, the ' sodium
chloride of the blood plasma. How then do the cells of the gastric
glands bring about the separation of this powerful acid from the per-
fectly neutral blood plasma ? In the first place, it is significant that the
mucous membrane of the stomach contains a higher percentage of
chlorine than the average of other organs and tissues, indicating that it
has the power of abstracting chlorine from the blood. The excess of
chlorine in the mucosa must, moreover, be but a very small fraction of
that actually secreted into the the gastric juice. The chlorine content
of the mucosa of the cardiac end is considerably greater than that of the
pyloric. These facts indicate that chlorine is attracted by the gastric
cells, but they throw no light on the question as to where the hydro-
chloric acid is really formed. Is it in the cells, or only in the lumen of
the gland tubes? That is to say, is it formed before or after the gastric
484 DIGESTION
juice has been secreted from the cells? After intravenous injection of
solutions of potassium ferrocyanide and some inert salt of iron, such as
one of the scale preparations, examination of the gastric glands has
shown that the prussian blue reaction, which requires the presence of
free mineral acid, is most pronounced in certain of the parietal cells. A
considerable amount of the precipitate is, however, also visible in the
lumen of the glands and in the stomach itself. Certain observers affirm
that, although some of the parietal cells may take -the stain, the vast
majority of them do not do so; and, moreover, that cells incapable of
forming hydrochloric. acid (e.g., of the liver) may also become stained,
and that the precipitation may occur in the blood and lymph.
The confusion in the results by these methods prompted A. B. Macal-
lum1* and Miss M. R Fitzgerald to investigate the distribution of the
chlorine in the cells by a rnicrochemical method, in which the chlorides
were precipitated with silver nitrate and the silver chloride then reduced
by exposing the section to light. It was found that both kinds of gas-
tric-gland cell, chief and parietal, but particularly the parietal, gave the
chloride reaction. Using as a stain a substance (cyaninine) which reacts
blue with acid and red with alkali, Harvey and Bensley,15 however, aver
that the secretion of the glands is practically neutral until the foveola is
reached, where the stain becomes blue, indicating an acid reaction.
This seems to show that the acid is not really secreted by the cells of
the gastric gland, but is formed after secretion.
According to the latter investigators, the chlorine is secreted by the
cells into the fovea as some weak chloride, such as ammonium chloride,
or it may be as an ester. Shortly after its secretion this weak chloride
undergoes a hydrolytic or other dissociation, during which free hydro-
chloric acid is liberated and ammonia or some other weak base set free.
Of these two products of the reaction the weak base is reabsorbed by
the gland cells, but the hydrochloric acid is left behind because the
cells are impervious to it. Indirect evidence in support of this view is
afforded by certain other instances in which hydrochloric acid is pro-
duced by the action of cells ; thus, the mold Penicillium glaucum when it
is grown in a medium containing ammonium chloride absorbs the am-
monia but leaves the hydrochloric acid. The high penetrating power
of the ammonia ion in practically all cells, and the fact that the mucosa
of the stomach contains a higher percentage of ammonia than any other
tissue in the body, must also be considered as circumstantial evidence
in favor of this view.
Whatever be the mechanism by which hydrochloric acid is produced,
there is no doubt that the1 epithelium is impenetrable to it. When the
vitality of the epithelium becomes lowered, as in anemia or after partial
THE BIOCHEMICAL, PROCESSES OF DIGESTION 485
occlusion of the arteries, the acid may penetrate the cells and cause
digestion of the stomach walls. Hyperacidity may on this account
become dangerous, as it lowers the resistance of the cell.
The digestive action of hydrochloric acid is closely linked with that of
pepsin, with which it will, therefore, be considered.
The Action of Pepsin
It is commonly believed that before its secretion pepsin exists in the
cells of the gastric glands as zymogen granules. The chief evidence for
this belief appears to be that after considerable activity the amount of
zymogen granules in the gland cells is found to be decidedly dimin-
ished. By such an hypothesis it is easy to explain certain interesting
results concerning the effect of weak alkali on the activities of extracts
of the mucous membrane of the stomach. When the mucous membrane
is extracted with weak acids, the extract is very active proteolytically.
If this so-called pepsin solution be made faintly alkaline, or even only
neutralized, and again made acid, it will be found to have lost much,
if not all, of its activity. On the other hand, an aqueous extract may be
rendered slightly alkaline for a short time and still display its digestive
activity on subsequent acidification. The extract made with water is
therefore much more resistant toward alkali than that made with weak
acid, and the difference is explained on the supposition that the watery
extract contains pepsinogen, whereas the acid extract contains pepsin.
It is believed that there are several varieties of pepsin, because the
optimum concentration of acid in which pepsin derived from the stomachs
of different animals acts is not always the same. Pepsin of the dog, for
example, acts best in a hydrogen-ion concentration corresponding to
that of a 0.05 N. hydrochloric acid solution, whereas that of the human
stomach works best at a concentration of 0.03 N. Different pepsin
solutions also show a difference with regard to the optimum tempera-
ture at which they act, and with regard to the nature of the protein
which they most readily attack. Thus, the pepsin of a calf's stomach
digests casein very rapidly, but coagulated egg white only slowly,
whereas the pepsin of the pig's stomach acts on both these proteins at
about the same rate.
It is well known that the activity of pepsin can proceed only in the
presence of acids, but this action of acids does not appear to depend on
the hydrogen-ion concentration alone, for when equal quantities of the
same pepsin are mixed with quantities of different acids so that the
hydrogen-ion concentration of the mixtures is uniform, it is found that
digestion proceeds most rapidly with hydrochloric acid and least rapidly
with sulphuric acid. The S04 ion seems, therefore, to be unfavorable
486 DIGESTION
for peptic activities. The acid seems to combine with the protein before
the pepsin attacks the latter; for, if we first combine the protein with
acid and then wash away all traces of free acid, the protein can be
digested in a neutral pepsin solution without the liberation of any free
acid.
There is evidence to show that pepsin itself also becomes combined
with the protein during the digestive process. If a piece of protein such
as fibrin be immersed in a solution of pepsin, and then taken out and
washed thoroughly to get rid of all adherent pepsin, it will be found, on
placing it in a hydrochloric acid solution of the proper strength, that
peptic digestion proceeds. Advantage may be taken of this fact to
separate pepsin from a solution, but the best protein to use for this pur-
pose is not fibrin but elastin. By such a method it has, for example,
been shoAvn that there is some pepsin in the intestinal contents, proving
thus that when the chyme passes into the intestine, the pepsin is not, as
used to be thought, immediately killed by the proteolytic enzyme.
With regard to the products of gastric digestion, little can be said
here. The first product is a metaprotein known as acid albumin or
syntonin. It is precipitated from the digestion mixture by neutraliza-
tion. The next product is known as primary proteose, being precipi-
tated by half saturation with ammonium sulphate. The third product
is secondary proteose, produced by complete saturation with the above
reagent ; and after all these bodies have been separated out, there re-
mains in solution the fourth product — peptone — which among other
things is characterized by the fact that with the biuret test it gives not
a violet but a rose-pink color.
It has often been claimed that along with these products a certain
amount of free amino acids may also appear in a peptic digestive mix-
ture. This, however, may be due to the action of erepsin, which is
usually present in pepsin preparations. It is important to note that the
term proteose is a general one, and that there are probably many varieties
of this substance, differing from one another according to the protein
from which they are derived.
The change produced by pepsin and hydrochloric acid is of the nature
of an hydrolysis, for it has been found that the amount of hydrogen and
oxygen in the digestive products is greater than that in the original
protein. It is by a similar process of hydrolysis that the other proteolytic
enzymes, such as pancreatin and erepsin, operate, but this does not
imply that the exact grouping that is split apart by the hydrolytic proc-
THE BIOCHEMICAL PROCESSES OF DIGESTION 487
ess is the same for each of these enzymes. Indeed, there is considerable
evidence that pepsin does not, like the other enzymes, break up the long
chain of amino acids that are linked together to compose the polypep-
tides, but that it only splits the big molecule of albumin or globulin
into several large groups, each of which is composed of long amino-acid
chains. Its action appears to be analogous with that of amylase on
starch, by which, it will be remembered, the big polysaccharide mole-
cule is split into smaller polysaccharide molecules, which then become
attacked by the dextrinase and split into disaccharide molecules (see
page 656). The evidence in support of this view is: (1) that pepsin is
unable to digest polypeptides, and (2) that it is able to digest certain
proteins upon which erepsin (see page 490) has no action.
The hydrolytic splitting of large into smaller protein molecules, like
that by which the chains of amino acids in the polypeptides are subse-
quently broken up, consists in a breaking* of amino-carboxyl linkings
(NHCO) (see page 598), with. the consequent liberation of a large num-
ber of unattached amino groups. The number of these free amino groups
can be determined quantitatively by the formaldehyde titration method
of Sorensen.* By this method it can be shown that from the very start
of peptic digestion the number of free amino groups increases, and pari
passu the power of the digestive products to combine with free hydro-
chloric acid. Indeed, when the experiments are done quantitatively and
the digestion allowed to proceed for a considerable time, the increase in
the formol titration is practically equal to the decrease in the free acids
as determined by the Giinsberg reagent.
The rate of peptic digestion is usually estimated by the law of Schiitz
and Borissow, according to "which the amount of coagulated albumin
that is digested in a Mett's tube is proportional to the square root of the
amount of pepsin, f *
The pepsin which leaves the stomach in the chyme is not all destroyed
in the intestine, as was at one time believed to be the case, for, as we
have seen above, some pepsin can be detected in the gastrointestinal con-
tents. A part of the pepsin may be absorbed into the blood and carried
back to the gastric glands to be used again. This would account for the
presence of antipepsin in the blood, and also for the presence of pepsin
in the urine. It is probable, however, that most of the pepsin is de-
stroyed after it enters the intestine.
*In this method the basic character of the amino acids is destroyed by the formaldehyde,, so
that a higher degree of acidity develops in the mixture. By determining the increased acidity by
titration with alkali, an estimate is oibtained of the number of amino groups. (See page 599.)
tThe amount of coagulated egg albumin digested is ascertained by measuring the length digested
away from the end of a column of coagulated egg white contained in a glass tube (Mett's method).
(See Cobb, P. W. : Am. Jour. Physiol., l'905, xiii, 448.)
488 DIGESTION
Clotting of Milk in the Stomach
Besides its power of digesting protein, the gastric juice is also endowed
with the property of clotting milk. This action is commonly attributed
to the presence of another enzyme besides pepsin, namely, rennin; but
in recent years considerable controversy has raged around the question
as to whether pepsin and rennin are not the same thing. One strong
argument in favor of this view is that all digestive juices that are capable
of digesting protein can also clot milk. In any case, when gastric juice
acts on milk, it splits the casein* of the milk into two portions, one of
which, called paracasein, immediately combines with calcium to form an
insoluble colloidal compound, which is precipitated and, by entangling
the fat of the milk, forms the clot; the other protein remains in solution
and is known as whey albumose. From studies on molecular weight it
is believed that the paracasein is produced from casein by the splitting
of the molecule of the latter into two, from which it would appear that
the action of this enzyme is nothing more than the first stage in the
hydrolysis of the casein molecule. ' The whey albumose, according to this
view, is a by-product.
There are many investigators, however, who believe that rennin and
pepsin are not identical, since an infusion of the stomach of a calf has a
powerful clotting action on milk but a very weak digestive one on egg
white, whereas a similar infusion from the stomach of a pig shows exactly
the reverse properties. This question is one of so controversial a na-
ture that it would be out of place to go into it further here. It
should be pointed out, however, that, when the gastric contents are acid
in reaction, milk will become clotted by the action of the acid itself
quite independently of any pepsin or rennin the juice may contain.
This acid clotting of milk is probably of a different chemical nature
from that produced by the enzymes.
On other foodstuffs than proteins the action of the gastric juice is
relatively unimportant, although polysaccharides may be considerably
broken down in the cardiac end of the stomach on account of the action
of swallowed saliva (see page 454), and disaccharides, as we have seen,
may become split by the hydrolyzing effect of the hydrogen ion. Fat
digestion also takes place in the stomach when the fat is taken in an
emulsified condition, as in milk and egg yolk, but not when in masses,
as in meat or butter. This action is due to the presence of a fat-splitting
enzyme, or lipase, in the gastric juice.
,*In the above nomenclature casein is the same as caseinogen, and paracasein the same as casein,
of the English physiologists.
CHAPTER LVI
THE BIOCHEMICAL PROCESSES OF DIGESTION (Cont'd)
DIGESTION IN THE INTESTINES
The further changes which the half-digested foodstuffs in the chyme
undergo in the intestinal canal depend on the enzymes present in the
secretion of the various glands and on the presence of bacteria. The
most important of the digestive juices are the pancreatic juice and bile.
The latter, however, does not contain any enzyme, its influence on diges-
tion being entirely adjuvant.
Pancreatic Digestion
When we were considering the mechanism of secretion of the pan-
creatic juice, we saw that the juice produced by the action of secretin on
the gland cells does not contain any active proteolytic enzyme, although
it contains one capable of acting on polysaccharides and another, on fat.
THE ACTION OF TRTPSIN
When this juice is mixed with the secretion of the duodenum or of
the upper part of the small intestine, it immediately develops powerful
proteolytic power. The same result may also be obtained by mixing it
with an extract of the mucous membrane of the duodenum made with
dilute bicarbonate solution. A very small amount of the extract is
capable of increasing the digestive activity of a very considerable quan-
tity of pancreatic juice, showing that the action depends on the presence
of an enzyme which has been called enterokinase. This influence of the
intestinal secretion is readily destroyed by heating.
Large quantities of alkali are contained in the pancreatic juice and
bile, so that in the upper reaches of the intestine the acidity of the
chyme is practically neutralized. A little lower down, however, an acid
reaction may again develop (see page 505). On account of these facts it
has been concluded that the activity of trypsin is most rapid in the pres-
ence of a slight excess of hydroxyl ions; i. e., in a weakly alkaline solu-
tion. It is interesting to note that, as a result of the great secretion of
alkali by the pancreas, extracts of this organ after death show a very
high degree of acidity in comparison with extracts from other organs
489
490 DIGESTION
and tissues. It has also recently been shown that the activity of trypsin
does not depend on the presence of free hydroxyl ions, but that it may
proceed in the presence of free acid, even up to a strength of CH = 1.5.
If pepsin is present together with trypsin in a distinctly acid solution,
the pepsin seems to destroy the trypsin, unless the mixture contains a
considerable quantity of protein, when ,the tryptic activity may persist
even for several hours. A practical conclusion that we may draw from
these results is to the effect that preparations of trypsin — the so-called
pancreatin, for example — if given with the food, may pass in an active
condition into the duodenum, where, in the more favorable environment
created by the neutralization of the excess of acid, it will develop its
proteolytic power. The therapeutic administration of pancreatin is,
therefore, justified (Long16).
The activated trypsin acts on proteins in very much the same way as
pepsin, except that the decomposition of the peptone and proteoses into
polypeptides is the chief feature of the process. Thus, after tryptic
digestion has proceeded for some time, only a trace of primary proteoses
but considerable quantities of leucine, tyrosine and other amino acids
will be found present. Some investigators believe that the thorough
nature of the digestive action of activated pancreatic juice may depend
on its also containing erepsin, an enzyme which we shall see to be pres-
ent in considerable amount in the mucous membrane of the intestine and
other tissues, and whose particular function is to split polypeptides into
the amino acids. From the autolytic digestion which takes place in
organs kept in a sterile condition after death, tryptic digestion differs
in that it produces only small quantities of ammonia. The large quanti-
ties of ammonia produced in autolytic digestion no doubt have a rela-
tionship to the acids simultaneously set free during this process.
In the products of tryptic digestion it is usually found that, although
there has been considerable splitting of the protein into amino acids,
there are still a good many amino-carboxyl (NHCO) linkages left un-
broken, indicating that certain polypeptides are left intact in the mix-
ture. . To split the polypeptides requires the aid of the erepsin, which is
present in the mucous membrane of the intestine. Interesting inves-
tigations have been made on the exact degree to which trypsin-entero-
kinase can split up the various known polypeptides. This seems to
depend on the structure of the polypeptide molecule and on the number
of amino acids present in the chain. For example, analylglycine, but
not glycylalanine is hydrolyzed, although both contain the same amino
acids but linked together in a different way; and tetraglycylglycine,
which contains five glycine radicles, is hydrolyzed, whereas diglycylgly-
cine, which contains only three, is not.
THE BIOCHEMICAL PROCESSES OF DIGESTION 491
The importance of the presence of erepsin in the mucous membrane
of the intestine is that it serves as a barrier to the passage of any unsplit
amino acids from the intestinal contents into the blood: It insures the
breaking up of the protein molecule into its ultimate units before absorp-
tion. The further fate of the absorbed amino acids will be considered
under the subject of protein metabolism.
THE ACTION OF LIPASE
Neutral fat is decomposed into fatty acids and glycerine by the lipase
present in the pancreatic juice. This enzyme may also be extracted from
the glands by means of 60 per cent alcohol. Its action is remarkably
accelerated by the presence of bile, and considerably depressed by inor-
ganic salts. It is also very dependent on the degree of alkalinity, the
optimum being a hydrogen-ion concentration of H x 10~8. The favoring
action of bile is undoubtedly owing to the bile salts (see page 493), and
it is probable that this action is dependent upon the influence which
these have in lowering surface tension and therefore bringing about a
more intimate contact between fat and water.
THE ACTION OF AMYLOPSIN
The action of pancreatic juice on carbohydrates depends on the
amylolytic enzyme called amylopsin. In animals having no active ptyalin
in the saliva, amylopsin serves as the only diastatic enzyme concerned
in the digestive process. In any case, at least for the first stages of the
disruption of the starch molecule — that is, its conversion into dextrines —
amylopsin is a more powerful enzyme than ptyalin. It does not appear
to be so efficient as ptyalin in the final stages of the hydrolysis, for it
does not produce so much reducing sugar as ptyalin does. Indeed ex-
tracts of pancreas will sometimes convert starch into soluble starch and
dextrine with great speed, but produce scarcely any reducing sugar.
On this account it is believed by many investigators that there are at least
two distinct and separate enzymes in amylopsin and also perhaps in
ptyalin, one a true amylase, which converts starch into dextrine, and
the other a dextrinase, which converts dextrine into maltose. In the
case of both ptyalin and amylopsin digestion proceeds best in a very
weak acid reaction. Amylopsin, as it is secreted in the pancreatic juice,
is fully activated; bile, apart from the alkali which it contains, having
no influence on its digestive power.
Besides amylopsin the pancreatic juice also contains maltase, and in
the case of young animals or of those that take milk with their food
throughout their lives, lactase also. After the suckling animal has dis-
492 DIGESTION
continued taking milk, the lactase disappears from the pancreatic juice.
Attempts have been made to bring it back by feeding the adult upon
milk, but without success. Occasionally the pancreatic juice also con-
tains invertase.
The Bile
Associated with the pancreatic juice in all its functions is the bile.
When this fluid is prevented from entering the intestine, the digestive
process becomes very imperfect, the absorption of fat being particularly
interfered with (see page 691). Bile is also an excretory product, and
its composition therefore is much more complex than that of the other
digestive fluids. This varies very much, however, according to the
method of collection. Bile from the gall bladder after death contains
much more solid material, particularly bile salts and mucin, than that
collected from a fistula of the bile duct or gall bladder during life.
These differences will be evident from the accompanying table.
Bile from
Gall bladder Fistula
100 parts contain — •
Water 86 97
Solids 14 3
Organic salts (bile salts) 9 0.9-1-8
Mucin and bile pigment 3 0.5
Cholesterol ' 0.2 0.06-0.16
Lecithin and fat 0.5-1.0 0.02-0.09
Inorganic salts 0.8 0.7-0.8
In general it may be said that bile obtained from a fistula in man
contains only about 3 per cent of total solids, of which from one-fourth
to one-half are inorganic, whereas bile from the gall bladder contains
10 to 20 per cent of total solids, of which only about one-twentieth are
inorganic. The chief cause for this difference appears to be that when
the bile goes to the intestine, a considerable proportion of its bile salts
is reabsorbed into the portal blood and reexcreted by the liver. Some
of the difference may also be caused by the fact that absorption of
water takes place from the gall bladder, and that mucin and possibly
cholesterol are secreted by this organ. These striking differences be-
tween fistula and gall-bladder bile are observed only when the com-
mon bile duct is occluded. If the bladder fistula is made with the com-
mon duct left open, some of the bile gains entry to the duodenum and
therefore becomes reexcreted. It is well known that a fistula of the gall
bladder in man after a time closes up and the bile again takes its usual
course along the bile duct into the duodenum.
THE BIOCHEMICAL PROCESSES OF DIGESTION 493
Interesting observations have been collected on the amount of the secre-
tion from a fistula both in man and in the lower animals. In man it is
commonly stated that about 500 c.c. of bile are secreted daily, the
amount varying considerably during the different hours of the day. The
secretion of bile is greatly reduced by hemorrhage. It is greater on a
meat diet than on one of carbohydrates. It is reduced during starva-
tion, but continues to be secreted up to the moment of death.
FUNCTIONS OF BILE
One of the main functions of the bile salts is that they greatly assist,
not only in the digestion, but also in the absorption of fats. When bile
is excluded from the intestine, the feces are loaded with fatty acids
which have been split off partly by the now less effective lipase and
partly by the action of bacteria. The fatty acid thus liberated in the
absence of bile salts is not absorbed, because the bile salts serve as the
carriers of fatty acids into the epithelial cells and lacteals. They com-
bine with the fatty acids, probably by forming some chemical compounds,
in which they carry them into the endothelial cells where the compounds
become disrupted, the fatty acid combining with glycerine to again form
neutral fat and the bile salts being carried to the liver and reexcreted.
The influence of bile salts in assisting the action of lipase is probably
due to a lowering of the surface tension, thus bringing water and fat
into closer union. This accelerating influence has also been demonstrated
when synthetic bile salts have been used, showing clearly that it is really
these and not any other constituent of the bile that are responsible for
its accelerating influence.
Bile also functionates as a regulator of intestinal putrefaction. This
it does apparently because of its slight laxative properties, by which
the intestinal contents are expelled before the bacteria have grown to
any great extent in them. Bile itself is a favorable culture medium for
certain bacteria, so that it can have no antiseptic action. Its assistance
in the action of trypsin and amylopsin depends very largely upon the
alkali which it contains.
As an excretory vehicle bile is important, because it possesses the
power of dissolving cholesterol. Toxins and metallic poisons of various
kinds are also excreted in it.
Although not directly concerned with the digestive function, it will be
convenient to say something here concerning the chemical nature and
derivation of the various biliary constituents.
494 DIGESTION
THE CHEMISTRY OF BILE
The Bile Salts
In most animals the bile salts consist of the sodium salts of glycocholic
and taurocholic acids. Each of these acids is composed of a part called
cholic acid which is more or less related to cholesterol, and of glycine
(CH2NH2COOH ammo-acetic acid) or taurine (C2H7NS03), a derivative
of cysteine, which is a-amino-/?-thiopropionic acid (CH2HS.CHNH2.
COOH). The exact form of cholic acid varies in different animals, that
of the pig, for example, being different from that of man. Bile salts are
an exclusive product of liver metabolism ; i. e., they are not formed in
any other part of the animal body. They give a very sensitive color
reaction known as Pettenkof er 's, which however is not specific of bile acids,
since it is also given by oleic acid and by many aromatic substances and
alcohols. It must be remembered that the part of the bile salts that is
characteristic of the liver is the cholic acid, the taurine and glycine
being present in other tissues and organs.
When cholic acid is given to animals mixed with the food, the amount
of taurocholic acid excreted with the bile is increased, indicating that
there must be a store of taurine available in the organism. This store
can not, however, be large, for if the feeding with cholic acid is repeated
several times, it will be found that the taurocholic acid diminishes and
glycocholic acid takes its place; and this increased excretion of glyco-
cholic acid goes on just as long as cholic acid is fed. The reserve of
taurine in the animal body appears therefore to be limited, although it is
used in preference to glycine when there is an excess of cholic acid to be
neutralized. On the other hand, the store of glycine seems to be inexhaust-
ible. That there is no reserve of cholic acid itself in the body is indicated by
the fact that no increase in taurocholic acid excretion by the bile results
when cystine, the mother substance of taurine, is given with the food.
If both taurine and cholic acid be fed, however, the excretion of tauro-
cholic acid increases.
The relative amounts of taurocholic and glycocholic acids in the bile of
different animals differ considerably. Human bile contains relatively
a small amount of taurocholic acid; on the other hand, the bile of the dog
contains a large excess of it.
Cholesterol
In human bile the percentage of this important substance is not high
(1.6 parts per 1000), but it is of great clinical importance because of the
fact that it may separate out as a precipitate forming gallstones. The
THE BIOCHEMICAL PROCESSES OF DIGESTION 495
percentage of cholesterol in these varies from 20 to 90 ; the remainder
being organic material such as epithelial cells, inorganic salts, pigment,
etc. The origin of cholesterol is partly endogenous and partly exoge-
nous. In the former case it comes from the envelope of red blood cor-
puscles and from the nervous tissues, where it is present In considerable
amount. The latter source is, of course, the food. The increase in;
cholesterol esters in the blood after feeding with food rich in this sub-
stance has been shown, particularly in rabbits.
That the bile should be the pathway through which cholesterol is
excreted depends no doubt on the fact that it contains bile salts, which
along with their other properties have a remarkable solvent action on
cholesterol. This solvent property depends on the cholic acid part of
the bile salts, which, as already remarked, is chemically very closely
related to cholesterol; indeed, the relationship is so close that some have
suggested that cholic acid is derived from cholesterol. This would mean
that the cholesterol of blood is excreted in two ways, as cholesterol and
as cholic acid. Other observers, however maintain that the cholesterol
is excreted mainly by the lining membrane of the gall bladder, and
that this explains why gall-bladder bile contains more of it than fis-
tula bile. This evidence is, however, not very strong, for the greater
excretion of cholesterol under conditions where the circulation of bile
is going on may be explained as due to the presence of bile salts, which
serve to carry the cholesterol out of the blood.
Many problems remain to be elucidated in connection with the metabolic
history of cholesterol. That some of it is absorbed when cholesterol is
contained in the food might seem to indicate that its source is entirely
exogenous, Against this view, however, stand two facts: (1) that the
cholesterol in the feces of herbivorous animals is of the same variety as
that present in those of carnivorous animals and not the phytosterol
which is present in plants; and (2) that the universal presence of
cholesterol in cells indicates that it must be manufactured there.
The Bile Pigments
The pigments of bile are bilirubin and biliverdin. The latter is pro-
duced from the former by oxidation. If the oxidation be carried a
stage further, a blue pigment called bilicyanin is formed. This process
of oxidation can be observed in the ring test for bile pigment with
fuming nitric acid. When bilirubin is reduced, urobilin, one of the
pigments in urine, is formed. Bilirubin must therefore be considered
as the mother substance of all these pigments, and it is of interest in
connection with its derivation to know that it has the same formula
496 DIGESTION
as iron-free hematin or hematoporphyrin, which is produced by treating
hemoglobin with concentrated sulphuric acid.
Chemical investigation has shown that bilirubin is built up from sub-
stituted pyrrols, probably four such being contained in the molecule.
The pyrrol group is also present in indole and tryptophane, and con-
sists of four carbon atoms and an NH group linked together as a ring
(see page 604). Similar pyrrol derivatives can be produced by decom-
posing chlorophyl, the green coloring matter of plants. It is important
to remember that bilirubin is acid in nature, and, therefore, can com-
bine with alkalies to form salts. The relative amounts of bilirubin and
biliverdin vary in the bile of different animals.
When these pigments enter the intestine they are reduced to urobilin,
part of which passes out with the feces, another part being absorbed into
the blood and excreted in the urine. Part of that excreted in the urine
exists, however, as a so-called chromogen named urobilinogen. The
urobilinogen is converted into urobilin by the action of oxygen.
The method by which urobilin is produced from blood pigment has
been studied by histologic examination of the liver particularly of birds
and amphibia, in which destruction of blood pigment goes on rapidly.
Increased destruction of blood pigment can be induced by poisoning
with certain substances such as arseniureted hydrogen. From such
studies it is usually believed that the bile pigments are a peculiar product
of hepatic activity, being produced from blood pigments that are de-
rived from erythrocytes which have been broken down either in the liver
itself or in some other viscus (e. g., the spleen). Whipple and Hooper20
have brought forward seemingly incontrovertible evidence against such
a view. They have found, for example, that the bile pigments are
formed just as readily in animals in which the circulation of the liver
was greatly curtailed by anastomosing the portal vein with the vena
cava (Eck fistula) as in normal animals. Even when the circulation
was limited to the anterior end of the animal (head and thorax) bile
pigment appeared in the blood when hemolyzed erythrocytes were in-
jected, and it was also formed when hemoglobin was placed in the pleural
and peritoneal cavities. The endothelial cells of the blood vessels and
elsewhere can evidently form the pigments, at least when the liver is
absent. When such a process occurs under normal conditions, it is quite
probable that the liver acts merely as an excretory organ for the pig-
ments in the same way as the kidney does for urea. Possessed of endo-
thelial cells, the liver might itself also produce some of the pigments,
but no more than other organs with a similar number of those cells.
Even the derivation of bile pigments from hemoglobin is called in
question, for the same workers have observed that, whereas the excre-
THE BIOCHEMICAL PROCESSES OF DIGESTION 497
tion of pigment from a biliary fistula is remarkably constant in a dog
fed on a fixed mixed diet, it became increased, sometimes by 100 per
cent, when the diet was changed to one of carbohydrates, and depressed
on a diet of meat. The question arises as to whether, after all, the bile
pigments are really derived from broken-down hemoglobin. May they
not be manufactured de novo out of other materials?
Whipple and Hooper have also shown that bile is a most important
secretion, for dogs rarely survive on an ordinary diet if bile is perma-
nently prevented from entering the intestine. Intestinal symptoms
soon supervene, and become progressively more severe until the death
of the animal. Feeding with bile does not relieve the condition, but
feeding with cooked liver seems to have a beneficial effect.
After extravasation of blood in the subcutaneous tissues, as in a bruise,
for example, a decomposition of hemoglobin proceeds quite like that
occurring in the liver, and leads to the production of blue and brown
and green pigments like those of the bile. When hemolysis is produced,
as by inhalation of arseniureted hydrogen or the injection of inorganic
or biological hemolysins, there is an immediate increase in the amount
of bile pigment in the bile. Even the injection of hemoglobin solutions
has this effect. Under these conditions of hemolysis, besides an increase
in urobilin, there may be considerable quantities of hemoglobin secreted
in the urine.
Bile salts and pigments usually accompany each other when any-
thing occurs to interfere with the free secretion of bile. For example,
after ligation of the bile duct both bile pigments and bile salts accumu-
late in the blood, in the serum of which they may be recognized by the
ordinary chemical tests in from four to six hours after the operation.
If the accumulation be allowed to proceed further, the bile pigments
become deposited in the tissues, giving them the peculiar yellowish ap-
pearance known as jaundice. Under these conditions the bile salts and
pigments also appear in the urine. The accumulation of bile salts in
the body affects certain physiologic processes; for one thing, it causes
a great lengthening in the clotting time of the blood.
If the blood supply to the liver is interrupted by ligation of the portal
vein and hepatic artery at the same time that the bile ducts are occluded,
not a trace either of bile salts or of bile pigment appears in the blood
during the six to eighteen hours that the animals survive the operation.
The amount of obstruction of the bile duct necessary to produce these
symptoms is very slight, since bile is secreted at a very low pressure.
Even a clot of mucus or a swollen condition of the mucous membrane
of the duct is sufficient to produce obstruction. In the discharge of bile
from the gall bladder into the duodenum it is claimed by Meltzer21 that a
498 DIGESTION
reciprocal relationship exists between the contraction of the bladder
musculature and the relaxation of the muscular fibers surrounding the
duct in the duodenum. If this reciprocal innervation fails to operate
properly, discharge of bile into the duodenum may become obstructed
so that a certain amount passes back into the blood, as in cases of bile-
duct obstruction.
Bile also contains a certain amount of lecithin and other phospholipins.
The amount varies considerably in the bile of different animals, even in
animals of the same species.- It is probably derived, as already men-
tioned, like the cholesterol, from the breaking-down of red blood cor-
puscles that goes on in the liver. It is no doubt digested by the ferments
of the intestinal tract, the liberated cholin, since it is toxic if absorbed,
being further attacked by bacteria so as to become converted into cer-
tain substances of a nontoxic nature.
CHAPTER LVII
BACTERIAL DIGESTION IN THE INTESTINE
On an average diet, in twenty-four hours the feces of man weigh
about 100 grams, or after drying, about 20 grams. About one-fourth of
the dry matter consists of the bodies of bacteria. If plated out by the
ordinary bacteriologic methods, however, it will be found that only a
small proportion of these bacteria are living. The greater number have
been destroyed, probably by the action of the mucin in the large intes-
tine. The nitrogen content of the feces amounts to about 1.5 grams a
day, of which about one-half is bacterial nitrogen. If the diet contains
large quantities' of cellulose material, as in green vegetable food and
fruit, the mass of feces as well as the bacterial content may be consid-
erably greater.'
The foregoing facts indicate that very extensive bacteriologic proc-
esses must be going on all the time in the intestinal contents, and the
question arises as to whether such action is beneficial or otherwise to the
animal economy. To answer this question interesting observations have
been made on the growth and well-being of animals excised from the
uterus under strictly sterile conditions and maintained thereafter on
sterile food. Such observations made on guinea pigs have shown that
the animals thrive and grow perfectly for a considerable time. Experi-
ments carried out on chicks have not, however, yielded similar results.
Chicks hatched out from the egg under strictly sterile conditions and
then fed on sterile grain, do not thrive, but do so if with the grain is
mixed a certain amount of fowl excrement. These experiments, appar-
ently contradictory in their results, show that for certain groups of
animals bacteria are required, but not for others.
The difference is probably dependent on the nature of the foods. It
will be remembered that the size of the large intestine varies consider-
ably according to the nature of the diet (see page 463). Animals taking
great quantities of cellulose foodstuffs have very large ceca and very
long large intestines; whereas those which, like the cat, live practically
entirely on cellulose-free food, have a rudimentary large intestine. The
size of the lower intestine is obviously dependent on the presence or
absence of cellulose in the food. It will be remembered also that the
forward movement of the contents of the large intestine is very slow;
indeed, special provision is made, by the presence of the so-called anti-
499
500 DIGESTION
peristaltic wave, to delay its movement. This suggests that an important
digestive process must be proceeding in this part of the gut. In these
ways conditions become established in the cecum for the active opera-
tion of bacteria. They attack the cellulose, and liberate the more diges-
tible foodstuffs contained in the vegetable cells, also producing out of
the cellulose itself materials of nutritive value. The acids that are also
produced by this process are neutralized by the carbonates secreted
by the mucosa.
In certain herbivorous animals — the ruminants — this process in the
cecum is not relatively of such importance, because it takes place in the
paunch. The animals swallow the food and it mixes in this part of the
stomach with the saliva, so that bacteria and ferments contained in it,
called cytases, attack the cellulose, liberating the more easily digested
foodstuffs inclosed within the cell walls. As this process goes on acids
accumulate in the digestive mixture. The food is then returned to the
mouth, chewed over again, and swallowed again into the main stomach,
where it is digested. The aid which bacteria render to digestion depends
therefore on the nature of the diet. Man, being omnivorous, stands mid-
way between the two groups of animals discussed above. Although the
cellulose contained in his food is not itself sufficiently digested to furnish
nutriment, yet it is so far acted upon as to permit the rupture of the
cell, the contents of which are then digested. The cellulose is, however,
of value in furnishing bulk to the intestinal contents — ''intestinal bal-
last," it is sometimes called.
In the small intestine in man there are bacteria capable of acting on
carbohydrates and producing from them organic acids, such as lactic,
acetic, etc. So long as a sufficiency of carbohydrate exists to encourage
the action of these bacteria, others having an action on protein do not
seem to thrive. It may be that this is to be accounted for partly by the
production of acid substances by the carbohydrate fermentation, and
partly by the fact that, as soon as the protein molecule is broken
down by the digestive enzymes, its building-stone ammo acids are ab-
sorbed. There are probably also bacteria in the small intestine capable
of splitting fat into fatty acid and glycerine, but practically nothing is
known of their action. In the large intestine of man, along with the
cellulose-digesting bacteria already mentioned, protein-digesting bac-
teria are also present. These bacteria belong to the class, Bacillus coli
communis, the various members of which are known as faculative anae-
robes because they can grow in the presence or absence of oxygen.
If bacterial growth is excessive or there is an insufficiency of carbohy-
drates in the small intestine, the bacteria attack the amino acids pro-
duced by the digestive enzymes and decompose them into products
that may be toxic if absorbed into the blood.
BACTERIAL DIGESTION IN THE INTESTINE
501
Bacterial Digestion of Protein
From a pathologic standpoint, the most important action of bacteria
is that which takes place on protein. Under anaerobic conditions the
intestinal bacteria have in general the power of splitting off the amino
group whereas under aerobic conditions they split off the carboxyl
group. This splitting off of the carboxyl group as carbon dioxide is per-
formed by the so-called carboxylase bacteria, and it may take place either
before or after deamidization (see page 615). If it happens after this
process, the products are not highly toxic and include phenol, cresol,
indole and skatole, which are partly absorbed into the blood and partly
excreted with the feces.
The fractions of those substances that are absorbed into the blood
have their toxicity removed by conjugation mainly with sulphuric acid
to form the so-called ethereal sulphates. A part is also combined with
glycuronic acid (see page 632). In the case of phenol and cresol this
conjugation occurs immediately after absorption, but in the case of
indole and skatole it is preceded by an oxidative process, converting
these substances into indoxyl and skatoxyl respectively. The detoxica-
tion process occurs in the liver, as has been shown by experiments in
which this organ was artificially perfused outside the body. They are
then removed from the blood by the kidneys and excreted in the urine.
The proportion of ethereal sulphates in this fluid is therefore an indica-
tion of the extent of intestinal putrefaction of protein (see page 632).
The indican, being readily detectable by the well-known color reaction
of Jaffe, serves as an indicator of the extent of intestinal putrefaction.
The indole and skatole which are not thus absorbed and detoxicated are
excreted with the feces, to which they give the characteristic odor.
The source of the phenol is tyrosine and that of the indole is trypto-
phane. The chemical processes involved are shown in the following
equations, in which the by-products of the reactions are in brackets.
C.OH
/\
HC CH
COH
/\
HC CH
HC
CH
HC
CH
CH2
CHNH2
COOH
(tyrosine)
'(NH3)
CH,
CH.,
HC CH
C
CH,
(CO2+H20)
COOH
(p-oxyphenyl-
propionic acid)
COOH
(p-oxyphenyl-
acetic acid)
(CO,)
COH COH
HC CH HC CH
HC
M
CH HC CH
>
/ ^ V
1
1E3(C02 + H20)
(paracresol)
(phenol)
502 DIGESTION
Putrefaction of tryptophane is probably preceded by deamidization :
CH CH
HC C C— CH.,.CHNH2.COOH HC ' C C— CH.,CHrCOOH
I il II — > I II I! — >
HC C CH (NH,) HC C CH (CO2 + H2O)
v/\/ \/\/
CH NH CH NH
(tryptophane) (indole-propionic acid)
CH CH CH
HC C C— CH...COOH HC C CH HC C C— CH,
HC C CH (CO, + 11,0) HC C CH HC C C
\X\X \X\X \/\/
CH NH CH NH (+CH3) CH NH
(indole-acetic acid) (indole) (skatole)
If, however, the carboxylase bacteria remove the carboxyl group be-
fore the amino group has been removed, highly toxic substances called
amines are produced. They are the so-called ptomaines. From alanine,
ethylamine is formed; from tyrosine, phenolethylamine; from histidine,
which it will be remembered is an important protein building-stone,
imidazylethylamine, and so on. The process of formation is illustrated
in the accompanying formulae:
1. CH3.CH(NH,).COOH — CO, -+- CH3.CH..(NH,)
Alanine Ethylamine
2. C6H,(OH).CH:,CH(NH2).COOH = C6.! + C6H,(OH).CR!.CH2.NH2
Tyrosine Phenylethylamine
3. C3N2H3.CHJ.CH(NH2).COOH=:CO.! + C3H3N2. CH2'.CH2.NH2
Histidine. Imidazylethylamine.
Similar substances are very common in the metabolic products of
plants; for example, they constitute the active principle of ergot. They
are also no doubt produced in the tissues of mammals, imidazylethyla-
mine, commonly called histamine, being thus produced, as well as the
closely related epinephrine, which is the active principle of the supra-
renal gland (see page 737), and may be described as a methylated ethyla-
mine derivative of tyrosine.
Phenylacetic acid produced by a similar process from tyrosine may
be excreted in the urine, where it forms the mother substance of homo-
gentisic acid, to which the dark brown color of the urine in alkaptonuria
is due.
The great importance attached to these decomposition products of
proteins depends on the fact that they have powerful pharmacologic
actions. These actions are developed very largely upon the vascular
system; histamine, for example, produces marked vasodilatation and
lowers the coagulability of the blood, whereas other substances of the
BACTERIAL DIGESTION IN THE INTESTINE 503
same class, like epinephrine, have the property of raising the blood pres-
sure. In larger doses, serious nervous symptoms and a condition of pro-
found collapse are produced. These observations have led several inves-
tigators to believe that the persistent occurrence of bacterial fermen-
tation and the absorption of the resulting decomposition products of
protein into the blood ultimately cause arteriosclerosis and the other symp-
toms that accompany senescence. It is difficult at the present time to
know how much of this one ought to believe, although it can not be
doubted that putrefaction has an unfavorable action on the arteries,
and that an excessive degree of it causes the symptoms of ptomaine
poisoning.
If the ptomaines have formed in the food before it is eaten, the symp-
toms develop in from one to five hours after the meal, but if the decomposi-
tion occurs in the intestine on account of bacteria that are taken at the same
time as the food, the ptomaines may not have developed sufficiently to
cause symptoms until from twelve to forty-eight hours; sometimes, how-
ever, they develop in an hour or so. Prominent among the symptoms is
usually diarrhea, which develops for the purpose of getting rid of the
offending bacteria and ptomaines.
Actual infection of food with bacteria of the paratyphoid-enteritidis
type is much more common than poisoning by substances (ptomaines} that
have been generated in food before it is taken (Jordan17). Meat, milk
and other protein foods are usually the carriers of the bacilli, and in most
of the accurately recorded cases the meat or milk was found to be
derived from animals suffering from enteritis or some other infection.
Sometimes, however, perfectly good food may become infected by
handling. Although the symptoms are usually acute, they may closely
simulate those of typhoid fever, and the effects of the attack may linger
for weeks or months.
BOTULISM
The commonest type of poisoning by substances actually present in the
food is that known as botulism. In this the gastrointestinal symptoms
are not pronounced, — indeed, paralysis of the intestinal tract with con-
stipation is the rule, — but those affecting the nervous system, dizziness,
diplopia and other visual disturbances, with difficulty in swallowing,
are very prominent. The temperature and pulse are usually normal.
In practically all of the reported cases of botulism, the source of infection
has been food which after having been subjected to some preliminary treat-
ment, such as smoking, pickling, or canning, had been allowed to stand
for some time and then eaten without cooking. The Bacillus botulinus,
which is responsible for the production of the poisons or toxins, is a
504 DIGESTION
strict anaerobe and is readily destroyed by cooking, as are also the
poisons. Antitoxins are formed by sublethal injections. Another but
now very rare example of poisoning by products formed in food is
that caused by "ergotoxin. "
The treatment in such cases is to encourage diarrhea by giving pur-
gatives. If the intoxication is of a more chronic character, the symptoms
are vague, consisting of drowsiness, lassitude, headache, and general de-
pression. The treatment here also is to clear out the intestines by a
good purge. There can be little doubt that many of the unhealthy condi-
tions of the skin leading to the formation of pimples, acnes, and boils,
are also caused by chronic intoxication with protein decomposition prod-
ucts. Again, purgation is the proper treatment.
It is unnecessary in a work of this character to go further into these
highly important questions. It is probable, however, that the importance
of the relationship of excessive protein putrefaction in the intestine to
many of the so-called minor diseases can not be overemphasized. On the
other hand, we must be careful not to attribute every sort of chronic
condition to this putrefaction. Toxemia is often a shibboleth of the
profession. "When a chronic disease can not be diagnosed, it is put down
as a toxemia. This, however, is not medical science — it is medical shirk-
ing. It is certainly unsafe at the present time to conclude that the
ordinary symptoms of senescence, such as hard arteries or increased blood
pressure, are invariably to be attributed to this cause. It will be re-
membered that Metchnikoff is largely responsible for such a view, and
also that he suggested, as the surest way to ward off the chance of such
intoxication, the taking of buttermilk, which would supply bacteria
through whose growth in the intestine the protein-destroying bacteria
would not be able to thrive. It is probable that the same result could be
attained in patients showing undoubted signs of suffering from intestinal
putrefaction by a change in diet in the direction of giving more carbo-
hydrate, for, as we have seen, if there is a plentiful supply of this food-
stuff in the small intestine, the bacteria do not tend to attack the protein.
Before leaving this subject it is interesting to consider for a moment
the cause of the severe symptoms that follow intestinal obstruction.
This question has recently been diligently investigated by Whipple,18
who found that the nonprotein nitrogen of blood (page 606) becomes greatly
increased in intestinal obstruction. The cause for this increase in non-
protein nitrogen is found to be an excessive breakdown of tissue protein
caused by the absorption into the blood of a proteose. When this pro-
teose isolated from obstructed loops of intestine was injected into fast-
ing dogs, profound symptoms of depression were produced, followed, in
cases in which the dose was sublethal, by recovery in from twenty-four
BACTERIAL DIGESTION IN THE INTESTINE 505
to forty-eight hours. Along with these symptoms the nitrogen elimina-
tion by the urine increased by 100 per cent. A very interesting fact is
that animals can be rendered immune to this proteose by progressively
increasing periodic administration. When they are thus immunized,
the toxic symptoms do not follow upon its injection, nor are the symp-
toms produced by artificially creating an intestinal obstruction. Con-
versely, when a chronic toxic condition is -kept up by a partial obstruc-
tion, such as that produced by making a gastrojejunal fistula and occlud-
ing the duodenum, the animals are less susceptible than normal ones to
proteose injection.
We have here and there incidentally referred to the reaction of various
parts of the gastrointestinal contents, but we would call attention once
again to this important subject, especially since many points of uncer-
tainty have recently been cleared up by the accurate observations of
Long and Fenger,19 who used the electrometric method for measuring
the hydrogen-ion concentration. The contents of the duodenum removed
by means of the Kehfuss tube in man showed a reaction varying from dis-
tinctly acid to slightly acid, depending upon the proximity of the tube
to the pylorus or papilla, this position being determined by x-ray exam-
ination. The slight degree of alkalinity is surprising. Lower down in
the duodenum the reaction was as frequently acid 'as alkaline, the de-
gree of acidity, however, being so slight as to favor rather than retard
the digestive powers of the pancreatic juice.
To determine the reaction lower down, the observations were made on
recently slaughtered animals (pigs, calves, and lambs), the small intes-
tine being tied off in loops of the upper, middle, and lower thirds. The
contents of the last loop were often alkaline, but might be more acid even
than those of the first, which were usually faintly of this reaction. Con-
siderable variations were, however, the rule. The mixed intestinal con-
tents of a recently fed dog, removed immediately after death, gave
PH = 6.79 ; i. e., very faintly acid.
DIGESTION REFERENCES
(Monographs)
iPavlov, J. P. : The Working of the Digestive Glands. Trans, by Sir W. H. Thomp-
son, London, Griffin, ed. 2, 1910.
sStarling, E. H. : Eecent Advances in the Physiology of Digestion, W. T. Keene &
Co., Chicago, 1907.
3Cannon, W. B.: The Mechanical Factors of Digestion, Internat. Med. Monographs,
London, Ed. Arnold, 1911.
^Carlson, A. J.: The Control of Hunger in Health and Disease, Univ. of Chicago
Press, 1917.
sTodd, T. Wingate: The Clinical Anatomy of the Gastrointestinal Tract, Manches-
ter, Univ. Press, 1915.
506 DIGESTION
(Original Papers)
^Cannon, W. B., and Cattell, McKeen: Am. Jour. Physiol., 1916, xli, 39.
caQesell, E.: Proc. Am. Physiol. Soc., Am. Jour. Physiol., 1918, xlv, 559.
7Dale, H. H., and P. P. Laidlaw: Proc. Phys. Soc., Jour. Physiol., 1912, xliv, pp.
12, 13.
7aBabkin, Bi P., Rubaschkin, W. J., and Ssawitsch, W. W.: Arch. f. mikr. Anatomic,
1909, Ixxiv, 68.
sMacallum, A. B.: Ergeb. der Physiol., xi, 598-657.
oMiller, F. E.: Quart. Jour. Exper. Physiol., 1913, vi, 57.
icEdkins, J. S.: Jour. Physiol., 1906, xxxiv, 133-144.
loaKeeton, E. W., and Koch, F. C.: Am. Jour. Physiol., 1915, xxxvii, 481; also
Popielski, L.: Arch. f. d. ges. Physiol., 1901, Ixxxvi, 215.
nMeltzer, S. J.: Am. Jour. Physiol., 1899, ii, 266.
i2Cannon, W. B.: Am. Jour. Physiol., 1898, i, 359.
isCannon, W. B., and Blake, J. B.: Am. Surg., 1905, xli, 686. Cf. No. 3.
i^Macallum, A. B.: See Fitzgerald, M. P., Proc. Roy. Soc., Ixxxiii, B, 56.
isHarvey, B. C. H., and Bensley, R. E.: Biol. Bull., Wood's Hole, 1912, xxiii, 225.
i^Long, J. H., et al.: Jour. Am. Chem. Soc., 1917, xxxix, 162 and 1493; also ibid.,
1916, xxxviii, 38.
17 Jordan, E. V.: Food Poisoning, Univ. of Chicago Press, 1917.
isWhipple, G. H., Cooke, J. V., and Stearns, T.: Jour. Exper. Med., 1917, xxv, 479.
Also Whipple, G. H., Stone and Bernheim: Ibid., 1913, xvii, 286 and 307.
isLong, J. H., and Fenger, F.: Jour. Am. Chem. Soc., 1917, xxxix, 1278.
soWhipple, C. H., and Hooper, C. W. : Am. Jour. Physiol., 1916, xl, 332 and 349 ; ibid.,
1917, xlii, 257 and 264; Hoope: Ibid., p. 280.
ziMeltzer, S. J.: Am. Jour. Med. Sc., 1917, cliii, 469.
CHAPTER LVIII
THE EXCRETION OF URINE
BY R. G. PEARCE, B.A., M.D.
It will be advisable to introduce the subject by a brief review of the
essential structural features of the kidney, in so far as they apply to
the excretory function of the organ.
STRUCTURE OF THE KIDNEY
The kidney is mainly derived from the surface of the celom, and is a
mesodermal structure. In this respect it differs from ordinary secreting
glands, which are endodermal in origin. Just as it is more or less
unique in its development as a gland, it is also unique in its method
of functioning. The physiologic theories of the mechanism of urinary
secretion are closely related to the highly characteristic structure of the
kidney. For this reason a brief survey of the structure of the different
parts of the uriniferous tubules and the epithelial cells with which these
are lined, is advisable.
The uriniferous tubule, which is the secreting unit of the kidney,
takes its origin in the capsule of Bowman, which may be likened to a
hollow sphere of very delicate epithelium, one side of which is
invaginated by a very much convoluted capillary mass, the glomerulus.
The capsule opens up by a narrow twisted neck into a tubule, which is
rather tortuous in the cortex (the proximal convoluted tubule), but soon
takes a sharp descending course in the medulla towards the pelvis of the
kidney, and doubles back (loop of Henle) in a straight course again to
the cortex, where it again makes a twisted course (the distal convoluted
tubule), and terminates in a collecting tubule, which, uniting with other
tubules, collects the urine and conducts it to the pelvis of the kidney.
The capsule is lined with very thin epithelial cells, especially over the
capillaries comprising the glomerulus. The proximal and distal tubules
507
508
THE EXCRETION OF URINE
contain epithelium showing a prominent striation. These striations are
rows of granules, which run towards the lumen of the cell, becoming
less distinct as they approach it and apparently standing in close rela-
tionship to the rather prominent internal (lumen) striated border of
the cell. Some histologists believe that the striations at the border are
Fig. 170. — Diagram of the uriniferous tubules (C) the arteries (A), and the veins (B) of the
kidney.
really cilia, which are described as being immobile. The cilia are shown
in Fig. 171. The descending limb of Henle's loop is lined with a thin
pavement epithelium with large bulging nuclei. The distal convoluted
tubule is lined with cells not unlike those found in the proximal tubules,
except that the inner border is not striated. The diameter of the lumen
THE EXCRETION OF URINE
509
of the capsule varies with the activity of the kidney, as is shown in
the following figures given by Brodie and Mackenzie.1
RESTING
KIDNEY
MM.
KIDNEY DURING
DIURESIS
MM.
Mean 'diameter of capsule
" " " glomerulus
' ' " space of capsule
Lumen of proximal convoluted tubule
" " distal « "
93.4
90.4
3.0
0.0
7.2
123.8
100.0
23.8
17.6
20.6
The urinary tubule has a remarkable blood supply. The renal arteries
arise directly from the abdominal aorta and are very short. They run
through the medulla to the cortex, and join with neighboring arteries to
B.
Fig. 171. — Cross sections of convoluted tubules from kidney of rat. A, during slight secretion; B,
during maximal secretion. (From Sauer.)
form arches from which proceed branches, that radiate into the cortex
and give off smaller branches each of which very shortly breaks up into a
small capillary tuft, — the glomerulus, — which lies in the invaginated sphere
of Bowman's capsule. The capillaries collect into an efferent vessel, which
appears to be smaller than the afferent artery, and this vessel in emerging
from the capsule again breaks up to form a capillary network about the con-
voluted tubules, forming their sole blood supply. These capillaries
coalesce to form the renal vein. The blood of the kidney must, accord-
ingly, pass through two sets of capillaries.
The kidney is richly supplied with nerves, which are for the most part
derived from the celiac ganglion and are in connection with the splanch-
510 THE EXCRETION OP URINE
nic and the vagus. Other branches from plexuses in the region of the
suprarenal body and the aorta join with those coming from the celiac
ganglion to form what is known as the renal plexus, which is arranged
in a network along the blood vessels and on the walls of the pelvis of
the kidney. These fibers are distributed to the very smallest blood ves-
sels, and nerve fibers have been observed among the cells of the tubules.
THE MECHANISM OF THE EXCRETION OF THE URINE
The great number as well as the variety of substances which are pres-
ent in both the blood and the urine makes it appear improbable that
urine excretion is dependent upon chemical combinations within the
renal cells, and leads us to seek a physicochemical mechanism to explain
the phenomenon. Can we discover the processes by which the kidney
fabricates a highly concentrated solution of salts from a very dilute
solution of the same salts in the blood plasma ? The problem is compli-
cated by the fact that the ratios existing between the concentration of
each urinary salt in the urine and the concentration of the same salt
in the blood are different. In other words, the urine is not merely
concentrated blood plasma freed from protein.
The passage of Avater and salts through the capillary wall and through
the basement membrane surrounding the renal cell probably takes place
by simple diffusion. If it were otherwise, an expenditure of energy
would be required, and it is difficult to understand how a basement
membrane could bring about energy changes. Any substance to which
the cell membrane is permeable will diffuse into the cell until an equi-
librium is established between its concentration within the cell and
that of the lymph or blood plasma. A nondiffusible substance will not'
enter the cell because it can not pass through the cell membrane, and
if it exerts an osmotic pressure, it will also tend to keep the water in
which it is dissolved from entering. If water does pass into the cell
under these conditions, it is due to the expenditure of energy opposed
to and greater than that which is offered by the osmotic pressure of the
nondiffusible substances. Possible sources for such energy are the pres-
sure of the blood in the renal capillaries, which would exert a force op-
posite to that of its osmotic pressure, and the presence within the cell of
a concentration of salts greater than is present in the blood, and able to
exercise a sufficient osmotic force to draw fluid into the cell against the
osmotic force of the nondiffusible salts. The passage of the urinary
constituents through the cell might also be due to- simple diffusion, the
substances passing through the cell to be extruded on the other side in
THE EXCRETION OF URINE 511
the same concentration as in the blood. In this case, the renal cells
would act merely as a filter, the urine having the same concentration
of each urinary salt as is present in the blood.
A comparison of the concentrations of the urinary salts in the urine
and the blood shows, however, that the urine is not merely a deprotein-
ized blood plasma, so that other factors must be sought to explain the
excretion. Since the concentration of the urine requires the expenditure
of much more energy than is provided by the known physical factors,
it is generally accepted that the renal cell in some manner supplies this
energy by its metabolic activity. It is impossible at present even to
surmise the nature of the process. Two possibilities may be considered.
One is that the urine is a filtrate of the blood which has passed through
a portion of the renal epithelium into the tubules as a very dilute fluid,
resembling the blood plasma minus its colloidal substances, and that
this dilute fluid is concentrated by the reabsorption of fluid and of salts
by other cells of the kidney, and again replaced in the blood stream. The
other is that the salts and fluid are each actively and individually ex-
creted by the kidney. Whichever condition is the true one, the fact
remains that the change in the concentration entails the expenditure
of a great amount of energy on the part of the renal cells.
The energy which the kidney must use in the actual work of concen-
trating the urine from the fluid of the blood plasma can not be com
puted from a comparison of the concentration of the urinary salts as a
whole in both the blood and the urine. Each constituent must be con-
sidered apart. We can not, for example, determine the molecular con-
centration of the blood plasma and the urine (by measuring A) (page
10) and estimate the work which is expended in producing the con-
centration from the observed difference. On the basis of such comparisons,
however, it is said that the excretion of 100 c.c. of urine requires at the
minimum 500 kilogrammeters of work (Cushny2). Even this conserva-
tive estimate may be wrong, for- it does not take into consideration the
possibility that the excretion of water by the kidney requires energy
expenditure on the part of the renal cells.
„ „ f. V .,,». a . fr , V rt *
Theories of Renal Function
For many years two rival hypotheses have dominated the teaching of
the mechanism of renal function. Bowman and Heidenhain postulated
that the constituents of the urine are secreted by the vital activity of
the epithelium of the capsule and the tubules. The glomerular capsule
secretes the water and the easily diffusible salts in a dilute solution, and
the uriniferous tubules add to this fluid the various organic and inor-
ganic salts to bring the urine to the necessary concentration. This
512 THE EXCRETION OF URINE
theory has been termed the vital theory. Ludwig, on the other hand,
advanced what is termed the physical theory, which holds that the
glomerulus and capsule act simply as a filter, which allows the fluid
of the blood plasma to pass through in a very dilute solution and in
large amounts. This fluid is concentrated by physicochemical processes
on its passage along the urinary tubules to the pelvis of the kidney.
Both of these theories are inadequate and fail to explain the phenom-
ena which research has shown to occur in the kidney, but they have
served to develop what Cushny terms a modern theory of urinary
excretion.
The Modern Theory of Urine Formation. — This theory accepts the
general scheme of filtration and reabsorption of Ludwig, but pays due
respect to the fact that the known physical forces are not adequate
to explain the reabsorption which must occur in the tubules. It therefore
supplements Ludwig 's theory by assuming a vital activity on the part
of the epithelium of the tubules in reabsorbing fluids and salts from
the dilute filtrate coming from the glomerulus and capsule. A large
amount of plasma fluid is filtered through the walls of the glomerular
vessels. This fluid has the same concentration of the salts to which the
capsule is permeable as does the blood plasma, but it is free of the col-
loidal substances normally present in the plasma. The blood leaving the
glomerulus is therefore a somewhat concentrated solution of plasma col-
loids, and must have returned to it the proper amount of water and
salts to make it an optimum fluid for the body cells. This is accomplished
by active absorption from the glomerular filtrate. The salts that are of
no use to the body are not reabsorbed and therefore appear 'in highly
concentrated form in the urine. These salts are termed nonthreshold sub-
stances, and since their presence in the plasma is unnecessary, they con-
tinue to be excreted as long as they are present in any concentration in
the blood. The salts that are necessary for the plasma are termed
threshold substances, and are reabsorbed until they are again present in
the plasma in optimal strength. For example, urea continues to be ex-
creted as long as any is present in the blood, while glucose is completely
reabsorbed so long as its concentration remains under a more or less
fixed level.
It is impossible to give a summary of the arguments which have been
advanced in support of any of the theories. However, since the modern
theory appears to offer a better explanation of the established facts, it
may be wise to recount some of the best experimental evidence in support
of it.
First, we must inquire as to the amount of deproteinized blood plasma
which the capsule must filter off from the blood in order to furnish the
THE EXCRETION OF URTNE 513
amount of the various salts excreted each day and the amount of water
absorbed by the epithelium of the tubules to account for the concentra-
tion in which the salts are found in the urine. In order to produce 20
grams of urea in 1200 c.c. of urine, 60 liters of blood-plasma fluid con-
taining 0.03 per cent of urea would have to be filtered through the cap-
| 20
sule [ — =6000), and 5.9 liters of water returned to the blood from
I 0.03
the uriniferous tubules. Since the bloodflow through the kidneys is very
great, at least 500 liters per day, only about 13 per cent of the fluid con-
tained in the blood passing" through the glomerulus would pass by
filtration through the capsule of Bowman.
The fact that such a large amount of fluid would have to be reab-
sorbed from the uriniferous tubules (59 liters) is a possible a priori
criticism of the theory, but Cushny points out that the amount each
tubule would have to absorb per hour would be very small (in his ex-
periment on a cat amounting to less than 0.014 c.c. per hour).
The filtration of the protein-free blood fluid through the renal capsule,
like that through any other membrane, depends on several factors. (1)
There must be a difference in the pressure between the blood and the
urinary filtrate. , In the laboratory the pressure used in filtering is
usually supplied by gravity, but in the case of the filtration of the urine
through the capsule the force is furnished by the pressure of blood in
the glomerular vessels. (2) The character of the filter determines what
substances shall pass. The renal capsule is a membrane normally im-
pervious to the proteins of the blood, but pervious to the other constitu-
ents. Under certain conditions it loses this character. (3) The char-
acter of the fluid determines how readily it will filter through the mem-
brane. If the fluid contains a substance which can not pass through the
filter and which exerts an osmotic pressure in opposition to the filtering
force, the rate of filtration as well as the amount filtered, will be reduced.
If the capsule acts as a filter it should be possible to alter the rate of
urine excretion by varying any of these factors, and experimentally this
is true. The factors can be varied in several ways. If the blood pressure
is raised by tying off several of the branches of the aorta, the urine is
appreciably increased, or if the blood pressure is decreased, as can be
done by compressing the renal artery by means of a screw clamp, the
amount of urine is decreased. In the artificially perfused kidney, the
fluid exuding from the ureter increases as the pressure of the perfusion
fluid is increased, arid decreases as the pressure is decreased. Whether
changes in the pressure in the blood are directly responsible for variations
in the rate of urine excretion, or whether they act indirectly by varying
the rate of the bloodflow in the kidneys, has been the subject of much
514 THE EXCRETION OF URINE
debate. Probably both factors are involved, as is shown by the follow-
ing observations. If the blood pressure is increased by vasoconstriction
in the splanchnic area produced by stimulation of the splanchnic nerves,
the flow of blood through the kidney is decreased and the excretion of
urine falls. Apparently, secretion can continue only as long as the col-
loids of the plasma are not notably increased, for, as the osmotic pressure
due to the indiffusible colloids rises, the pressure in the capillaries is no
longer able to oppose it. The same point has been beautifully shown by
Starling and his pupils, who found that the secretion of urine ceases
when the capillary pressure in the glomerulus fell below that exerted by
the osmotic pressure of the blood proteins, the critical pressure being
from 30 to 40 mm. Hg. They also found that dilution of the blood with
saline solution by reducing the osmotic pressure of the proteins in the
plasma, was accompanied by an increase in the rate of excretion; excre-
tion in such cases being maintained at a blood pressure below the normal
critical pressure. If the dilution of the blood was made with saline con-
taining gelatin or gum arabic, on the other hand, the diuretic effect was
greatly decreased, and any fall in the blood pressure was followed by a
suppression in the urine (Knowlton9). These experiments evidently
indicate that the saline produces its diuresis by diluting the plasma
proteins and loAvering their osmotic pressure, since when the osmotic
pressure of the blood is maintained by the addition of colloids in which
this is present, no diuresis occurs. The significance of these facts, in
connection with the raising of lowered blood pressure after hemorrhage,
has already been alluded to (page 139).
This view is confirmed by the experiments of Barcroft and Straub,10
who showed that the oxygen consumption is often not appreciably
raised during the diuresis produced by the injection of saline. If the
diuresis produced by this means was due to an actual increase in the
work of the kidney, the oxygen consumption would have been increased.
In the frog, the glomerulus and the tubules are supplied with blood
by the renal artery, as is the case in the mammal, but the tubules cu-
riously enough are also supplied with some of the blood coming from the
lower extremities and the trunk through a vessel which has no counter-
part in the mammal — the renal portal vein. The blood, therefore, which
is supplied to the tubule is a mixture from the glomerulus and the renal
portal system. By ligating the renal vessels it is possible to cut off the
blood supply of the glomerulus while leaving the tubules supplied by the
renal portal vein. Normally the pressure in the renal portal system is
not sufficient to force blood back through the glomerular vessels. Liga-
ture of the renal vessels at once results in a suppression of the urine.
If the glomerular vessels are perfused with Ringer's solution at a
THE EXCRETION OF URINE
515
pressure equal to that found in the aorta, a considerable flow of fluid
may be secured from the ureters, but no fluid is obtained when the renal
portal vein is perfused at a pressure equal to that normally present in
this vein. Rowntree and Geraghty11 found that phenolsulphonephthalein
added to the perfusion fluid passed through the renal portal vein, did not
cause secretion, but when urea was added to the perfusate, fluid con-
taining the dye was obtained from the ureter. Unfortunately the pres-
sure employed in these experiments may have allowed some fluid to be
forced backward into the glomeruhis, so that the results may be due to
filtration through the capsule.
Renal
artery
jMalpighian
corpuscle
Renal-portal vein
Fig. 172. — Diagram of blood supply of Malpighian corpuscle and of convoluted tubules in amphibian
kidney. (Redrawn from Cushny.)
It is generally accepted that the proof that the capsule acts as a filter
is fairly complete. Unfortunately such decisive experimental facts can
not be offered to prove the assumption that the epithelium of the tubules
reabsorbs the excess of water and salts which are filtered off through
the capsule. If the modern theory of urine excretion is correct, the cells
of the tubules must not only absorb large amounts of water, but they
must also allow for the reentrance into the blood, either completely or
partially, of certain salts, while they must reject others entirely.
"We have called attention above to the fact that the glomerular filtrate is
very different from the urine that is finally passed. The urine contains a
very high percentage of small molecules, and the proportion in which they
516 THE EXCRETION OF URINE
are present is entirely -different from that in the blood plasma or in the
glomerular filtrate. This is shown in the following figures, which give an
average normal value for the urea, uric acid, chlorine, and glucose in 100
c.c. of protein-free blood plasma and 100 c.c. of urine. In the third col-
umn is given the change in concentration which has occurred in the
kidney.
100 C.C. PROTEIN-
FREE BLOOD
PLASMA CONTAINS
100 C.C. URINE
CONTAINS
CHANGE IN
CONCENTRATION
IN TlIE KIDNEY
Urea
Uric Acid
Chlorine
Glucose
.033
.0022
.41
.1
2.
.05
.6
60
22.7
1.5
Here the blood plasma fluid contained but 0.033 per cent of urea, and
the urine 2 per cent. Accordingly, 6 liters of glomerular filtrate would
f2
be required to furnish 100 c.c. of urine, \- - = 6000). Six liters of
I 0.33
glomerular filtrate would contain 6.6 grams of sugar, 0.132 grams of
uric acid, and 24.6 grams of chlorine. But 100 c.c. of urine contains no
glucose, 0.05 grams of uric acid and 0.6 grams of chlorine. According
to the modern theory, these figures indicate that during the passage of the
urine through the tubules 5900 c.c. of water, 6.6 grams of sugar, 24 grams
of chlorine and 0.067 grams of uric acid would have to be absorbed by
the renal epithelium in the production of 100 c.c. of urine containing
the concentration given above.
Among the most convincing experiments that can be offered in sup-
port of the absorption of fluid and salts by the tubules, are those in
which the pressure of the urine in the tubules is slightly increased by
partial closure of the ureter (Cushny). In these experiments the ureter
of one kidney is partly closed with a clamp and the excretion obtained
from this kidney is compared with that of the opposite normal kidney.
In general, obstruction of the ureter results in a decrease in the amounts
of water, chloride and urea excreted. But, curiously, the urea content is
decreased relatively less than is the chloride and water content. These
results can be explained on the basis that any pressure acting to oppose
the head of pressure producing filtration in the glomerulus will reduce
the amount of the glomerular filtration, and accordingly the time allowed
for 'the passage of this filtrate along the tubules is increased and absorp-
tion becomes more complete. Since urea is probably not absorbed at all
and chloride is, the discrepancy in the effects on the excretion of urea
and chlorine in the partially obstructed kidney can be explained.
When very large amounts of water are taken by mouth, it often hap-
THE EXCRETION OF URINE 517
pens that the urine excreted has a concentration of salts less than that
present in the fluid of the blood. Some investigators believe that such a
condition is possible only on the assumption that water is actively ex-
creted, but a more plausible explanation based on the modern theory
is that the water that is absorbed from the alimentary tract reaches the
kidney as a dilute saline solution, and is rapidly filtered off in a form
somewhat more dilute than the optimal solution which blood plasma must
have for the well-being of the tissues. The tubules reabsorb the amounts
of water and of those salts, such as chlorides, uric acid, and sugar, nec-
essary to restore the plasma to the optimal concentration, but do not
absorb the nonthreshold substances, such as urea.
It is impossible to analyze the forces that are responsible for such a
degree of absorption by the epithelium of the tubules. For the present
Ave must classify them, for want of a better term, as vital forces. The
questions that await immediate investigation are whether absorption
actually takes place, and, if it does so, what factors cause it to vary.
Many attempts have been made, by destroying the capsules or the
tubules by means of poisons or by operation, to determine directly or
indirectly the question of the function of the tubules.
In such experiments, however, the number of factors involved con-
fuse the issue and make the results practically valueless so far as de-
termining the normal function of the tubules. Other experimenters
have attempted to show absorption in the tubules by injecting diffusible
substances, such as chemicals and dyes, into the ureter under what they
deemed sufficient pressure to force the solution into the tubules, and by
an examination of the blood or the tissues to determine whether or not
the injected substances had been absorbed. The results obtained by
this method are not convincing, probably chiefly because of the difficulty
in reaching the tubules. Indeed, it is very questionable Avhether it is
possible to inject a substance into the tubules from the ureter.
Years ago Heidenhain, the exponent of the vital theory of excretion,
believed that he had demonstrated the ability of the renal cells to ex-
crete dye substances injected intravenously. Since he failed to find
evidence of dye excretion in the capsule, but found masses of dye in the
tubules and stained granules in the cells of the tubules, he concluded
that the cells of the tubules had the power to excrete the dye, and from
analogy he believed that the tubules must likewise excrete the water
and the various urinary salts. Subsequent work, however, has failed
to confirm his belief that the capsule is not concerned in the excretion
of the dye, and it is as reasonable to explain the results of the experi-
ments with the dyes by assuming that the masses of dye substances
found in the tubules and in the cells are due to the reabsorption of
518
THE EXCRETION OF URINE
water and perhaps of some of the dye from the dilute glomerular filtrate,
as to accept Heidenhain's hypothesis.
In the following table taken from Cushny the movements of the con-
stituents of the plasma may be followed through the kidney. The ulti-
mate destination of each is indicated in the enclosures.
67 LITERS PLASMA
CONTAIN
62 LITERS
FILTRATE
61 LITERS
REABSORBED FLUID
CONTAIN
1 LITER URINE
CONTAINS
PER
CENT TOTAL
IN ALL
PER
CENT TOTAL
PER
CENT TOTAL
Water
92 62 1.
62 1.
61 1.
95 950 c.c.
Colloids
| 8 5360 gm.|
— —
— —
Dextrose
0.1 67 gm.
67 gm.
0.11 67 gm.
— —
Uric acid
Sodium
Potassium
Chloride
0.002 1.3
0.3 200
0.02 13.3
0.37 248
1.3
200
13.3
248
0.0013 0.8 "
0.32 196.5 "
0.019 11.8 "
0.40 242 ' <
0.05 0.05 gm.
0.35 3.5 '
0.15 1.5 '
0.6 6.0 '
Urea
Sulphate
0.03 20
0.003 1.8
20
1.8
2.0 2.0 '
0.18 1.8 '
(From Cushny.^)
It will be noted that the dextrose alone is completely absorbed, and
that the urea and the sulphate are not absorbed at all from the glom-
erular filtrate. The other salts are partly absorbed.
As already mentioned, Barcroft and Straub have shown that the
diuresis which results from the injection of saline into the blood is not
accompanied by any increase in the oxygen consumption of the kidney.
This observation, coupled with the fact that the total amount of chloride,
urea, and sulphate which is excreted during saline diuresis, is greater than
under normal conditions indicates that the excretion of these salts is
not due to any vital secretory power of the kidney, but rather to factors
that are extrarenal in origin.
The diuresis produced by adding urea or sodium sulphate to the blood,
on the other hand, is accompanied by an increase in the oxygen con-
sumption of the kidney. This increase can not be due to active elimina-
tion of these salts by the tubules, the work of which requires oxygen,
for no increase in oxygen consumption accompanies the increased ex-
cretion of the same salts under saline diuresis. Sulphate and urea are
nonthreshold substa'nces, and are not absorbed by the tubules. The
explanation of the oxygen consumption is probably that the osmotic
pressure which these bodies in the glomerular filtrate exert makes it
necessary for the epithelium to oppose a greater absorbing force to con-
centrate the urine, and hence a greater expenditure of energy is requird.
Diuretics. — The action of the xanthine compounds — caffeine, theo-
bromine and theophylliiie — in the production of diuresis is unexplained.
THE EXCRETION OP URINE 519
It may be due in part to vascular changes and in part to reduction in
the resistance to nitration brought about by alteration in the permea-
bility of the capsule.
According to the modern theory the polyuria in diabetes is produced
by the excessive amount of water taken and by the inability of the
kidney to concentrate the urine against the osmotic pressure offered by
the concentrated sugar solution in the tubules. The presence of the hy-
perglycemia in an amount higher than is present in the optimal blood
plasma in this disease makes sugar a nonthreshold substance, so to speak,
and none is absorbed. The diuresis following the injection of sugar is
therefore of the same type as that produced by sulphate and urea. The
diuretic action of the digitalis group is dependent upon its influence on
the circulatory system. If the circulation is already sufficient, digitalis
does not cause diuresis. The cause of the diuresis produced by pituitary
extract is not known. It may be owing in part to its action on the cir-
culation and in part to a direct action on the kidney.
Albuminuria. — The plasma proteins ordinarily do not obtain entrance
into the tubules of the kidney. In disease such as acute nephritis and
cardiac failure, the plasma colloids are filtered off through the capsule,
probably because of some change that has occurred in the permeability
of its membrane due to inflammation or asphyxia. In these cases the
urine is usually reduced in amount. Probably there is no purely glom-
erular or tubular type of nephritis, both structures sharing in the dis-
ability. While it can not be said that any of the so-called renal tests
that have been advanced in recent years are free from criticism, they
nevertheless have contributed very useful information.- The fact that
the kidney of the chronic nephritic excretes a urine of more or less fixed
low specific gravity would suggest that here there is an impairment of
the resorbing mechanism, and the failure of a kidney to excrete the
proper amount of dye, as in the phenolsulphonephthalein test, suggests
an impairment in the filtering apparatus. Hard and fast rules can not
be applied, however, and probably the tests must at present be inter-
preted for the kidney as a whole.
The Influence of the Nervous System on the Secretion of Urine. — In
spite of numerous and repeated attempts to demonstrate that a nervous
mechanism governs the excretion of urine, no proofs \yhich are above
criticism have been forthcoming. Stimulation of the splanchnic nerves
results in a diminution in the excretion of urine, probably because of a
diminution in the blood supply of the renal vessels owing to the vasocon-
striction. Stimulation of the vagus nerves below the level of the cardiac
branches has been said to result in the augmentation of the rate of urine
excretion (Asher and Pearce12). The results are doubtful, however, since
520
THE EXCRETION OF URINE
there is no increase in the oxygen absorption under the above conditions
(Pearce and Carter13). In the light of the modern theory this vagal diure-
sis would be interpreted as due to an inhibition of the absorption in the
tubules rather than an augmentation in the actual excretion of urine.
There is no doubt that the renal nerves profoundly affect the excretion
of urine, but that they do so directly is very improbable, since perfectly
Si
SZ
Fig. 173. — Nerve supply of the kidney. K, kidney; S\, S», major and minor splanchnic nerves; V,
vagus; C.G., Celiac ganglion; A, aorta. (From Cushny.)
adequate renal function c.an be maintained in animals that have had the
kidnej's entirely removed and then replaced. There are numerous re-
flexes that affect the rate of urine excretion by constriction of the renal
vessels. Injury to the bladder or ureter, abdominal injuries to the kid-
ney, or even cold applied to the skin, may result in incomplete suppres-
sion of the urine.
CHAPTER LIX
THE AMOUNT, COMPOSITION, AND CHARACTER OF URINE
BY R. G. PEARCE, B.A., M.D.
In the chapters on digestion and metabolism, we have followed the
course which food takes with especial reference to the nutrition of the
body. The excretion of these elements of nutrition is taken up under a
number of the subdivisions of physiology, viz., respiration, digestion,
kidney function and the skin. In the chapters on digestion attention was
called to the fact that the feces, besides containing the indigestible resi-
due of the aliment, contain several excretory products which at one
time or another have actually been within the body proper. These in-
clude normally the pigments of the body and many of the heavier mineral
salts, such as iron, magnesium, lime and phosphates; and under abnormal
conditions, as when the metals are given as medicine, bismuth and mer-
cury. The respiratory system excretes most of the oxygen and carbon.
In this chapter we shall take up the manner in which the body rids itself
of the nitrogenous and some of the mineral waste materials. Even at
the risk of repetition, it will be advantageous to recapitulate certain facts
concerning the essential chemical structure of the urinary constituents,
so that we may be in a position to appreciate the kidney function in
health and disease.
We now know that the kidney does not form any of the specific con-
stituents of its secretion (except hippuric acid). These substances are
formed in the various tissues of the body, and are brought to the kidneys
by the blood, where they are eliminated. But while the constituents are
.unchanged in chemical composition in the urine from that in which they
are found in the blood, they do occur in greatly changed proportions.
It is this variation in the concentration of the urinary constituents in
the blood and the urine which presents the most important and at the
same time the most difficult question in the physiology of the kidney.
In the following table the percentage composition of the blood plasma is
compared with that of an average sample of human urine. The third
column gives the change in concentration which each constituent under-
goes in passing through the renal filter.
521
522
THE EXCRETION OF URINE
BLOOD PLASMA
PER CENT
URINE
PER CENT
CHANGE IN
CONCENTRATION
Water
90-93
95
—
Proteins, fats and other colloids
Dextrose
7-9
0.1
z
z
Urea
0.03
2
60
Uric acid
0.002
0.05
25
Creatinine
Ammonia
0.001
0.04
40
Sodium
0.32
0.35
1
Potassium
0.02
0.115
7
Calcium
0.008
0.015
2
Magnesium
Chlorine
0.0025
0.009
0.006
0.27
2
30
Phosphates (PO4)
Sulphates (SO4)
Amino acids
0.003
0.18
60
The Amount of Urine
The amount of urine passed in twenty-four hours varies with the
amount of fluid ingested and the proportion of fluid retained by the body
or excreted by other channels. Under ordinary conditions a twenty-four-
hour sample amounts to from 1000 to 1800 c.c. of urine. On a constant
water intake the volume of urine is extremely variable for any single
day or part of the day (Addis and Watanabe3). The average volume of
urine excreted by twenty individuals on the third, fourth and fifth days
of a constant diet in which the fluid intake was 2,070 c.c., varied from
1,013 to 1,712 c.c. for a twenty-four-hour period, from 684 to 1,195 c.c.
for the first twelve hours of the day, and from 501 to 788 c.c. for the
first eight hours of the day. In normal subjects the amount of urine
excreted during the night is usually less than that during the day. This
is such a constant finding that in cases where more than 50 per cent of
the urine is excreted in the twelve hours of the night, suspicions of renal
disease should be aroused.
The Specific Gravity of Urine
In urine collected at different times of the day the specific gravity may
show a variation of ten points. Indeed, the specific gravity of the urine
has been taken as a functional test by clinicians. With a constant food
and water intake the variations found in the specific gravity of samples,
of urine taken at two-hour periods in normal and pathologic conditions
are very useful as criteria of the functional state of the kidney. Fixa-
tion of the specific gravity at either a low or a high figure is not the
usual normal finding. The following figures will illustrate:
AMOUNT, COMPOSITION, AND CHARACTER OF URINE
523
DAY
NIGHT
8-10
A.M.
10.12
A.M.
12-2
P.M.
2-4
P.M.
4-6
P.M.
6-8
P.M.
8-8
P.M.-A.M.
Normal person
In Hypertensive Nephritis
In Myocardial Decompensation
1.016
1.010
1.018
1.019
1.009
1.020
1.012
1.010
1.019
1.014
1.009
1.018
1.020
1.019
0.020
1.010
1.010
1.021
1.020
1.009
1.022
(Compiled from Mosenthal's figures.)
The proportion of water to total solids is often very similar in plasma
and urine, but when water is taken in large quantities the urine shows
much greater changes than does the blood, and the solids may sink to a
very low concentration. On the other hand, when little fluid is taken or
when the skin and bowel eliminate a large amount of fluid, the urine
may become very concentrated without any change in the blood plasma.
The total solids in urine can be determined with approximate accuracy
by multiplying the last two figures of the specific gravity by the con-
stant coefficient 0.233 (Haeser).
The Depression of Freezing Point
While the solids of the blood consist, for the most part, of proteins
and colloids, those of the urine are made up of inorganic salts and small
organic molecules. The molecular concentration — that is, the total number
of molecules in a given quantity of fluid — is under ordinary conditions
much greater in the urine than in the blood. The molecular concentra-
tion may be determined by the depression of the freezing point of a fluid
below that of distilled water (see page 10). Blood freezes almost con-
stantly at -0.56° C., while urine may freeze at variations of temperature
between -1° C. and -2.5° C. ; if very concentrated it may freeze at a
temperature as low as -5° C., or if dilute the freezing point may be as
high as -0.075° C.
The variability of the freezing point and the specific gravity of the
urine lead us to a consideration of the relationship of the urinary volume
to its concentration. In the first place, the volume of water ingested is
more frequently than otherwise in excess of the minimum absolutely re-
quired by the body, and is subject to greater variation than the sub-
stances excreted in the urine. The kidney is able to eliminate one con-
stituent of the plasma which may be present in excess without involving
any changes in others. For example, when salt is added to the food and
excreted in the urine, the total chlorides are increased, but the amount
of urine and the other constituents may remain unchanged; or, again, as
may happen, excess of salt leads to an increase in the volume of the
urine, but the salt concentration remains constant while that of the
other urinary bodies is decreased. Similarly, although the rate of urea
524 THE EXCRETION OF URINE
excretion is not demonstrably augmented by an increase in the volume
of the urine, an increase in the rate of urea excretion induced by the
ingestion of urea is accompanied by a larger volume of urine. That these
two factors may not stand in a causal relationship to each other is sug-
gested by recent work of Addis and Watanabe,3 who find no quantitative
relationship between the rate of increase in urea excretion and the
increase in urine volume, and who believe that the apparent relationship
is due to a common cause, such as alteration in the rate of circulation or
change in the activity of the kidney cells. Nevertheless, there appears to
be a limit set to the power of the kidney to take the urinary salts or water
from the plasma and to place them in the urine in quite different propor-
tions. The definite amount of water required to hold the urinary salts
has been termed the "volume obligative" (Ambardr>). These limits of
concentration may be fixed by the energy which the kidney can bring to
act against the osmotic resistance.
The inconstancy in the behavior of the kidney toward ingested salts is
probably due to the fact that the salts reach the kidney in the concen-
tration in which they are held by the blood plasma, and not as they were
ingested. If salt is absorbed rapidly enough to disturb the salt equilib-
rium of the tissues and plasma, then water will be abstracted from the
tissues, and the plasma on reaching the kidney will eliminate the salt
and water together. The difference in the reaction arises from the
varied activity in the tissues in general rather than in the kidney itself.
The Reaction of Urine
In man and the carnivora this reaction is generally acid to litmus or
phenolphthaleih. The cause is found in the fact that the end products
of protein metabolism give rise to sulphuric and phosphoric acids the
acidity of which gives the urine an acid reaction. In the herbivorous
animals the alkaline reaction is due to the fact that vegetables and
fruits contain salts of dibasic or polybasic acids, such as acid potassiuiv
malate, citrate, acetate, and tartrate. Oxidation of these in the bod"
gives rise to carbonates. Some of the carbonic acid is excreted through
the lungs, and hence the associated base, generally sodium or potassium,
is combined so as to form a weak basic salt.
The measurement of the acidity of the urine in terms of gram anions
or cations, like the same measurement in blood, requires the use of the
rather difficult electrical or indicator method, the principle of which has
been described in Chapter V. Expressed in terms of CH, the acidity
varies between 4.7 x 10-7 and 100 xlO"7. The total potential acidity —
that is, the number of. II ions which will be formed in the face of a con-
AMOUNT, COMPOSITION, AND CHARACTER OF URINE 525
tinual neutralization of those in solution — may be obtained fairly accu-
rately by titrating the urine with %0 normal alkali in the presence of
neutral potassium oxalate, using phenolphthalein as an indicator (Folin).
The results may be expressed in acidity per cent in terms of c.c. N/10
NaOII required to neutralize 100 c.c. of urine. If the ammonia excretion
is added to the titration results, the total potential acidity is very closely
measured.
The urine is more alkaline shortly after meals than at other times,
since acid is being excreted by the gastric glands. It is more acid on a
meat than on a vegetable diet, and is acid during starvation because
protein is then the chief metabolite. In disease there is no characteristic
variation, save that the urine is more generally acid, which may be ex-
plained by the fact that in serious illness the diet is restricted. When
the acidity is increased, the excretion of ammonia is usually greater,
since ammonium carbonate, the forerunner of urea, acts as an alkali and
neutralizes the acid radicles. This rise in ammonia, however, is not
always proportional to the acid radicles present, since the fixed alkali
derived from fruits and vegetables may be sufficient to neutralize the
acid formed.
THE SOLID CONSTITUENTS
For practical reasons we shall divide the constituents of the urine into
normal and abnormal. The former are present in the average urine in
amounts sufficient to be detected by ordinary means; the latter only
rarely appear in detectable quantities. In a person eating an ordinary
diet the most important organic and inorganic constituents of the urine
are as follows:
TOTAL SOLIDS (40 TO 60 GRAMS) IN ONE LITER OF NORMAL URINE
ORGANIC CONSTITUENTS, "25-40 GM. INORGANIC CONSTITUENTS, 15-25 GM.
Urea, 20-35 gm. Sodium chloride (NaCl), 8-15 gm.
Creatinine, 1.011.5 gm. Phosphoric acid (P2OS), 2.5-3.5 gm.
Uric acid, 0.5-1.25 gm. - Sulphuric acid, (SO3), 2-2.5 gm.
Hippuric acid, 0.1-1.7 gm. Potassium (K,O), 2-3 gm.
Other constituents (ethereal sulphates, Sodium (Na2O), 4-6 gm.
oxalic acid, urinary pigments, etc.), Calcium (CaO), 0.1-0.3 gm.
1.5-2.3 gm. Magnesium (MgO), 0.2-0.5.
Ammonia (NH3), 0.3-1.2 gm.
Iron (in pigme"nt), 0.001-0.010.
(Compiled from Mosenthal's* figures.)
These urinary salts are present in the blood, and are excreted only by
the kidney. An investigation of the mechanism of renal secretion must
therefore include a study of the relationship existing between the con-
centration of the urinary salts in the blood and in the urine.
526 THE EXCRETION OF UEINE
The Normal Organic Salts of the Urine
Nitrogenous Constituents.— The greater number of the organic salts of
the urine are made up of bodies which contain nitrogen, and which are
derived from the protein element of nutrition. The proteins, which form
the chief building material of the body, are broken up into their con-
stituent amino acids in the intestinal tract and absorbed as such by the
blood. Portions of these acids are taken up by the tissues to repair and
to replace those proteins which have been discarded, and the remaining
protein, in excess of the body need for amino acids, is deamidized, the
major portion of the carbon, oxygen and hydrogen being oxidized to
form C02 and water, and the lesser portion of these elements being com-
bined with the nitrogen to form urea, ammonia, uric acid, etc. A similar
fate later awaits the nitrogen moiety which found a place in the tissues,
and which is replaced in turn by new nitrogenous bodies.*
Since all the ingested nitrogen, except a small and rather constant
amount which is lost by the feces and the sweat, is excreted in the urine,
the total nitrogen of the urine has been taken as a measure of the nitro-
gen or protein metabolism of the body. In normal conditions the protein
metabolism is adjusted in such a manner that the nitrogen intake is
equal to the nitrogen output, a condition known as nitrogenous equilib-
rium. If the nitrogen intake is reduced below the actual body needs,
the excretion of nitrogen is greater than the intake which indicates that
the body protein is replacing the protein usually furnished by the food.
The minimum amount of protein that the body must have to maintain
equilibrium varies in individuals, but is on the average between 5 and 6
grams of nitrogen a day, which corresponds to about 40 grams of pro-
tein. With the ordinary diet it is usually between 12 and 20 grams a
day, or represents from 75 to 125 grams of protein. Since protein is not
stored by the body except in periods of growth or after periods of undsr-
nutrition, an increase in the protein food is accompanied by an increase
in the nitrogen excreted in the urine. For this reason, unless the amount
of nitrogen ingested is known, the study of the total nitrogen of the urine
gives no information concerning the nature of the nitrogen metabolism
of the body. The total output of nitrogen per day usually amounts to
10 to 15 grams — from 1 to 2 per cent of the urine by weight.
All the nitrogenous bodies of the urine are normally nonprotein, and
arise from similar bodies in the blood, where they exist in concentra-
tions of from 20 to 30 mg. per 100 c.c. In excreting the nitrogen of the
urine the kidney therefore takes it from a solution in which it is found
in a concentration of 0.03 per cent on the average and delivers it to a
*For further details see page 610.
AMOUNT, COMPOSITION, AND CHARACTER OF URINE 527
solution containing an average of 1.00, or concentrates it at least 30
times.
Urea. — The chief of the nitrogenous bodies of the urine is urea, the
origin of which has been fully described in the chapters on metabolism.
No constituent of the urine is subject to greater variation both in abso-
lute and in relative amounts. On an average diet containing 120 grams
of protein per day, the absolute urea excretion may amount to about 30
grams ; on a low protein diet it may be only a few grams. When the pro-
tein intake is high, the nitrogen eliminated as urea may be 90 per cent
of the total nitrogen; but when the protein intake is low, this proportion
may fall to 60 per cent. The difference is because on a low protein diet
the greater percentage of nitrogen eliminated is endogenous in origin,
and urea, which is the chief constituent of the exogenous nitrogen moiety
of the urine, is accordingly decreased on low diets.
In recent years the importance of the relationship between the con-
centration of the urinary constituents in the blood and the urine has
been much insisted upon, and since the estimation of the amount of
urea in the blood and the urine is relatively simple, most of the work
has been done by using these values. Ambard and Weil5 believe that a
quantitative relationship exists between .the rate of urine excretion and
the concentration of urea in the blood and the urine, since the urea in
the blood acts as a stimulus to the renal cells. By comparing the rate
of urea excretion and the concentration of urea iri the blood and urine
in a mathematical formula, they have obtained a value which they be-
lieve is more or less fixed for the normal kidney. This expression is
known as Ambard' s coefficient and formula* and has been used as a
means of evaluating the functional capacity of the kidney.
Whatever the value of the formula may be in expressing the relationship
existing between the rate of urea excretion and the concentration of this
salt in the blood, it is certain that, in diseased conditions where impair-
ment of the kidney is certain, the concentration of urea in the blood re-
mains permanently at an abnormally high average level, although the
*Ambard and Weil's formula is:
Ur
K = , in which:
70 vT"
D x — x — —
P V2S
K = coefficient of urea excretion (Constant of Ambard).
Ur = grams of urea per liter of blood.
D — output of urea in grams per 24 hours.
P == weight of the patient.
C = grams of- urea per liter of urine.
70 = standard weight.
25 = standard concentration of the urine.
The average value for this constant in normal individuals is said to lie between .06 and .09. -
Critical reviews of the work have been published recently by Maclean6 and by Addis and
Watanabe.*
528 THE EXCRETION OF URINE
amount of urea excreted during twenty-four hours may be exactly the
same as under normal conditions. Probably the increased concentration
of urea in the blood under these conditions is a compensatory measure
to provide sufficient pressure to cause its excretion through a damaged
outlet. It is this increase in urea of the blood which is indicated by the
term iirea retention in nephritis.
It must not be lost sight of, however, that the approximate constancy
of the combined formula is due in large part to the mathematical con-
struction, and also to the fact that any increase in the concentration of
urea in the blood is usually accompanied by an increased rate of urea
excretion. The factors which are most variable occur as the square or
the square roots of their values, and thus the disturbing effect they pro-
duce on the constancy of the resultant of the formula is greatly re-
duced, while the most constant factor, the concentration of urea in the
blood, is used with modification. In such a complex mechanism as the
renal function it is very probable that other factors are of great im-
portance in controlling the rate of urinary excretion. . Many of these
factors can not admit of mathematical expression. The writer seriously
doubts the advisability of adopting an empirical formula as a means
of expressing .unknown physiologic laws. Such measures are apt to
give a sense of knowledge altogether false, and thus hinder research
progress.
The upper limit of blood urea-nitrogen is about 20 mg. per 100 c.c..
which would correspond .to about 0.45 gm. of urea per liter of blood.
The average figure is half of this amount. The maximum concentration
of urea in the urine is seldom over 8 per cent. On this basis the kidney
can raise the concentration of the urea in the urine, at a conservative
estimate, from 100 to 200 times. Normally the daily output of urea
nitrogen may range from 8 to 12 gm., and the nitrogen which it contains
is roughly 80 per cent of the total excretion for the day.
Ammonia. — The chief source of ammonia in the body is from the ni-
trogenous portion of the deamidized amino acids. The ammonia found
in excess in the portal blood is derived from ingested ammonium salts
and from ammonia resulting from bacterial action on proteins in the
intestinal tract. The ammonia of the body is present chiefly in the form
of ammonium carbonate, and it is this salt that is the precursor of urea.
Because ammonium carbonate is so readily converted into urea by the
tissues of the body, little ammonia is normally present in the systemic
blood. The greater portion of the ammonia that finds its way into the
urine serves as a base to transfer acid radicles either ingested or formed
within the body. The amount of ammonia in the urine, therefore, is an
indirect measure of the extent of urea formation and of the acid bodies
AMOUNT, COMPOSITION, AND CHARACTER OF URINE 529
of the blood. For the latter reason the determination of the ammonia
excretion in urine is of some clinical importance. The ingestion of
mineral acids increases the ammonia excretion, while alkalies tend to
reduce it. During fasting and in diseases such as diabetes, where there
is an abnormal metabolism, the amount of ammonia in the urine is in-
creased. Ordinarily the daily output of ammonia nitrogen does not
exceed 0.5-0.6 gm., constituting 3-5 per cent of the total amount of
nitrogen.
Creatinine. — On a meat-free diet the daily excretion of creatinine is
remarkably constant, amounting to from 7 to 11 mg. per kilogram of
body weight. For this reason its determination is accepted as an in-
dispensable feature in metabolism investigations involving urine an-
alysis.
Any gross variation from the normal amount indicates the certain
failure of the attendants to collect all of the twenty-four-hour specimen
of urine. Normally the blood contains from 1 to 2 mg. per 100 c.c.
The creatinine is one of the last of the urinary constituents to accumu-
late in the blood during renal insufficiency, and for this reason affords
a reliable prognostic indication concerning the patients' condition. A
rise in the creatinine concentration of the blood is evidence of serious
renal disease, patients with concentrations of 5 mg. never recovering
(Chase and Meyers)7 The concentration of creatinine in the urine is
about 100 times greater than in the blood.
In adult man creatine does not appear in the urine save during starva-
tion or wasting diseases. In woman it is absent save after postpartum
resolution of the uterus. Children commonly excrete creatine along
with creatinine until the middle years of childhood.
The Purine Bodies and Uric Acid. — The most important purine in
human urine is uric acid. Xanthine is the next in importance, and small
amounts of hypoxanthine, guanine, and adenine are found. Among the
most interesting of the salts of the urine to the clinician are the urates,
because an accumulation of uric acid in the body was believed to be
responsible for many obscure clinical conditions. It is quite true that
the salts of uric acid are found in higher than normal amount in some
diseases, especially gout, leukemia, and chronic nephritis, but the many
vague theories associated with uric acid and disease have long ago been
exploded.
The human body has the almost unique distinction among mammals
of not being able to destroy any of the uric acid it produces, and hence
all the uric acid formed during metabolism must be excreted in the urine.
Unfortunately the kidney appears to be less competent to rid the body
of this waste than it is of the other urinary metabolites, and one of the
530 THE EXCRETION OF URINE
earliest signs of renal insufficiency is now held to be a failure of the
kidney to prevent the uric acid of the blood from increasing. Perhaps
the reason for the inability of the kidney to excrete uric acid readily
lies in the fact that its salts are among the least soluble of those in the
urine. It is on this account that when the urine cools, a red sediment of
urates containing certain pigments often separates out.
The uric acid of the urine is possibly derived entirely from the purine
metabolism of the body, in which the nucleins either of the body cells or
of the exogenous food take part. It is decreased during starvation and
increased by eating food rich in nucleins, such as liver and sweet-
breads.
Under ordinary conditions the excretion of uric acid amounts to from
0.3 to 1.2 gm. per day (0.02 to 0.10 per cent), the variation being de-
pendent upon the state of health, diet, or personal idiosyncrasy. The
blood of a normal individual contains on the average 1.8 mg. of uric
acid per 100 c.c. The kidneys are therefore able to concentrate the
uric acid in the urine from 30 to 60 times over its concentration in the
blood plasma.
The purines found in coffee and tea (caffeine, etc.) are excreted in
the urine as salts not of uric acid but of methylated xanthines.
Hippuric Acid. — This is a constant constituent of the urine of her-
bivorous animals, and is usually present in small amounts in human
urine. The amount rarely exceeds 0.7 gm. a day, but on a diet rich in
fruits and vegetables it may exceed 2 gm. It is interesting, since it is
the only urinary constituent that is synthesized by the renal cells.
Amino acids are always present in small amounts in the urine, con-
stituting, according to D. D. Van Slyke, about 1.5 per cent of the total
nitrogen. The estimation of the amino-acid nitrogen of the urine has
not been found to be of any clinical significance.8
The aromatic oxyacids are normally present in the urine in varying
amounts. These include phenol, indoxyl, skatoxyl, and phenylacetic,
paraoxyphenyl, propionic, oxymandelic and homogentisic acids. These
bodies are derived from phenylamino acids, such as tyrosine, tryptophane,
and phenylalanine. It is believed that the putrefactive decomposition
of proteins in the large intestine results in the production of these toxic
bodies. The body protects itself by oxidizing them and uniting them
to sulphuric acid to form the ethereal or conjugated sulphates, which
are found in the urine in the form of sodium or potassium salts. The
determination of the amounts of these bodies in the urine has therefore
been taken as an index of the putrefaction going on within the bowel.
The chief of these bodies is urinary indican, which is found usually as
a potassium salt. The test for indican in the urine consists in oxidiz-
AMOUNT, COMPOSITION, AND CHARACTER OF URINE 531
ing the indoxyl in an acid solution by means of ferric chloride to indigo
blue, and shaking out the indigo blue with chloroform. The depth of
the color of the chloroform affords a rough means of determining
the amount of indican present. The fact that the indican test is nega-
tive must not be taken to mean that the intestinal processes are normal,
for if the intestine fails to contain phenylated amino acids, or the proper
bacteria are not present, no indican will be found. On the other hand,
the putrefactive process of the large bowel may not be very extensive,
yet the amount of indican in the urine be increased, because of greater
absorption due to constipation.
Skatole, a fecal-smelling substance, is formed by certain kinds of bac-
teria. The greater proportion of this substance is excreted by the bowel,
but if the person is constipated, some of it may find its way into the
blood to impart a fecal odor to the breath and urine. Its presence
therefore has some diagnostic importance.
A very interesting body which is sometimes found in the urine is
homogentisic acid. It is thought to be an intermediate step in the metab-
olism of tyrosine, and is found in the urine of people suffering from
alkaptonuria. The disease is remarkable in that it appears to run in
families and produces no ill effects. Homogentisic acid is a strong
reducing agent, and for this reason may be confused with sugar in
Fehling's test.
The inorganic constituents of the urine include the acids: chlorides,
sulphates and phosphates; and the bases: sodium, potassium, magnesium,
and calcium.
The Acids of the Urine. — The chlorides compose the bulk of the acid
radicles in the urine. Although they appear to be necessary constituents
of the living cell, they do not, so far as known, enter into combinations
with the organic constituents. The tissues appear to require a rather
definite concentration of sodium chloride in order to carry on their
work, for reduction in the sodium-chloride intake of the body results
in a reduction in the chloride excretion by the urine. In salt starvation
the chlorides may disappear entirely from the urine, the amount of
chloride excreted appearing to be closely related to the amount of salt
ingested. When the intake is constant, the rate of excretion is likewise
more or less constant, but a sudden reduction in the salt of the diet may
be accompanied by a slight decrease in the salt content of the blood,
with an attendant loss of water. On the other hand, when the salt is
again taken, there is a retention of salt and of water, with a consequent
increase in body weight, until equilibrium is re-established on the old
level. While the above is the usual reaction, a considerable retention of
salt without an increase in the \vater content of the body may occur in
532 THE EXCRETION OF URINE
some apparently normal cases. This is due probably to the deposition
of salt in the tissues.
Careful studies fail to confirm the idea that there is a fixed relation-
ship between the salt and the water of the body. As with the nitroge-
nous constituents, however, there appears to be a relationship between
the rate of excretion of chlorides and the amount of chloride in the blood.
Ambard believes that this relationship, like that of the excretion of urea
to the blood urea, is capable of being expressed mathematically (see
page 527), if allowance is made for the fact -that NaCl is not excreted
after it falls beloAV a certain concentration in the blood equal to about
5.62 gm. per 1000 c.c. This level is more or less constant for normal
individuals, but is considerably increased in disease of the kidney. This
is known as the threshold of chloride excretion.
The amount of sodium chloride excreted in the urine in twenty-four
hours varies between 8 and 20 gm. a day, according to the intake. It
is therefore apparent that the kidney is able to concentrate the salts
of the plasma from ten to twenty times.
The Sulphates. — Since the inorganic sulphates do not form an im-
portant constituent of the food, the greater portion of the sulphates of
the urine are derived from the sulphur found in the protein, molecule.
For this reason the sulphates of the urine, like the nitrogen, are a meas-
ure of protein metabolism. An increase in the nitrogen excretion is
accompanied by an increase in the sulphur excretion, the ratio being
about 5 to 1. The daily output of sulphur is between 1 and 3 gm. The
greatest output is in the form of the alkaline sulphates, about 10 pe
cent in combination with aromatic bodies, and a small amount in com-
bination with amino acids and neutral organic salts.
The phosphates of the urine are derived from the food and from the
oxidation of phosphorus-containing bodies in the tissues such as
nuclein, lecithin, etc. The daily excretion varies between 1 and 5 gm.,
calculated as P203. When calcium or magnesium is present in the
food, they are excreted by the bowel as phosphate, and proportionately
less is found in the urine. The amount usually excreted in the feces
equals about 30 per cent of the total.
Since phosphates in the urine exist as a mixture of the mono- and di-
sodium hydrogen phosphates, - they have an important bearing on the
reaction of the urine, the amount of each varying with the degree of
the acidity of the urine.
On a heavy protein diet the urine is acid on account of the sulphuric
and other acids formed from the meat, and in this case there is a greater
amount of phosphoric acid and the mono-sodium hydrogen phosphate.
When the urine is alkaline or less acid, as it is on a vegetable diet, there
AMOUNT, COMPOSITION, AND CHARACTER OF URINE 533
is a large amount of the disodium hydrogen phosphate. Since calcium
and magnesium phosphates are more soluble than the diphosphates of
the same metals, deposits of the earthy phosphates are often found in
neutral or alkaline urines. When the urine is heated, the diphosphate
of calcium breaks up into the mono-calcium and a tri-calcium phos-
phate, Avhich accounts for the fine turbidity often taken for albumin in
the flame test. Addition of acid will cause this to disappear. The crys-
tals of triple phosphates which occur in alkaline urine are ammonium
magnesium phosphate, NH4MgP04.
KIDNEY REFERENCES
(Monographs)
Beddard, A. P. : Recent Advances in Physiology, Longmans, Green & Co., London,
1906.
Cushuy, A. R.: Secretion of Urine, Longmans, Green & Co., London, 1917.
(Original Papers)
JBrodie, T. G., and Mackenzie, J. J.: Proc. Roy. Soc., 1914, Ixxxvii, B, 593.
sCuslmy, A. R.: Secretion of Urine, 1917, p. 48.
3Addis and Watanabe: Jour. Biol. Chem., 1916, xxiv, 203.
JMosenthal, II. O.: Arch. Int. Mod., 1915, xvi, 733.
5Ambard .and Weil: Physiologic normale et pathologique des reins, Paris, 1914,
J. B. Bailliere et fils.
eMaclean, F. C. : Jour. Exper. Med., 1915, xxii, 212.
"Chase and Meyers: Jour. Am. Med. Assn., 1916, Ixvii, 931.
s\ran Slyke, D. D., and Meyer, G. M.: Jour. Biol. Chem., 1912, xii, 399; and 1913,
xvi, 197, 213 and 231.
oKnowlton, F. P.: Jour. Physiol., 1911, xliii, 219.
loBarerof t, J., and Straub, H. : Jour. Physiol., 1910, xli, 145.
uRowntree and Geraghty: Jour. Pharm. and Exper. Therap., 1910, i, 579.
i2Asher and Pearce, R, G.: Zeitschr. f. Biol., 1913, Ixiii, 83.
i-Pearce, R. G., and Carter, E. P.: Am. Jour. Physiol., 1915, xxxviii, 350.
PART VII
METABOLISM
CHAPTER LX
METABOLISM
Introductory. — The object of digestion, as we have seen, is to render
the food capable of absorption into the circulatory fluids — -the blood and
lymph. The absorbed food products are then transported to the various
organs and tissues of the body, where they may be either used at once
or stored away against future requirements. After being used, certain
substances are produced from the foods as waste products, and these pass
back into the blood to be carried to the organs of excretion, by which they
are expelled from the body. By comparison of the amount of these ex-
cretory products with that of the constituents of food, we can tell how
much of the latter has been retained in the body, or lost from it. This
constitutes the subject of general metabolism. On the other hand, we may
direct our attention, not to the balance between intake and output, but to
the chemical changes through which each of the foodstuffs must pass be-
tween absorption and excretion. This is the subject of special metabolism.
In the. one case we content ourselves with a comparison of the raw. ma-
terial acquired and the finished product produced by the animal factory;
in the other we seek to learn something of the particular changes to which
each crude product is subjected before it can be used for the purpose of
driving the machinery of life or of repairing the worn-out parts of the
body.
In drawing up a balance sheet of general metabolism, we must select
for comparison substances that are common to both intake and output. In
general the intake comprises, besides oxygen, the proteins, fats and car-
bohydrates; and the output, carbon dioxide, water and the various nitrog-
enous constituents of urine. This dissimilarity in chemical structure be-
tween the substances ingested and those excreted limits us, in balancing the
one against the other, to a comparison of the smallest fragments into which
each can be broken by chemical agencies. These are the elements, and of
them carbon and nitrogen are the only ones which it is possible to measure
534
METABOLISM 535
with accuracy in both intake and output. From balance sheets of intake
and output of carbon and nitrogen and from information obtained by ob-
serving the ratio between the amounts of oxygen consumed by the animal
and of carbonic acid excreted; we can draw far-reaching conclusions re-
garding the relative amounts of protein, fat and carbohydrate that have
been involved in the metabolism.
As has already been stated, the essential nature of the metabolic proc-
ess in animals is one of oxidation — that is, one by which large unstable
molecules are broken down to those that are simple and stable. Dur-
ing this process of catabolism, as it is called, the potential energy locked
away in the large molecules becomes liberated as actual or kinetic energy —
that is, as movement and heat. It therefore becomes of importance to
compare the actual energy which an animal expends in a given time with
the energy which has meanwhile been rendered available by metabolism.
"We shall first of all consider this so-called energy balance and then pro-
ceed to examine somewhat more in detail the material balance of the body.
ENERGY BALANCE
The unit of energy is the large calorie (written C.), which is the amount
of heat required to raise the temperature of one kilogram of water through
one degree (Centigrade) of temperature.* We can determine the calorie
value by allowing a measured quantity of a substance to burn in com-
pressed oxygen in a steel bomb placed in a known volume of water at a
certain temperature. Whenever combustion is completed, we find out
through how many degrees the temperature of the water has become
raised and multiply this by the volume of water in liters. Measured
in such a calorimeter, as this apparatus is called, it has been found that
the number of calories liberated by burning one gram of each of the proxi-
mate principles of food is as follows :
Carbohydrates .......................
| Sugar ....................... 4.0
Protein .................................... 5-0
Fat ........................................ 9.3
The same number of calories will be liberated at whatever rate the com-
bustion proceeds, provided it results in the same end products. When
a substance, such as sugar or fat, is burned in the presence of oxygen, it
yields carbon dioxide and water, which are also the end products of the
metabolism of these foodstuffs in the animal body ; therefore, when a gram
of sugar or fat is quickly burned in a calorimeter, it releases the same
*The distinction between a calorie and a degree of temperature must be clearly understood. The
former expresses quantity of actual heat energy; the latter merely tells us the intensity at which the
heat energy is being given out.
536
METABOLISM
amount of energy as when it is slowly oxidized in the animal body. But
the case is different for proteins, because these yield less completely oxi-
dized end products in the animal body than they yield when burned in
oxygen; so that, to ascertain the physiologic energy value of protein, we
must deduct from its physical heat value the physical heat value of the
incompletely oxidized end products of its metabolism. It is obvious that
we can compute the total available energy of our diet by multiplying the
quantity of each foodstuff by its calorie value.
Methods. — In order to measure the energy that is actually liberated
Fig. 174. — Respiration calorimeter of the Russell Sage Institute of Pathology, Bellevue Hospital,
New York. At the right is seen the table with the absorption tubes; and in the middle, at the
back, the electric control table for regulating the temperature of the double walls of the calorimeter.
At the extreme left is the oxygen cylinder. (Lusk's Science of Nutrition.)
in the animal body, we must also use a calorimeter, but of somewhat dif-
ferent construction from that used by the chemist, for we have to provide
for long-continued 'observations and for an uninterrupted supply of oxy-
gen to the animal. Animal calorimeters are also usually provided with
means for the measurement of the amounts of carbon dioxide (and water)
discharged and of oxygen absorbed by the animal during the observation.
Such respiration calorimeters have been made for air sorts of animals, the
most perfect for use on man having been constructed in America (see Fig.
174). As illustrating the extreme accuracy of even the largest of these,
METABOLISM 537
it is interesting to note that the actual heat given out when a definite
amount of alcohol or ether is burned in one of them exactly corresponds
to the amount as measured by the smaller bomb-calorimeter. All of the
energy liberated in the body does not, however, take the form of heat. A
variable amount appears as mechanical work, so that to measure in calories
all of the energy that an animal expends, cue must add to the actual cal-
ories given out, the calorie equivalent of the muscular work which has
been performed by the animal during the period of observation. This can
be measured by means of an ergometer, a calorie corresponding to 425
kilogram* meters of work. That it has been possible to strike an accurate
balance between the intake and the output of energy of the animal body,
is one of the achievements of modern experimental biology. It can be
done in the case of the human animal ; thus, a man doing work on a bicycle
ergometer in the Benedict calorimeter gave out as actual heat 4,833 C.,
and did work equalling 602 C., giving a total of 5,435 C. By drawing up
a balance sheet of his intake and output of food material during this
period, it was found that the man had consumed an amount capable of
yielding 5,459 C., which may be considered as exactly balancing the actual
output,
It would be out of place to give a full description of the respiration
calorimeter here. The general construction will be seen from the accom-
panying figure of the form of apparatus in use for patients in the Russell
Sage Institute, New York. One of the most interesting details of its con-
struction concerns the means taken to prevent any loss of heat from the
calorimeter to the surrounding air. This is accomplished in the following
way: The innermost layer of the wall is of copper; then, separated from
this by an air space, is another wall of copper, outside of which are two
wooden walls separated from each other and from the outer copper walls
by air spaces. The two copper walls arc connected through thermoelectric
couples, so that an electric current is set up whenever there is any differ-
ence in their temperatures. The current is observed by means of a gal-
vanometer placed outside the calorimeter, and from its movements the ob-
server either heats up or cools down the outer copper walls so as to cor-
rect the difference of temperature causing the current. This is done by an
electric heating device or by cold water tubes placed between the outer-
most copper and the innermost wooden walls. Since the temperature of
the two copper walls is the same, there can be no exchange of heat between
them, and consequently none of the heat that is absorbed by thfe inner cop-
per walls is allowed to be carried away. All the heat given out by the
animal is absorbed by the stream of cold water flowing through the coils
*A kilogram meter is the product of the load in kilograms multiplied by the distance in meters
through which it is lifted.
538 METABOLISM
of pipe in the chamber. The heat used to vaporize the moisture from
skin and lungs must of course also be measured. This is done by collect-
ing the water vapor in a sulphuric-acid bottle placed in the ventilat-
ing current. By multiplying the grams of water by the factor for the
latent heat of vaporization, we obtain the calories of heat so eliminated.
"The calorimeter contains a comfortable bed and is provided with two
windows, a shelf, a telephone, a fan, a light, and a Bowles stethoscope for
counting the pulse. The ordinary experiment takes about as long as a trip
from New York to New London. Patients, as a rule, doze from time to
time or else try to work out some scheme by which they can amuse them-
selves without moving. After three or four hours they are rather bored
by the quiet, and the observations are not prolonged beyond this time.
They are allowed to turn over m bed once or twice an hour, but reading
and telephoning are discouraged, since these increase the metabolism.
The air in the box is fresh and pure, the patient suffers no discomfort, and
objections to the procedure are very infrequent. Most of the patients
are only too glad of the extra attention, and they insist that the calor-
imeter has a marked therapeutic value." (Du Bois.)
Normal Values. — Having thus satisfied ourselves as to the extreme
accuracy of the method for measuring energy output, we shall now con-
sider gome of the conditions that control it. To study these we must first
of all determine the basal ~h.eat production — that is, the smallest energy
output that is compatible with health. This is ascertained by allowing a
man to sleep in the calorimeter and then measuring his calorie output
while he is still resting in bed in the morning, fifteen hours after the last
meal. When the results thus obtained on a number of individuals are
calculated so as to represent the calorie output per kilogram of body weight
in each case, it will be found that 1 C. per kilo per hour is discharged
— that is to say, the total energy expenditure in 24 hours in a man of 70
kilos, which is a good average weight, will be 70X24 = 1,680 C.
When food is taken the heat production rises, the increase over the
basal heat production amounting for an ordinary diet to about 10 per
cent. Besides being the ultimate source of all the body heat, food is there-
fore a direct stimulant of heat production. This specific dynamic action,
as it is called, is not, however, the same for all groups of foodstuffs, being
greatest for proteins and least for carbohydrates. Thus, if a starving
animal kept at 33° C. is given protein with a calorie value which is equal
to the calorie output during starvation, the calorie output will increase by
30 per cent, whereas with carbohydrates it will increase by only 6 per
cent. Evidently, then, protein liberates much free heat during its as-
similation in the animal body; it burns with a hotter flame than fats or
carbohydrates, although before it is completely burned it may not yield
METABOLISM 539
so much energy as is the case, for example, when fats are burned. This
peculiar property of proteins accounts for their well-known, heating qual-
ities. It explains why protein composes so large a proportion of the diet of
peoples living in cold regions, and why it is cut down in the diet of those
who dwell near the tropics. Individuals maintained on a low protein diet
may suffer intensely from cold.
If we add to the basal heat production of 1,680 C. another 168 C. (or
10 per cent) on account of food, the total 1,848 C. nevertheless falls far
short of that which we know must be liberated when we calculate the
available energy of the diet, which we may take as 2,500 C. What be-
comes of the extra fuel? The answer is that it is used for muscular work.
Thus it has been found that if the observed person, instead of lying down
in the calorimeter, is made to sit in a chair, the heat production is raised
by 8 per cent, or if he performs such movements as would be necessary for
ordinary work (writing at a desk) it may rise 29 per cent — that is to say,
to 90 C. per hour. There is, however, practically no difference in the en-
ergy output of a person lying flat or lying in a semi-reclining posi-
tion, as in a steamer chair. Allowing eight hours for sleep and sixteen
hours for work, we can account for about 2,168 C., the remaining 300 odd
C. that are required to bring the total to that which we know, from statis-
tical tables of the diets of such workers, to be the actual daily expenditure,
being due to the exercise of walking. If the exercise is more strenuous,
still more calories will be expended ; thus, to ascend a hill of 1,650 feet at
the rate of 2.7 miles an hour requires 407 extra calories. Field workers
may expend, in 24 hours, almost twice as many calories as those engaged
in sedentary occupations.
Standard for Comparison
When the energy output per kilo body weight is determined in animals
of varying size, the values are greater the lighter the animal. This is
evident from the following results obtained on dogs:
Weight of dog Heat production in calories
per Tctio per day
(1) 31.2 35.68
(2) 18.2 46.2
(3) 9.6 65.16
(4) 0.5 66.07
(5) 3.19 88.07
(Rubner)
When, on the other hand, instead of body weight, the area of the sur-
face of the body is taken as the basis of calculation, results that are almost
constant are obtained. Following are the results in the above animals on
this basis :
540
METABOLISM
Surface in. square cm.
(1) 10,750
(2) 7,662
(3) 5,286
(4) 3,724
(5) 2,423
Ileat production in calories
per square meter of sur-
face per day
1036
1097
1183
1153
1212
(Rubner)
Such results have prompted observers to conclude that the determining
factor in the calorie output of warm-blooded animals is the relative sur-
face of the animal. This is greater the smaller the animal, with the con-
sequence that heat is more rapidly lost to the surrounding air from the
surface, thus requiring more active combustion. Until quite recently it has
been generally believed that such a relationship between body surface and
heat production did actually exist, but, thanks to the work of F. G. Bene-
dict7 and E. F. and D. Du Bois6, it is now known that the calculations were
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Fig. 175. — Chart for determining surface area of man in square meters from weight in kilo-
grams (Wt.) and height in centimeters (Ht.) according to the formula: Area (Sq. Cm.) := Wt.
0.425 XHt. 0.725 X71.84. (From Dubois and Dubois, Arch. Int. Med., 1917, vol. 17.)
based upon incorrect computations of the body surface. In the older re-
searches the calculation was made by using a formula known as Meeh 's, in
which weight was multiplied by a certain factor (viz., 12.312 x T^Aveight).
Du Bois, however, has shown that an average error of 16 per cent is in-
curred in using this formula. For accurate measurement the body was
covered with thin underwear, which was then impregnated with melted
paraffin and reinforced with paper strips to prevent it from changing in
area when removed. This model of the surface was afterwards cut up
into flat pieces and photographed on paper of uniform thickness, the pat-
METABOLISM 541
terns being then cut out, and weighed. From the results it was easy to
calculate the actual surface area.
Where the height and weight are known, a fairly accurate computation
of the surface can be secured by using the following formulas : A="W°-425
XH°-725X71.84; A being the surface area in square centimeters; H the
height in centimeters; and W, the weight in kilograms. Based on this
formula, a chart has been plotted from which the surface area may be de-
termined at a glance (Fig. 175). Another method recently employed by
Benedict is based on measurements made from photographs of the subject
in various poses.
By the use of these more accurate measurements of body surface, it is
now known that, although the surface-area law gives us constant results
for the energy output of different individuals of similar build, and offers
us a much more accurate basis for comparing those of different laboratory
animals, yet it breaks down when applied to men in widely differing states
of body nutrition. Thus, in the case of a man who starved for a month,
the calorie output per square meter of surface decreased towards the end
of the fast by 28 per cent. Obviously, therefore, it would be incorrect to
draw conclusions regarding possible changes in energy output of a series of
emaciated or corpulent individuals by comparison of their calorie output
per square meter of surface with that of normal individuals.
The determining factor of energy output is undoubtedly the general
condition of bodily nutrition — the active mass of protoplasm of the body
(Benedict). That there is a relationship between the body surface and
metabolism is undoubted, but the relationship is not -a causal one. At
present, therefore, the only safe method to employ in comparing the
metabolism of normal and diseased individuals is that called by Benedict
"the group method," in which the metabolism of groups of persons of
like height and weight is compared, it being assumed that such individuals
have the same general growth relations. For the application of this group
method, however, more extensive data will be required than exist at pres-
ent, and although some of the conclusions drawn from results computed on
the surface-area basis may have to be revised, it is probable that they
are in general correct.
Influence of Age and Sex
The energy output is low in the newly born ; it increases rapidly during \
the first year, reaching a maximum at about three to six years of age, and
then rapidly declining to about twenty, after which' it declines much more
slowly. The decline in the earlier years does not proceed steadily, how-
ever, for at the period just preceding the onset of puberty a decided in-
crease becomes evident, indicating that at this period the metabolism of
542 METABOLISM
the growing organism is being stimulated. Females have a lower energy
output than males, and the stimulating influence of puberty is less marked
in them.
In round numbers, 40 C. per square meter of surface per hour is the
energy output of normal men, a 1.5 per cent deviation being considered
as decidedly abnormal. The average metabolism of fat and thin subjects is
the same, but that of women is 6.8 per cent lower than that of men. The
basal metabolism of a group of men and women between the ages of forty
and fifty was 4.3 per cent below the average for the larger group between
the ages of twenty and fifty; and that of a group between fifty and sixty
years was 11.3 per cent lower.
Influence of Diseases
The measurements have been made by the direct method which has just
been described, but since the much simpler indirect method (page 554)
yields comparable results, it is being adopted for clinical purposes. These
results were obtained by making parallel determinations of energy out-
put by both methods, in disease as well as in health. Some of the ob-
servations that have been made on the energy output in various diseases
are as follows: In very severe cases of exophthalmic goiter, heat produc-
tion may be increased by 75 per cent over the normal ; in severe cases, by
50 per cent. The warmth of the skin and the sweating, which are promi-
nent symptoms of this disease, are therefore accounted for by the in-
creased elimination of heat, and it is considered possible that the other
symptoms would be produced in any normal individual were his metabo-
lism maintained for months or years at the high level which it occupies in
goiter. In the opposite condition of myxedema, the energy output is
markedly reduced, but rises slowly during treatment with thyroid extract,
or much more rapidly with the very active thyroid hormone recently iso-
lated by Kendall. In diabetes it has often been thought that the rapid
emaciation and loss of strength were dependent upon an excited state of
metabolism, or a useless burning up of the energy material. The most
recent work, however, clearly shows that this is not the case, the basal
metabolism as calculated per unit of body surface being within, the limits
indicated above. During the starvation treatment the energv output may
be much below the normal. In uncompensated cases of cardiorenal dis-
ease, there is increased energy output. In pernicious anemia the metabo-
lism is normal, although in severe cases there may be an increased demand
for oxygen.
Even at the risk of repetition, it is important to point out that in all
these diseases the energy output is the same whether measured directly or
by the indirect method about to be described.
METABOLISM
543
THE MATERIAL BALANCE OF THE BODY
We must distinguish between the balances of the organic and the in-
organic foodstuffs. From a study of the former we shall gain information
regarding the sources of the energy production whose behavior under
various conditions we have just studied. From a study of the inorganic
balance, although we shall learn nothing regarding energy exchange —
for such substances can yield no energy — we shall become acquainted
with several facts of extreme importance in the maintenance of nutrition
and growth.
To draw up a 'balance sheet of organic intake and output requires an
accurate chemical analysis of the food and of the excreta (urine and ex-
pired air).
Methods for Measuring Output
The principle by which the output is measured will be understood by
referring to Fig. 176, from which it will be seen that the calorimeter
is connected with a closed system of tubes provided with an air-tight ro-
Water to absorb ha at
Hll
~ta-
Chamber for antmol
window
Fig. 176. — Diagram of Atwater-Benedict respiration calorimeter. As the animal uses up the O2,
the total volume of air shrinks. This shrinkage is indicated by the meter, and a corresponding
amount of O2 is delivered from the weighed O2-cylinder. The increase in weight of bottles
II and III gives the CO2; that of" I, the water vapor.
tary blower or pump to maintain a constant current of air, as indicated by
the arrows. Following the air stream as it leaves the chamber, we note
a side tube connecting with a meter to indicate changes in volume of the
544 METABOLISM
air in the system. Beyond this and the pump is a specially constructed
bottle containing concentrated H2S04, then one containing soda lime, and
lastly another H2S04 bottle. The first H2S04 bottle absorbs all the water
vapor contained in the air coming from the chamber; the soda lime bottle
absorbs the C02, and the second H2S04 bottle absorbs water that is pro-
duced in the chemical reaction involved in the absorption of the C02 by
the soda lime (2NaOH+C02=H20+Na2C08). By weighing these ab-
sorption bottles before and after an animal has been for some time in the
chamber, the weight of H20 and of C02 given out can be determined. An-
other side tube leads to an oxygen cylinder, the valve of which is manip-
ulated so as to cause oxygen to be discharged into the system at such a
rate as to compensate exactly for that used up by the animal, as indicated
by the behavior of the meter. The amount of oxygen required is de-
termined either by weighing the oxygen cylinder before and after the ob-
servation or by measuring the volume of oxygen used by passing it through
a carefully calibrated and very sensitive water meter inserted on the side
tube that connects the 02 cylinder with the main tubing of the system.
Since muscular activity causes pronounced changes in the rate of me-
tabolism, means are usually taken to secure graphic records of any move-
ments made during the observation.
The growing importance in clinical investigations of measurements of
the respiratory exchange and the necessity for having methods that are as
simple as is consistent with accuracy, have led to the introduction of
several other forms of apparatus, of which those of F. G. Benedict and of
Tissot* are the most important. In the former a tightly fitting mask,
applied over the nose and mouth is connected, by a short T-piece, with
the same tubing as that used in the respiration calorimeter. The patient
thus breathes in and out of the air stream that is passing along the tubing
without any of the obstruction experienced when the breathing has to be
performed through valves, as in the older (Zuntz) forms of portable
respiratory apparatus. It is particularly for studies on man that this
apparatus has been devised. The Tissot and Douglas methods are shown
in Figs. 179 and 180.*
To complete the investigation, it is necessary that the urine and feces
be collected and the nitrogen excretion measured. "When the respiratory
excreta are measured over a considerable period of time, as in the large
calorimeter, the urine is collected for the same period, but when shorter
respiratory measurements are made, the urine of the twenty-four hours
is usually taken.
Principles Involved in Calculating1 the Results. — Provided with the an-
alyses furnished by the above methods, we proceed to ascertain the total
"The Tissot method will be found described in full elsewhere (page 554).
METABOLISM 545
amounts of nitrogen and carbon excreted and to calculate from the
known composition of protein how much protein must have undergone
metabolism. We then compute how much carbon this quantity of pro-
tein would account for, and we deduct this from the total carbon excre-
tion. The remainder of carbon must have come from the metabolism of
fats and carbohydrates, and although we can not tell exactly 'which, yet
we can arrive at a close approximation by observing the respiratory quo-
tient (R. Q.), which is the ratio of the volume of carbon dioxide exhaled
CO
to that of oxygen retained by the body in a given time, i. e., n 2 . By ob-
<J?
serving this quotient, therefore, we can approximately determine the
source from which the nonprotein carbon-excretion is derived.
Having in the above manner computed how much of each of the proxi-
mate principles has undergone metabolism, we next proceed to compare
intake and output with a view to finding whether there is an equilibrium
between the two, or whether retention or loss is occurring.
It may serve to make clear the methods by which these calculations are
made to study the following example :
Example of a Metabolism Investigation. — It is desired to know whether a diet con-
taining 125 grams protein, 50 grams fat, and 500 grams carbohydrate is sufficient for a
man doing a moderate amount of work.
INTAKE
Carbon Nitrogen Calories
Protein, 62 gm. 20 gm. 512.5
Carbohydrate, 200 205D.O
Fat, 38 465.0
Total, 300 gm. 20 gm. 3027.5
OUTPUT
Carbon Nitrogen
In urine, 11 gm. (16.5x0.67) 16.5 gm.
In feces, 5 1.0
In the breath, 254
Total, 270 gm. 17.5 gm.
Eetained in Body. — 30 gm. carbon and 2.5 gm. nitrogen. This amount of nitrogen repre-
sents 2.5 X 6.25 z= 15.6 gm. protein or 75 gm. muscle. Now, this amount of protein will
account for 8.25 gm. carbon; so that 30 - 8.25 ;= 21.75 gm. carbon represents 21.75 X
1.3 = 28.3 gm. fat. On this diet, therefore, the subject retains in his tissues 15.6 gm.
protein and 28.3 gm. fat per diem.
Furnished with these data we may now proceed to compute how much
energy must have been liberated in the body.
To express the above result in terms of energy liberated, we know that
3027.5 C. were supplied and that all these have been used except 15. 6 X
4.1=64 retained as protein, and 28.3X9.3=263.2 retained as fat; or in
tolo 327.2 C. We find, therefore, that 3027.5 — 327.2 = 2,700 C. have been
required.
546 METABOLISM
This is called the method of indirect calorimetry, and it has been clearly
established by numerous observations that the results agree exactly with
those secured by the method of direct calorimetry described above. For
most purposes the indirect method is quite satisfactory, and it is espe-
cially valuable in cases in which there are considerable and sudden
changes in body temperature. That the results by the two methods should
agree shows clearly that the law of the conservation of energy must apply
in the animal body, for it is evident that if any energy were derived from
outside the body other than that taken with the food, the results by the
direct method would be higher than those by the indirect.
CHAPTER LXI
THE CARBON BALANCE
Before proceeding to discuss the special metabolism of proteins, fats
and carbohydrates, it will be advantageous to consider briefly some gen-
eral facts concerning the excretion of carbon dioxide and the intake of
oxygen. In the first place, it is important to note that the extent of the
combustion process in the animal body is proportional to the amount of
oxygen absorbed and of carbon dioxide produced, whereas the nature of
the combustion is indicated by the ratio existing between the amounts of
carbon dioxide expired and of oxygen retained in the body. An investi-
gation of the carbon balance, in other words, is partly quantitative and
partly qualitative — quantitative in the sense that it indicates how in-
tensely the body furnaces are burning, and qualitative in the sense that
it tells us what sort of material is being burned at the time.
THE RESPIRATORY QUOTIENT
Influence of Diet. — The respiratory quotient is determined by com-
parison of the volume of carbon dioxide expired with the volume of oxy-
gen meanwhile retained in the body or, as a formula,
Vol. C02 expired
Vol. 02 retained
For the sake of brevity the respiratory quotient is often written R. Q. That
it serves as an indicator of the kind of combustion occurring will be evi-
dent from the following equations:
I. Carbohydrate : C(.H]2O6 + 6O2 = 6CO2 + 6H2O
(Dextrose.)
2. Fat: C,Hr(C,;H.,3O2)34- 80O2 = 57CO2 + 52H..O
(Olein.)
3. Protein : C.2H112N]8O,2S + 77O2 — 63CO2 + 38H.O + 9CO (NH,) 2 + SO3
[Empirical formula for
albumin (Lieberkiihn).]
... E.Q>== «W«|_=o.82
547
548 METABOLISM
4. Conversion of fat into carbohydrate:
2G,H6(018HJS01), + 6402 = 16C6H12O6 + 18CO2 + 8H2O
(Olein.)
5. Conversion of carbohydrate into a mixed fat:
13C6H12O6 = C55H104O6 + 23CO2 + 26H2O.
( Oleostearopalmit in. )
Taking carbohydrates first, the general formula may be written CH20,
from which it is plain that, to oxidize the molecule, oxygen will be re-
quired to combine with the carbon alone, according to the equation,
CH20 + 02 = C02 + H20. In other words, the volume of carbon dioxide pro-
duced by the combustion will be exactly equal to the volume of oxygen
used in this process, in obedience to the well-known gas law that equi-
molecular quantities of different gases occupy the same volume. The
respiratory quotient is therefore unity (Equation 1). With fats and pro-
teins, however, the general formula must be written CH2-f-0, indicating
therefore that for its complete oxidation the molecule must be supplied
with oxygen in sufficient amount to combine not only with all of the car-
bon, but also with some of the hydrogen, forming water; so that the vol-
ume of C02 produced will be less than the volume of oxygen retained,
and the respiratory quotient will be less than unity. As a matter of fact,
as the above equations show (2 and 3), the respiratory quotient for fats
and proteins lies somewhere between 0.7 and 0.8, being usually nearer
0.7 in the case of fats, and nearer to 0.8 in the case of proteins.
That the conditions hypothecated in the equations exist in the animal
body during the combustion of the foodstuffs can easily be shown by ob-
serving the respiratory quotient of animals on different diets. An her-
bivorous animal, such as a rabbit, when it is well fed gives invariably a
respiratory quotient of about 1, whereas a strictly carnivorous animal,
such as the cat, gives a respiratory quotient of about 0.7. Even more
striking perhaps is the comparison of the respiratory quotients in an
herbivorous animal while it is well fed and after it has been starved for a
day or two. In the latter case the respiratory quotient will fall to a low
level because, by starvation, the animal has been compelled to change its
combustion material from the carbohydrate of its food to the protein and
fat of its own tissues.
As already explained (page 545), it is from the respiratory quotient
that we are enabled to tell what proportions of fat and carbohydrate,
respectively, are undergoing metabolism. A useful table showing the
percentage of calories produced by each of these foodstuffs, after allow-
ing for protein, is given by Graham Lusk (see page 565).
THE CARBON BALANCE 549
Influence of Metabolism. — Apart from diet, the respiratory quotient
may often be altered by changes in the metabolic habits of the animal.
These are most conspicuously exhibited in the case of hibernating
animals. In the autumn months, when the animal is eating voraciously
of all kinds of carbohydrate food and depositing large quantities of
adipose tissue in his body, the respiratory quotient may be considerably
greater than unity, indicating therefore either that relatively more
carbon dioxide is being discharged or less oxygen retained. As a matter
of fact, it can easily be shown that it is the former of the causes that
is responsible for the higher quotient, the explanation for the increased
production of C02 being that, as the carbohydrate changes into fat, the
relative excess of carbon in the former is got rid of as C02, as indicated
in Equation 5. On the other hand, if the animal is examined while in
his winter sleep, it will be found that the respiratory quotient is now
extremely low, often not more than 0.3 to 0.4, which may be interpreted
as indicating either an excessive absorption of oxygen or a markedly
decreased excretion of carbon dioxide. As a matter of fact, there is a
great diminution in both the excretion of carbon dioxide and the intake
of 02, because the whole metabolic activity of the animal is extremely
depressed, but this diminution affects the oxygen to a much less degree,
indicating therefore a relative increase in the oxygen retention. The
explanation is that the oxygen is being used in the chemical process in-
volved in the conversion of the- fat back into carbohydrate.
Whatever may be the relationship between fat and carbohydrate in
the nonhibernating animal, there is no doubt that during hiberna-
tion, before the fat stores are burned, fat is converted into something
closely related to carbohydrates, the equation for the process being rep-
resented as given above (No. 4).
In man and the higher mammalia, the only condition, apart from diet
which can affect the nature of the combustion process is disease; thus
in total diabetes (page 678) the organism loses, the power of burning
carbohydrate, so that whatever the diet may be, the respiratory quotient
is very low, never higher than that representing combustion of fat and
protein. It has been claimed by certain investigators that in diabetes
the respiratory quotient may fall considerably below 0.7, indicating, as
in hibernating animals, that fat is being converted into carbohydrate.
The most recent and carefully controlled observations, however, deny
this claim, and for the present we must assume that in the body of man
fat is not converted into carbohydrate (see page 664). In numerous other
diseases investigated by.Du Bois and others6 no qualitative change in
the combustion processes in man has been brought to light.
550 METABOLISM
THE MAGNITUDE OF THE RESPIRATORY EXCHANGE
It is evident that the amount of carbon dioxide expired and of oxy-
gen retained will be proportional to the energy liberation in the animal
body. Even at the risk of repetition it should be noted that the
energy exchange can be very accurately calculated from the result of
the material balance sheet — indirect calorimetry, as it is called (page
562). On account of the comparative simplicity of measuring the carbon
dioxide output and oxygen intake, it is natural that many of the obser-
vations that have been made on energy production in the animal body
depend on the use of this method, justification for which is found in the
complete agreement between the results of direct and indirect calorim-
etry in a great variety of diseases and conditions in man (Du Boisr>).*
In the first place, it is interesting to compare the respiratory ex-
changes of different animals computed per kilo body weight. This is
shown in the following table.
ANIMAL
WEIGHT
GM.
OXYGEN AB-
SORBED PER KILO
AND HOUR
GM.
CARBON DIOXIDE
DISCHARGED
PER KILO
AND HOUR
GM.
VOL. C02
TEMPERA-
TURE OF
AIR
VOL. 02
Insecta
Field cricket
0.25
2.305
—
—
Amphibia
Edible frog
0.0(5:?
0.060
0.69
15°-19°
(44.2 c.c.)
(30.76 c.c.)
0.105
0.1134
0.78
—
(73.4 c.c.)
(57.7 c.c.)
Aves
Common hen
1280
1.058
1.327
0.91
19°
.
900 ocn
(740 c.c.)
(675 c.c.)
q oq<*
A igeon
Sparrow
^O^-OoU
22
9.595
O.43O
10.492
0.79
18°
(6710 c.c.)
(5334.5 c.c.)
"Mammalia
Ox
6,18,000
0.389-0.485
—
—
660,000
Sheep
66,000
0.490
0.671
0.99
16°
(343 c.c.)
(341 c.c.)
Dog
6213
1.303
1.325
0.74
15°
(911 c.c.)
(674 c.c.)
Cat
2464
1.356
1.397
0.75
-3 2°
3047
(947 c.c.)
(710 c.c.)
> >
0.645
0.766
0.86
29.6°
(450 c.c.)
(389 c.c.)
Rabbit
1433
1.012
1.354
0.97
18°-20°
Guinea pig
444.9
1.478
1.758
0.86
22°
Rat (white)
80.5
3.518
7°
(1789 c.c.)
ATousp ^ ^
«?PC
"8.4
17°
Man
CHJ
66,70
0.292
0.327
—
(Modified from Pembrey.)17
*For the convenience of those who may desire to know more about the methods of analysis
that are suitable in the clinic, a chapter on the subject will be found beginning on page 554.
THE CARBON BALANCE 551
Several factors operate to explain these differences, and of these the
following are of importance:
1. The Body Temperature. — Increase in body temperature entails in-
creased combustion. This explains why the metabolism of a bird is
greater than that of a mammal of the same size, for, as is well known, the
temperature of a bird is two or three degrees centigrade above that of
other animals. Rise in body temperature also explains, in part at least,
the increased metabolism observed in fever.
2. The Temperature of the Environment. — In considering this we must
distinguish between the effect produced on warm-blooded and on cold-
blooded animals. Since the body temperature of a cold-blooded animal
is only one or two degrees Centigrade above that of its environment, it
follows that the metabolic activity will be directly proportional to the
temperature of the latter. In a warm-blooded animal, on the other hand,
the body temperature remains constant Avhatever changes may occur
in that of the environment, this constancy of body temperature being
dependent on the fact that the intensity of the combustion processes is
inversely proportional to the cooling effect of the atmosphere. Thus,
suppose the external temperature should fall, then the loss of heat from
the body will tend to become greater, and to maintain the body tempera-
ture at a constant level, the body furnaces must burn more briskly, with
the result that an increased excretion of carbon dioxide and intake of
oxygen will occur.
This influence of the surrounding atmosphere on the metabolic activ-
ity of w-arm-blooded animals has, as already pointed out, been used by
several investigators to explain the greater combustion per kilo body
weight of small as compared with large animals. The argument is that,
since the surface of small animals relatively to their mass is much greater
than in large animals, the cooling of the small animals will be proportion-
ately greater. The relationship between surface and mass is shown by tak-
ing two cubes and putting them together; the mass of the two cubes is
equal to double that of either cube, whereas the surface is less than
double, since two aspects of the cubes have been brought together. To
prove the contention, the respiratory exchange has been computed per
square meter of surface instead of per kilo body weight, with the result
that a very close correspondence in the metabolism of different animals
has been observed ; but this question has already been discussed, and we
now know that the law of cooling can not be the only one that determines
extent of the respiratory exchange (see page 541).
3. Muscular Exercise. — This has a most important influence on the ex-
change and it is particularly in connection with it that studies in carbon-
dioxide output and oxygen intake have been of great practical value, par-
552 METABOLISM
ticularly when the investigations are undertaken on men doing ordinary
types of muscular exercise, such as walking or climbing. It is true
that the influence of muscular exercise on the energy metabolism may
also be studied by having a person in the calorimeter do exercises on an
ergometer, but the results thus obtained are in many ways not nearly so
valuable as those which can be secured by observing the respiratory
exchange of persons doing ordinary types of muscular exercise in the
open. The folloAving table of observations on horses is of interest in this
connection.
CONDITION
AIR EXPIRED
CARBON DIOXIDE
OXYGEN ABSORBED
C02
IN LITERS
DISCHARGED IN
IN LITERS PER
02
PER MINUTE
LITERS PER
MINUTE
MINUTE
Best
44
1.478
1.601
0.92
Walk
177
4.342
4.766
0.90
Trot
333
7.516
8.093
0.93
It will be observed that the metabolism increases extraordinarily for
even a moderate degree of work, but that at the same time the respiratory
quotient remains constant. From observations on the respiratory ex-
change of working men and animals, extremely important facts concern-
ing the efficiency of muscular work have been secured. The form of
respiratory apparatus (Zuntz or Douglas) employed for this purpose
must be capable of being strapped on the man's back without causing
any embarrassment to his bodily movements. By a comparison of the
respiratory exchange with the amount of work done, the efficiency of the
work can readily be determined. It has been found, for example, that
the efficiency is much greater after the man or animal has got into the
swing of the work, his energy expenditure per unit of work being much
greater during the first half hour's work in the morning than it is
later on. This indicates that after a little practice the muscles can ex-
ecute a given movement and perform a given amount of work much
more smoothly than when they are not in training. Another interesting
outcome of the investigations has been to show that work done under ab-
normal conditions that tend to produce any kind of muscular strain is
done inefficiently. It has been found in marching soldiers, for example,
that the slightest abrasion of the foot greatly increases the energy
expenditure, for the man, in trying to avoid the pain produced by the
abrasion, brings into operation muscular groups that are really not
required for the efficient performance of the movement, but are used
instead to avoid pressure on the sore. Fatigue also causes inefficient
performance of work; that is to say, the fatigued person, on attempting
THE CARBON BALANCE 553
the same amount of work as he performed before becoming fatigued,
will do so at a much greater expenditure of energy.
There is a diurnal variation in the respiratory exchange, which is in
general parallel with the body temperature ; it rises during the day, the
time of activity and work, and falls during the night, the time of rest
and sleep. Food also affects respiratory exchange, but it will be unnec-
essary to go into this further after what has been said on page 547.
CHAPTER LX1I*
A CLINICAL METHOD FOR DETERMINING THE RESPIRATORY
EXCHANGE IN MAN
BY R. G. PEARCE, B.A., M.D.
Principle. — Since the determination of the respiratory exchange in
man is of some importance in the study of certain diseases of the respira-
tion, circulation and metabolism, and also because directions for carry-
ing out the necessary procedures are not generally available, we have
thought it might be of assistance to include here brief directions for the
Tissot and the Douglas methods. These methods have been found to
compare favorably in accuracy with others in use at present, t and be-
cause of their adaptability and simplicity they are specially suited for
clinical work.
By these methods the energy metabolism of the body is calculated from
oxygen consumption or carbon dioxide excretion per minute (indirect
calorimetry) (page 546), the figures for which are determined from the
volume and percentile gaseous composition of the expired air.
The subject breathes through valves which automatically partition the
inspired and expired air. The expirations from a number of respirations
are collected in a spirometer or bag, and the volume of the respirations
per minute is determined. The gaseous composition of the expired air
is determined by gas analysis, and the oxygen consumption and energy
output of the body are calculated from the data obtained.
Description and Use of Parts of the Apparatus: 1. THE MOUTHPIECE
AND VALVES. — The mouthpiece is made of soft pure gum rubber, and con-
sists of an elliptical rubber flange having a hole in the center 2 cm. in
diameter, to which on one side a short rubber tube is attached. On the
opposite side of the hole, at right angles to the rubber flange, are at-
tached two rubber lugs. The rubber flange is placed between the lips,
and the lugs are held by the teeth. The rubber tube of the mouthpiece is
connected to the tube carrying the valves. The nose must be tightly
closed if mouth breathing is used. This is accomplished by a nose clip,
which consists of a V-shaped metal spring, the ends of which are pro-
vided with felt pads. A toothed rachet is attached to the ends of the
*This chapter is added for the convenience of workers in this subject.
tCarpenter: Carnegie Institution of Washington Reports, No. 216, 1915.
554
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN
555
spring, and serves to hold the spring tightly clamped on the nostrils in
the proper position (see Fig. 177).
Some individuals experience great distress when made to breathe
through the mouth. For these it is best to use a face mask. Unfortu-
nately at the present time no mask is entirely satisfactory. Perhaps the
best is one sold by Siebe, Gorman & Co.,* which is pictured in the cut.
Fig. 177. — A, Nose clip; B, Face mask; C, Mouth piece.
After being placed in position the face mask should be tested for leaks,
which can be done by putting soap around the edges.
2. THE VALVES. — The valves of Tissot are probably the best for the
purpose, but they are expensive and difficult to obtain. We have made
perfectly satisfactory valves from the prepared casings used in the
manufacture of bologna sausage. These can be obtained preserved in
salt, and they will keep indefinitely on ice. When needed a short piece
*This mask has been used extensively by Carpenter. The agent in this country is II. X. Klmer,
1140 Monadnock Bldg., Chicago.
556
METABOLISM
is taken, washed free from salt by allowing water from the tap to run
through it, and softened in a weak glycerine solution. The gut becomes
very soft and pliable, and does not dry quickly. A piece of the casing
about 10 cm. long is threaded through a glass tube of about 15 mm. bore
and 4 to 6 cm. long. One end of the casing is brought around the outside
of the tubing and secured by means of a thread. The lower end of the
membrane is pinched off and the casing is then cut a little more than
half way across its middle, so that the opening will lie just within the
free end of the tube when the casing is drawn back through it. The
loose end of the casing is slightly twisted — an essential procedure — and
is then secured by a thread on the outer side of the tube. If properly
made, the valve will work freely without vibration, and the opening be
sufficiently large to allow a good current of air to pass. It should col-
lapse instantly and be air-tight when the current of air is reversed. The
back lash, or lag of closure, of these valves is extremely small, and
they will open or close with a pressure of air not exceeding the pressure
Fig. 178. — Diagram of respiratory valves.
changes in normal respiration. When not in use, the valves should be
kept in glycerine water on ice. Valves prepared in this way have been
in use a month without loss of efficiency. They are, however, made with
so great ease that new valves are provided for each subject, and they are
therefore especially adapted to ward work (Fig. 178).
The valves are inserted in reverse order into a supporting metal
T-piece, and the joints made air-tight by tape. The stem of the T is
connected with the mouthpiece. Through a rubber tube of about 3/4
inch bore, the expired air is collected in the spirometer, or Douglas Bag.
3. THE TISSOT SPIROMETER is pictured in Fig. 179. We have found the
100-liter size to be very serviceable in the clinic. This instrument is
mounted on a platform having rubber wheels, and can be moved about
the wards with ease. The bell of the spirometer is made of aluminum
and is suspended in a water-bath between the double walls of a hollow
cylinder made of galvanized iron. The height of the bell is 72 cm.
and the diameter 42 cm. An opening at the bottom of the
cylinder connects through a three-way stopcock with the rubber tube
leading from the expiratory valve of the mouthpiece (see Fig. 177).
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN
557
The bell is counterpoised by means of a weight. In the original Tissot
spirometer an automatic adjustment permitted water in amount equal
to the w,ater displaced by the bell to flow from the spirometer cylinder
into a counterpoise cylinder as the bell ascended out of the water.
Fig. 179. — The Tissot spirometer. In actual experiment, subject is reclining or lying down and
the valves and mouthpiece are held with a clamp.
The bell, being heavier out of water than when it is immersed, is accord-
ingly counterpoised in any position, although Carpenter has shown that
this refinement is unnecessary. An opening in the top of the spirometer
permits the insertion of a rubber stopper, through which are passed a
thermometer, a water manometer, and a stopcock with tube for drawing
558
METABOLISM
the sample of air. A scale on the side of the instrument gives the vol-
ume of the air.
During an observation the subject sits in a reclining position or lies
upon a couch. When the bell of the spirometer is placed at zero, the
mouthpiece adjusted in the mouth, and the nose clamped, respiration is
started, the expirations being passed through the stopcock, which is
so turned as to allow them to pass to the outside air. After a few
minutes the stopcock is turned so that the expirations are passed into
Fig. 180. — The Douglas bag method for determining the respiratory exchange. The arrange-
ment of mouthpiece, valves, and connecting tubes shown here has been found to be more con
venient than that recommended by Douglas.
the spirometer for a definite length of time. At the end of the period
the cock is again turned, and after the barometric pressure, temperature,
and volume of the air have been noted, the composition of the air is
determined in the Haldane gas analysis apparatus.
4. THE DOUGLAS BAG. — The Douglas bag is made of rubber-lined cloth,
and is capable of holding from 50 to 100 liters. It is especially useful
for investigations during exercise, since it is fitted with straps so that
the bag can be fastened to the shoulders (Fig. 180). It is then connected
with the A7alves, the mouthpiece of which is placed between the lips.
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN
559
Respirations are commenced with the three-way valve turned so as to
allow the expirations to pass directly outside. After respiratory equi-
librium is established, the three-way valve is turned during an inspira-
tory period so that the succeeding expirations may pass into the bag.
The time required to fill the bag comfortably is determined with a stop-
watch. The air which has been collected in the bag during the period
is thoroughly mixed and passed through a meter, the temperature and
barometric pressure are noted, and a sample analyzed in the Haldane
A. B.
Fig. 181. — Haldane gas apparatus {A) and Pearce sampling tube (B).
gas-apparatus. The bag should be emptied completely by rolling it up
when nearly empty.
5. The Haldane Gas-analysis Apparatus. PRINCIPLE. — The Haldane
method of analysis of expired air is simple and easily learned. The ap-
paratus (Fig. 181) consists of a gas burette, a control burette of the
same size (both surrounded with a water jacket), and bulbs containing
dilute caustic potash or soda solution for the absorption of the carbon di-
oxide and an alkaline pyrogallate solution for the absorption of the
560 METABOLISM
oxygen. The gas burette is connected with the bulbs by a two-way
stopcock, which allows a sample of gas to pass into either bulb. A con-
trol tube (10) is put into connection with the burette through a manometer
tube, which is connected with the alkali bulb, and can be made to com-
pensate for any changes in temperature that may occur during the course
of the analysis. For an analysis the gas is transferred to the burette
from the sampling tube, saturated with water vapor over mercury, and
then measured, after which it is transferred into the caustic solution to
free it from C02, and returned to the burette to determine the loss of
volume due to C02 absorption. It is then transferred into the alkaline
pyrogallate solution, which frees it from oxygen, after which it is again
brought back to the burette to determine the loss in volume due to the
absorption of the oxygen.
THE APPARATUS. — The detail of the Haldane apparatus is shown in
the accompanying cut. The measuring burette (1) holds 21 c.c. The bulb
is of 15 c.c. capacity, and the graduated stem, which is about 4 mm. in
bore and 60 cm. in length, is graduated to 0.01 c.c. from 15 c.c. to 21
c.c. The stopccck at the top of the burette is double-bored, so that in
one position air can be drawn in from a gas sampler (2) and in another
sent into the absorption bulbs (3). The lower part of the burette ex-
tends through the rubber cork at the bottom of the water jacket (4).
A piece of rubber tubing is attached to the bottom of the burette and
is passed through a metal tube, furnished- on its inside with a metal disc
which presses against the rubber tubing, the pressure being controlled by
means of a fine adjusting screw (6). Below this a glass stopcock (7) con-
nects with rubber tubing to the mercury leveling bulb (5). The absorption
bulb for C02, containing 20 per cent NaOH or KOH (9), is put in con-
nection with the burette by suitably turning stopcocks (3 and 8).* The
control burette (10) is also in connection with this bulb through the
manometer tube (11). t Any variation in temperature which may occur
during the analysis will cause the level of the alkaline solution in the
manometer to change.
When final readings of the shrinkage of volume are made, the level of
the caustic solution is returned to the level of that in the manometer.
By so doing any error due to temperature changes is avoided, since
change in temperature must be equal in the two burettes.
The absorption bulb for oxygen (12) is filled with a solution made by
dissolving 10 grams of pyogallic acid in 100 c.c. of a nearly saturated
KOH solution. The specific gravity of the KOH should be 1.55, which is
obtained approximately by dissolving the sticks (pure by alcohol) in an
*The. stopcock (8) is double-bored, so that the tube leading from the burette can be brought into
connection with either 9 or 12.
tThis tube also has a three-way stopcock (19), so that it may be opened to the outside.
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 561
equal weight of water. The mark (13) on the stem of the bulb indi-
cates the level at which the solutions should stand. Enough pyrogallate
solution is introduced through tube 15 to fill bulbs 12 and 14 two-thirds
full. Then pyrogallate solution is poured into tube 16 until the* differ-
ence in level of the fluids is sufficient to produce enough pressure to
raise the level of the pyrogallate solution in 12 to the level 13 on the
stem. Stopcock 8 must be open during this procedure. It may be neces-
sary to add or take away a little pyrogallate solution through 15 to at-
tain the above level.
Care must be taken to allow for complete absorption of oxygen from
the air that is entrapped between 14 and 16 before an analysis is made ;
otherwise changes will be produced in the level of the pyrogallate solu-
tion. The air in the capillary tubing connecting the burettes with the
absorption bulbs must also be freed of C02 and 02. This can be accom-
plished by making a dummy analysis of atmospheric air before the real
analysis. Great care must be taken to have atmospheric pressure in all
the tubes at the start of the analysis. This is accomplished by opening
the stopcock in the burette first to atmospheric air and then to the ab-
sorption bulbs, until -no further change in the level of the fluids in the
stems of the absorption bulbs occurs. This level is then marked and
used as the standard. A small amount of water in the burette over the
mercury assures saturation of the air with water vapor. Time for drain-
age must be allowed before making readings.
A very serviceable sampling tube for the transfer of air can be made
from a 30 c.c. ground-glass syringe, to which is attached a two-way
stopcock. A cut of this is shown in Fig. 181. The dead space in these
syringes is washed out by working the piston back and forth several
times. A thin coating of vaseline prevents leakage of the gas. We have
found that these sampling tubes will retain a sample of expired air with-
out change up to eight hours.
MANIPULATION OF APPARATUS. — The sampling syringe (20) is attached
to opening 2 of the burette, and its stopcock ( 17) opened to atmospheric air.
The level of the mercury is raised to the level of the stopcock of the syringe
and is then turned so that syringe and burette are in communication. The
bulb of mercury is lowered so that the mercury falls in the burette. This
draws the piston of the syringe with it, and fills the burette with air
from the syringe. It is advisable to put a little positive pressure on the
piston of the syringe in the maneuver to prevent possible leakage. When
all of the air is in the burette a slight positive pressure is produced in
the burette by gently pressing on the piston, and immediately there-
after the stopcock on the syringe (17) is again turned to the original
position. This allows the pressure of air in the burette to come to that
562 METABOLISM
of the atmosphere. The height of the mercury is now adjusted to a con-
venient height in the burette by closing cock 7 and turning the milled
screw 6. The cock 18 is now made to communicate with the absorption
bulbs. -If the air in the burette is at atmospheric pressure, no change
will occur in the level of the .fluids. The reading is then taken on the
burette.
The next step in the analysis consists in turning stopcock 8 to com-
municate with the caustic soda solution in bulb 9, and the leveling tube
(5) is raised, forcing mercury into the burette and the air into bulb 9.
The gas is passed back and forth several times until absorption is com-
plete, as can be determined by the fact that the level of the mercury in
the burette remains constant when the fluid in the bulb is returned to
its original level (13} on the stem. In this adjustment it is convenient
to make the gross leveling by the mercury bulb and the fine leveling by
closing 7 and turning 6 until the fluid in 9 is at the original height.
The reading on the burette indicates the loss in volume due to the C02
absorbed.
The oxygen is removed by a similar procedure, the gas being passed
into the alkaline pyrogallate solution by turning cock 8 to communicate
with bulb 12. The absorption of oxygen is slower than for C02, and
more care must be taken to get complete absorption. The air in the
tubing between the fluid in 9 and stopcock 8 must be washed out sev-
eral times in order to get the oxygen which is left in it after the absorp-
tion of the C02. When this is complete, the final reading on the burette
is made and the loss in volume from the second reading represents the
oxygen.
THE CALCULATIONS
The calculation of the percentile composition of the air and of the re-
spiratory quotient is represented in the following example of an actual
analysis:
(The temperature and barometric pressure as taken at the time of the
experiment were 20° C. and 747 mm. Hg.)
CO, analysis —
1st reading of burette 20.00
2nd reading of burette after absorption of CO, 19.20
CO2 absorbed 0.80
0.80 -4- 20 = 4.0 per cent CO2 in expired air.
02 analysis —
2nd reading of burette 19.20
3rd reading of burette after absorption of O2 15.90
O2 absorbed 3.30
3.30 -=- 20 = 16.50 per cent of 02 in expired air.
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 563
Determination of E.Q. —
O2 in atmospheric air — 20.94%
O2 - CO2 in expired air (1(5.50 + 4) = 20.50%
100-20.94 = 79.06%," N in atmospheric air.
100-20.50 = 79.50%, N in expired air.
Since the nitrogen is not changed in volume, the last figure shows that
more oxygen must have been taken in during inspiration than 02 + C02
has been given back in expiration. This obviously must 'be taken into
account in the calculations. The amount of 02 actually inspired for each
100 c.c. of air expired is found as follows:
20.94 (% O2 in atmospheric air)
79.06 (% N2 in atmospheric air) X 79'50 <% N' in exPired air) > or °'265 (con
stant factor X 79.5 (% N found for this observation) =21.07, the volume of O2 which
would have been present in expired air to account for N present. t
21.07-16.50 = 4.57% O2 actually absorbed.
4.00 - 0.03 (CO2 in inspired air) = 3.97% CO2 excreted.
3.97
' — 0.87, the respiratory quotient, or ratio of CO, excreted to O., absorbed.
Total Gas Exchange. — The volume of air expired in 15 minutes into
the Tissot spirometer was found to be 100 liters measured at 20° C. and
747 mm. Hg (brass-scale barometer). This volume of gas must be cor-
rected so as to give the volume of dry air at 0° and 760 mm. Hg. To do
this two things must be taken into account. (1) Since the expired air is
saturated with water, the pressure due to water vapor must be subtracted
from the observed barometric pressure to obtain the true pressure. The
vapor tension of water for various temperatures is given in Table II
on page 564.. (2) The barometer tube lengthens, or contracts with heat
or cold, and therefore the barometric readings must be corrected.
The corrections for ordinary barometric readings are found in Table III,
page 565. The figure corresponding to the temperatures is subtracted
from the barometric reading in order to obtain correct barometric pres-
sure.
In the above experiment, the correction for the barometer is 2.41 mm.
(see Table III, page 565), and that for vapor tension at 20° C. is 17.4
(see Table II, page 564).
Actual Barometric Pressure.— 747 - (17.5 + 2.39) = 727.21 mm. The
coefficient of expansion of gases is taken as 0.003665) or 1/273 ; therefore
the volume of 0° equals the volume at 1° divided by 1-0.003665 t; and
hence
"This is the constant O percentage :n air.
tThis calculation can be simplified by using an abbreviated table (page 564) giving the O» figure
corresponding to the various percentages of N in the expired air.
564 METABOLISM
Vx273 V
V° = 273+T=l + 0.003665 t ' When V° =
VP
The volume of gas being inversely as the pressure, Vo — _ _ , where V = volume at
760
P pressure; or working both corrections together,
VPx273 VP
Vo_
760 x (273 + t)~760 (1 + 0.003665 t)
This formula applied to the present problem reads:
Vo = 100 x 727.2 _ _ = 89.2 liters.
760 (1 + 0.003665x20)
The latter calculation can be considerably simplified by using standard
tables which give constants for corrections of gas volumes. These are
easily obtainable and are given in part in Table IV.
According to these tables for 20° C. and 727.21 mm. Hg B.P., the
factor is 0.89124; therefore:
0.89124 x 100 — 89.124 liters, 0°C. and 760 mm. Hg.
0.89124x4.57 = 40.7 liters ^of O2 in 15 min., or 16.28 L. per hour.
The Caloric Value Calculated from the Gas Exchange. — By reference
to Table V giving the heat value of 1 liter of 02 at various respiratory
quotients, it is found that at a R.Q. of 0.87, 4.888 calories are expended ;
16.28 liters of 02 is therefore equivalent to 18.4 x 4.888 = 79 calories.
The results must be calculated for surface area as well as body weight.
Suppose the subject weighed 85 kg. and was 170 cm. in height; by refer-
ence to the chart for determining the surface area of man (page 540),
this would be found to be 1.96 square meters. The caloric expenditure
79
per square meter in the above case is therefore ~~ = ^
TABLE I
THE PERCENTAGE OF OXYGEN WHICH is EQUIVALENT TO THE NITROGEN FOUND IN THE
EXPIRED AIR
To obtain the nitrogen in the expired air, add the percentage of CO2 and O2 found
and subtract the sum from 100. The table gives the percentage for O2 corresponding to
this figure:
78.7 78.8 78.9 79.0 79.1 79.2 79.3 79.4 79.5 79.6 79.7 79.8
%O, 20.86 20.88 20.90 20.93 20.96 20.98 21.01 21.04 21.07 21.10 21.12 21.14
79.9 80.0 80.1 80.2 S0.3 80.4 80.5 80.6
21.16 21.19 21.22 21.25 21.28 21.31 21.35 21.38 _
TABLE II
TENSION OF AQUEOUS VAPOR IN MILLIMETERS OF MERCURY
To obtain the dry barometer pressure, subtract the mm. Hg. corresponding to the
temperature of the air from the barometer pressure at the time of the experiment:
Temp. 15° 16° 17° 18° 19° 20° 21° 22° 23° 24° 25°
Mm. 12.7 13.5 14.4 15.4 16.3 17.4 18.5 19.7 20.9 22.2 23.5
METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 565
TABLE III
TEMPERATURE CORRECTIONS TO EEDUCE EEADINGS OF A MERCURIAL BAROMETER WITH A
BRASS SCALE TO 0°C.
Subtract the appropriate quantity as found in table from the height of the barometer.
The table is for a barometer with a brass scale, and the values are a little lower (about
.2 mm.) than for the glass scale. The corrections for intermediate temperatures can be
approximated.
Temp.
700
mm.
710
mm.
720
mm.
730
mm.
740
mm.
750
mm.
760
mm.
770
mm.
15°
20°
25°
1.69
2.26
2.83
1.72
2.22
2.87
1.74
2.32
2.91
1.77
2.36
2.95
1.79
2.39
2.99
1.81
2.42
3.03
1.84 .
2.45
3.07
1.86
2.48
3.11
TABLE IV
TABLE FOR REDUCING GASEOUS VOLUMES TO NORMAL TEMPERATURE AND PRESSURE
The observed volume, when multiplied by the factor corresponding to the temperature
and pressure, will give the volume of the expired air reduced to 0° and 760 mm.
Mm.
15°
16°
17°
18°
19°
20°
21°
22°
23°
24°
25°
720
730
740
750
760
770
.898
.910
.922
.935
.947
.960
.894
.907
.919
.932
.944
.957
.891
.904
.916
.928
.941
.953
.888
.901
.913
.925
.938
.950
.885
.897
.910
.922
.934
.948
.882
.894
.907
.919
.931
.945
.880
.891
.904
.916
.928
.940
.877
.888
.901
.913
.925
.936
.873
.885
.897
.910
.922
.933
.870
.882
.894
.907
.919
.930
.867
.879
.891
.904
.916
.927
R. Q. CALORIES FOR 1 LITER O2 KELATIVE CALORIES CONSUMED AS
Number Carbohydrate Fat
per cent per cent
0.707
4.686
0
100
0.71 '
4.690
1.4
98.6
0.72
4.702
4.8
95.2
0.73
4.714
8.2
91.8
0.74
4.727
11.6
88.4
0.75
4.739
15.0
85.0
0.76
4.752
18.4
81.6
0.77
4.764
21.8
78.2
0.78
4.776
25.2
74.8
0.79
4.789
28.6
71.4
0.80
4.801
32.0
68.0
0.81
4.813
35.4
64.6
O.S2
4.825
38.8
61.2
0.83
4.838
42.2
57.8
0.84
4.850
45.6
54.4
0.85
4.863
49.0
51.0
0.86
4.875
52.4
47.6
0.87
4.887
55.8
44.2
0.88
4.900
59.2
40.8
0.89
4.912
62.6
37.4
0.90
4.924
66.0
34.0
0.91
4.936
69.4
30.6
0.92
4.948
72.8
27.2
0.93
4.960
76.2
23.8
0.94
4.973
79.6
20.4
0.95
4.985
83.0
17.0
0.96
4.997
86.4
13.6
0.97
5.010
89.8
10.2
0.98
5.022
93.2
6.8
0.99
5.034
96.6
3.4 .
1.00
5.047
100.0
0.0
(From Lusk.)
CHAPTER LXIII
STARVATION
In order to furnish us with a standard with which we may compare
other conditions, we shall first of all study the metabolism during starva-
tion. A valuable chart compiled from observations made in the Carne-
gie Institution of Washington on a man who fasted for thirty-one days
is reproduced in Fig. 182.
The Excretion of Nitrogen. — When an animal is starved, it has to
live on its own tissues, but in doing so it saves its protein, so that the
excretion of nitrogen falls after a few days to a low level, the energy
requirements being meanwhile supplied, so far as possible, from stored
carbohydrate and fat. Although always small in comparison with fat,
the stores of carbohydrate vary considerably in different animals. They
are much larger in man and the herbivora than in the carnivora. Dur-
ing the first few days of starvation it is common, in the herbivora, to find
that the excretion of nitrogen is actually greater than it was before
starvation, because the custom has become established in the metabolism
of these animals of using carbohydrates as the main fuel material, so
that when carbohydrates are withheld, as in starvation, proteins are
used more than before and the nitrogen excretion becomes greater. We
may say that the herbivorous animal has become carnivorous. The same
thing may occur in man when the previous diet was largely carbohy-
drate; thus, almost invariably in man the nitrogen output is larger on
the third and fourth days of starvation than on the first and second.
Another factor influencing the nitrogen excretion during the early
days of the fast is the amount of previous intake of nitrogen; the greater
this has been, the greater the excretion. By the seventh day, however, a
Uniform output of nitrogen will usually be reached irrespective of the
individual's protein intake. During the greater part of starvation, most
of the energy required to maintain life is derived from fat, as little as
possible being derived from protein. This type of metabolism lasts until
all the available resources of fat have become exhausted, when a more
extensive metabolism of protein sets in, with the consequence that the
nitrogen excretion rises. This is really the harbinger of death — it is often
called the premortal rise in nitrogen excretion. It indicates that all the
ordinary fuel of the animal economy has been used up, and that it has
566
STARVATION
567
[NUTRITION LABORATORY OF THE CARNEGIE INSTITUTION OF WASHINGTON, BOSTON, MASSACHUSETTS]
METABOLISM CHART OF A MAN FASTING 31 DAYS
APRIL 14 -MAY 15.1913
23456789 10 II 12 13 14 15 16 17 IB 192021 22232425262725299031
OXYGEN AND CARBON
f
6»
V
4.00
3.90
3.80
3.70
ALVEOLAR CO, TENSION, mm.—* 3'60
38.0 3.50
36.0 3.40
34.0 3.30
32.0 3.20
30.0 3.10
81000 PRESSURE, mm. ^l'° 3'00
132 2.90
128 2.80
124 2.70
120 2.60
1 16 2.50
HEAT PER 24 HRS.CAL5. ' ' 2 Z?°
108 1700
104 1650
100 1600
BODY TEMPERATURE, *C x6 I55°
36.8 1500
36.4 1450
' I.T2 1350
1.08 1300
1.04 1250
1.00 1200
RESPIRATORY QUOTIENT — 1
.92 0.80
RESPII1ATION RATE ^5L -75
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\l/2 3 4 5 6 7 8 9 10 I.I 12 l> 14 15 16 17 18 19 20 21 Zl 23 24 25 26 2.7 26 Z? 30 3.1
Fig. 182. — Curve constructed from data obtained from a man who fasted for thirty-one days.
The days of the fast are given along the abscissae, and the various measurements along the or-
dinates. (From F. G. Benedict.)
568 METABOLISM
become necessary to burn the very tissues themselves in order to obtain
sufficient energy to maintain life. Working capital being all exhausted,
an attempt is made to keep things going for a little longer time by liq-
uidation of permanent assets. But these assets, as represented by pro-
tein, are of little real value in yielding the desired energy because, as
we have seen, only 4.1 calories are available against 9.3, obtainable
from fats.
These facts explain why during starvation a fat man excretes daily
less nitrogen than a lean man, and why the fat man can stand the starva-
tion for a longer time. The premortal rise is, however, not prevented by
feeding oil, which would seem to indicate that death may be due not so
much to the absence of fuel as to serious nutritional disturbance of es-
sential organs; e. g., there may be no available material to supply the
glands of internal secretion with the building stones they must have
(see page 580).
Not only is there this general saving of protein during starvation,
but there is also a discriminate utilization of what has to be used by the
different organs, according to their relative activities. This is very
clearly shown by comparison of the loss of weight which each organ un-
dergoes during starvation. The heart and brain, which must be active if
life is to be maintained, lose only about 3 per cent of their original
weight, whereas the voluntary muscles, the liver and the spleen lose
31, 54 and 67 per cent, respectively. No doubt some of this loss is to
be accounted for as due to the disappearance of fat, but a sufficient
remainder represents protein to make it plain that there must have been
a mobilization of this substance from tissues where it was not absolutely
necessary, such as the liver and voluntary muscles, to organs, such as the
heart, in which energy transformation is sine qua non of life. The vital
organs live at the expense of those whose functions are accessory.
The energy output per square meter of body surface steadily declines.
In the man examined by Benedict, it was 958 C. per square meter of
surface at the end of the first twenty-four hours, but only 737 on the
thirty-first day of the starvation period. The oxygen intake and carbon-
dioxide output correspondingly diminish.
The behavior of the nitrogenous metabolites in the urine is of par-
ticular interest, the following facts being of significance: Urea nitrogen
relatively falls and NH3 - N rises. For example, on the last day of feeding
the percentage output of NH3 - N in relation to total nitrogen was 3.16 ;
on the eighth day of the fast it was 14.88 (Cathcart).2 Acidosis is the
cause. The total amount of c'reatinine and creatine shows only a slight
fall, but creatinine relatively decreases and creatine increases (Cathcart).
Since creatine is a substance peculiar to muscle tissue, it is possible by
STARVATION 569
comparing the creatine and creatinine output with that of nitrogen to
determine whether all of the nitrogen liberated by the breakdown of
muscle has been excreted, or whether some has been retained either for
resynthesis in the muscle itself or for use elsewhere. If the muscle
breakdown as calculated from the creatine-creatinine output is greater
than that calculated from the nitrogen, synthesis of the noncreatine
remainder must be occurring ; whereas if the breakdown calculated from
nitrogen is greater than that calculated from creatine, etc., other tis-
sues than muscle must be contributory. Stored nitrogen or free nitro-
gen in transit from tissue to tissue for utilization is the most likely
source of such excess nitrogen.
That transference of nitrogenous substances from place to place in the
body in starvation is proved (1) by the constant presence of amino ni-
trogen in the blood and tissues (Van Slyke) ; and (2) by the effect of
copious water drinking. The latter causes a decided increase in the out-
put of nitrogen, but it does not appear that the extra nitrogen is due to
increased protein breakdown. It is probable, however, that in such cases
there would also be an increase in endogenous protein metabolism, since
the washed-out free nitrogen would have to be replaced.
Excretion of Purines. — Although at first they fall somewhat, the total
amount increases as the fast progresses. Perhaps the first decline is
due to general using up of hypoxanthine of muscle and the later rise
to the breakdown of nuclei (page 638).
Excretion of Sulphur.— It is important to compare the excretion of
sulphur and nitrogen. In the early days of starvation a ratio of 17 N : 1 S
has been found, but later one of 14.5: 1, which is practically the same
as that in muscle (i.e., 14; 1), indicating that late in fasting the main
source of protein supply is muscle.
Several of the changes observed during starvation can be attributed
to the condition of acidosis which supervenes. The acids are derived
from incomplete combustion of fat (see page 683), and are represented
by ^-oxybutyric, the amount being sometimes considerable (10-15 grams
a day), especially in obese individuals. The large ammonia excretion
(sometimes 2 grams a day) is evidently for the purpose of neutralizing
the excess of acid. Another consequence of the acidosis is the decline
in the alveolar tension of C02 (page 354), and it is possible that some of
the circulatory changes shown in the chart may also be dependent on
it. The method of repeated fasting used for reducing obesity is quite
safe if the acidosis is carefully watched.
Many secondary changes also occur in the starving organism. Thus,
the mobilization of fat is often responsible for a pronounced increase in
the fat content of the blood (see page 698), and that of protein explains
570 METABOLISM
the presence of an amount of ammo nitrogen not much below that of
normal animals (viz., 4 mg. per 100 c.c. of blood). Similarly with
carbohydrates, early in the condition the blood sugar becomes much
lower than normal, but then remains steady. This is significant when
we remember that after two or three days of starvation all of the avail-
able glycogen has been used up. It indicates that carbohydrate must
be essential for life, and that it is produced in starvation from proteins
(see page 667).
Starvation ends in death in an adult man in somewhat over four
weeks but much sooner in children, because of their more active metab-
olism. At the time of death the body weight may be reduced by 50 per
cent. The body temperature does not change until within a few days
of death, when it begins to fall, and it is undoubtedly true that if means
are taken to prevent cooling of the animal at this stage, life will be
prolonged.
Death from starvation must be due either to a general failure of all
the cells or to injury of certain organs that are essential for life. Since
the loss of protein from the body as a whole may vary between 20 and
50 per cent at the time of death by starvation, it is unlikely that general
failure can be its cause. If it were so, death would always occur when
some fixed loss of protein had occurred. Certain organs evidently cease
to perform their function, either because they are deprived of raw mate-
rial for the elaboration of some substance (hormone) necessary for life,
or because the organs themselves wear out from want of nourishment.
NORMAL METABOLISM
Apart from the practical importance of knowing something about the
behavior of an animal during starvation, such knowledge is of great
value in furnishing a standard with which to compare the metabolism
of animals under normal conditions. Taking again the nitrogen balance
as indicating the extent of protein wear and tear in the body, let us
consider first of all the conditions''ilnder which equilibrium may be re-
gained. It would be quite natural 'to suppose that, if an amount of pro-
tein containing the same amount of nitrogen as is excreted during
starvation were given to a starving animal, the intake and output of
nitrogen would balance. We are led to make this assumption because we
know that any business balance sheet showing an excess of expenditure
over income could be met by such an adjustment. But it is a very differ-
ent matter with the nitrogen balance sheet of the body; for, if we give
the starving animal just enough protein to cover the nitrogen loss; we
shall cause the excretion to rise to a total which is practically equal to
STARVATION 571
the starvation amount plus all that we have given as food; and although
by daily giving this amount of protein there may be a slight decline in
the excretion, it will never come near to being the same as that of the
intake. The only effect of such feeding will be to prolong life for a
few days.
Nitrogenous Equilibrium. — To attain equilibrium we must give an
amount of protein whose nitrogen content is at least two and one-half
times that of the starvation level. For a few days following the estab-
lishment of this pure protein diet, the nitrogen excretion will be far in
excess of the intake, but it will gradually decline until the two practically
correspond. Having once gained an equilibrium, AVC may raise its
level by gradually increasing the protein intake. During this progres-
sive raising of the ingested protein, it will be found, at least in the car-
nivora (cat and dog), that a certain amount of nitrogen is retained by
the body for a day or so immediately following each increase in pro-
tein intake. The excretion of nitrogen, in other words, does not immedi-
ately folloAv the dietetic increase. The amount of nitrogen thus retained is
too great to be accounted as a retention of disintegration products of
protein ;" it must therefore be due to an actual building up of new pro-
tein tissue — that is, growth of muscles.
Nitrogenous equilibrium on a protein diet alone is readily attainable
in the cat, and less readily in the dog. But in man and the herbivorous
animals, it is impossible to give a sufficiency of protein alone to maintain
equilibrium; there will always be an excess of excretion over intake.
Indeed it scarcely requires any experiment to prove this, for it is self-
evident when we consider that there are less than 1000 C in a pound of
uncooked lean meat, and that there are few who could eat over three
pounds a day, an amount, however, which would scarcely furnish all of
the required calories. A person fed exclusively on flesh is therefore
being partly starved, even although he may think that he is eating
abundantly and be quite comfortable and active. This fact has a prac-
tical application in the so-called Banting cure for obesity.
Protein Sparers. — Very different results are obtained when carbohy-
drates or fats are freely given with the protein to the starving animal.
Nitrogen equilibrium can then be regained on very much less protein,
so that we speak of fats and carbohydrates as being "protein sparers."
Carbohydrates are much better protein sparers than fats; indeed they
are so efficient in this regard that it is now commonly believed that car-
bohydrates are essential for life, and that when the food contains no
trace of carbohydrates, a part of the carbon of protein has to be con-
verted into carbohydrate. This important truth is supported by evi-
dence derived from other fields of investigation (e. g., the behavior of
572 METABOLISM
diabetic patients, in whom the power to use carbohydrates is greatly
depressed). The marked protein-sparing action of carbohydrates is il-
lustrated in another way — namely, by the fact that we can greatly
diminish the protein breakdown during starvation by giving carbo-
hydrates. In this way we can indeed reduce the daily nitrogen excre-
tion to about one-third its amount in complete starvation. Carbohy-
drate starvation is said to entail a failure of the muscles to use again in
their metabolism certain of the products (e. g., creatine) which result
from their disintegration. At any rate it has been found that creatine
is excreted in the urine when no carbohydrates are available.
In the case of man living on an average diet, although the daily nitro-
gen excretion is about 15 grams, it can be lowered to about 6 grams
provided that in place of the protein that has been removed from the
diet enough carbohydrate is given to bring the total calories up to the
normal daily requirement. If an excess of carbohydrate over the energy
requirements is given, the protein may be still further reduced with-
out disturbing the equilibrium. It has been found that it is not the
amount of carbohydrate alone that determines the ease with which the
irreducible protein minimum can be reached; the kind of protein itself
makes a very great difference. This has been very clearly shown by
one investigator, who first of all determined his nitrogen excretion while
living exclusively on starch and sugar, and who then proceeded to see how
little of different kinds of protein he had to take in order to bring him-
self into nitrogenous equilibrium. He found that he had to take the
following amounts: 30 gm. meat protein, 31 gm. milk protein, 34 gin.
rice protein, 38 gm. potato protein, 54 gm. bean protein, 76 gm. bread
protein, and 102 gm. Indian-corn protein. The organism is evidently
able to satisfy its protein demands much more readily with meat than
with vegetable proteins.
This variability in the food value of different proteins depends on their
ultimate structure — that is, on the proportion and manner of linkage
of the various ammo acids that go to build up the molecule. In no two
proteins are these building stones, as they are called, present in exactly
the same proportions, some proteins having a preponderance of one or
more and an absence of others, just as in a row of houses there may be
no two that are exactly alike, although for all of them the same build-
ing materials were available. Albumin and globulin are the most im-
portant proteins of blood and tissues, so that the food must contain the
necessary units for their construction. If it fails in this regard, even to
the extent of lacking only one of the units, the organism will either be
unable to construct that protein, and will therefore suffer from partial
starvation, or it will have to construct for itself this missing unit. It
STARVATION 573
is therefore apparent that the most valuable proteins will be those that
contain an array of units that can be reunited to form all the varieties
of protein entering into the structure of the body proteins. Naturally,
the protein which most nearly meets the requirements is meat protein,
so that we are not surprised to find that less of this than of any other
protein has to be taken to gain nitrogen equilibrium.
The most exact information regarding the "food value" of different
proteins has been secured by observations on the rate of growth of young
animals. This method yields more reliable information than can be
secured by studies on the nitrogenous balance, because it is not usually
possible to keep up the latter observations for a sufficient period of
time, or to secure an adequate number of data. During growth the
building-up processes are in excess of the breaking-down, so that the
effect is an increase in bulk of the tissues, thus permitting us, by the sim-
ple expedient of observing the body weight, to draw conclusions as to
the influence of various foodstuffs on tissue construction.
CHAPTER LXIV
NUTRITION AND GROWTH
In the growth of animal tissues two factors are concerned, one being
the property of the cell to grow, the growth factor; and the other, the
availability of suitable material to grow upon, the food factor. Concern-
ing the groAvth factor little is known; its variability in different species
of animal, its irregularity despite proper adjustment. of the food factors,
its abnormality leading to tumor formation, etc., are all well-known but
apparently inexplicable facts (Mendel8).
THE FOOD FACTOR OF GROWTH
Our knowledge is constantly increasing concerning the food factor of
growth, and many facts of extreme practical importance have been ac-
cumulated in recent years. In seeking for the relationship of food to
growth, we must first of all consider whether this process entails a
greater expenditure of energy than is necessary for mere maintenance
in adult life. Important results bearing on this question have been se-
cured by observations on the basal metabolism of young children. In
computing the energy supply of fasting adult animals of different sizes,
it will be remembered that the smaller the animal, the greater is the
energy exchange in relationship to the body weight, although when
computed in relationship to body surface tolerably constant values are
obtained. When the calorie output per square meter is determined in
growing children, there is, as we have already seen, clear evidence of
greater energy expenditure (see page 541), particularly marked in boys
just before puberty. An increased energy metabolism has also been de-
scribed in the case of infants, but the uncontrollable muscular activity,
the psychic disturbances, etc., may explain the result. Even after dis-
counting these factors, however, it is possible that there may be a cer-
tain influence, depending probably on the active mass of growing proto-
plasmic tissue, which stimulates the energy expenditure. The question
is not yet finally settled.
The Relationship of Proteins to Growth and Maintenance of Life. —
Since protein constitutes the fundamental chemical basis of the cell, it
is natural to devote attention in the first place to this food principle.
574
NUTRITION AND GROWTH
575
In the pioneer investigations, studies on the nitrogen balance in young
animals yielded results from which it was concluded that the conditions
for the disintegration of protein are less developed in young animals
than in adults, so that the growing organs rapidly withdraw circulating
protein and build it into tissue protein.
In consideration of the accumulation of data extending over several
decades, Rubner denied these conclusions, and showed that the diet of
the growing infant is by no means relatively rich in protein. He con-
cluded that "growth is not proportional to the quantity of protein in the
diet." Important though this pioneer work may have been in the de-
velopment of our present-day conception, the viewpoint of the men who
carried it out was very much narrowed on account of the paucity of
knowledge concerning the structure of the protein molecule. No allow-
ance was made for the fact, which has recently been firmly established,
that the protein molecule may vary extremely in regard to the units
of which it is composed, and that the growing tissues may demand, not
so much an abundance of protein as such, but rather a proper supply of
all the building stones which are required for growth (Mendel).
QUANTITATIVE COMPARISON OF AMINO ACIDS OBTAINED BY HYDROLYSIS OF PROTEINS*
(Compiled by T. B. Osborne, 1914) t
CAS3IN
OVAL-
BUMIN
GLIADIN
ZEIN
EDESTIN
LEGUMIN
ox
MUSCLE
Glycocoll
Alanine
0.00
1.50
0.00
2.22
0.00
2.00
0.00
13.39
3.80
3.60
0.38
2.08
4.0
8.1
Valine
7.20
2.50
3.34
1.88
6.20
?
2.0
Lcucine
9.35
10.71
6.62
19.55
14.50
8.00
14.3
Proline
6.70
3.56
13.22
9.04
4.10
3.22
8.0
Phenylalanine
Glutaminic acid
3.20
15.55
5.07
9.10
2.35
43.66
6.55
26.17
3.09
18.74
3.75
1.3.80
4.5
10.6
Aspartic acid
Serine
1.39
0.50
2.20
?
0.58
0.13
1.71
1.02
4.50
0.33
5.30
0.53
22.3
?
Tyrosine
Cystine
Histidine
4.50
?
2.50
1.77
?
1.71
1.61
0.45
1.84
3.55
? .
0.82
2.13
1.00
2.19
3.55
?
2.42
4.4
?
4.5
Arginine
Lvsine
3.81
5.95
4.91
3.76
2.84
0.93
1.55
0.00
14.17
1.65
10.12
4.29
11.5
7.6
Tryptophane, about
Ammonia
1.50
1.61
present
1.34
1.00
5.22
0.00
3.64
present
3.28
present
1.99
present
1.07
65.49
• 48.85
85.68
88.87
82.28
57.43
102.87
*These analyses are combinations of what appear to be the best determinations of various
chemists.
tThe figures for the more recent analyses of gliadr'n are inserted.
From the accompanying ' table giving the percentage of the various
amino acids, etc., present in certain proteins, it will be evident that there
are very marked variations in the units of which different proteins aro
composed. If any one of these units should be essential for growth and
576
METABOLISM
the organism be unable to manufacture the missing unit for itself, it
is clear that growth could not .proceed- however much protein not contain-
ing the necessary unit we might feed to the animal. It is an application
of the law of the minimum, and is analogous with the failure of growth
which has long been known to ensue when certain inorganic substances
are withheld from the growing animal. A diet might be perfectly bal-
anced as judged by comparison of the nitrogen intake and output, and
yet if it should fail to contain even one of the essential units and the
organism should be incapable of supplying this unit, then would the
diet be inadequate for growth.
These important facts are the outcome of modern work, and they
have been established by observations on the growth of young animals
fed with a ' ' basal ration ' ' to which were added mixtures of amino acids
Days
Each division = 20 days.
Fig. 183. — Curves of growth of rats on basal rations plus the various proteins indicated. The
normal curve may be taken as that with casein (I). (Adapted from Lafayette B. Mendel and
T. B. Osborne.)
or various proteins which differ considerably from one another in the
nature of the units entering into their make-up. In such experiments
the periods during which growth is observed must be prolonged, since
a transient increase in weight might depend merely on repair processes
occurring in tissues which had previously for some reason been brought
below par.
Among the most important observations have been those of Lafayette B.
Mendel and T. B. Osborne8 and of McCollum and his collaborators. The
animals chosen for Mendel and Osborne 's experiments were young white
rats. Large batches of these animals were fed on a basal rat-ion consisting of
protein-free milk (containing the inorganic salts, the sugars, traces of
protein, and unknown substances having an important influence on
NUTRITION AND GROWTH
577
growth — vitamines?), to which were added more carbohydrate, purified
fat, and the protein whose influence on growth it was desired to study.
The same diet was fed at regular intervals to a given batch of rats, and
the weight of each rat was periodically taken, the observation being pro-
longed until the animals grew to maturity and produced young, and these
again grew to maturity, reproduced, and so on. By plotting the re-
sults in curves, with the time periods along the abscissae and the average
weight of the rats of each batch along the ordinates, the extent of the
influence of a given diet on the curve of growth was obtained. A normal
curve of growth is shown in No. 1 of Fig. 183. It was obtained from re-
sults secured by adding liberal amounts of casein to the basal diet.
Wt
100
Mo
HO
VH
Each division -*o dayi
Days
Each division -20 days
Lafayette B. Mendel and T. B. Osborne.)
Similar curves were obtained with lactalbumin of milk and ovalbumin
and ovovitellin of egg. Perhaps the most interesting substances capable
of producing the normal curve of growth are certain of the proteins that
T. B. Osborne has succeeded in separating in crystalline form from
vegetable foodstuffs. These are edestin (hempseed), globulin (squash
seed), excelsin (Brazil nut), glutelin (maize), globulin (cottonseed),
glutein (wheat), glycinin (soy bean), cannabin (hempseed).
That growth proceeds normally with any one of these proteins when
fed abundantly does not, however, necessarily indicate that each con-
tains in adequate proportion all of the necessary units to meet the pro-
tein demands of growing tissues. In the case of casein, for example,
one of the units, namely, glycocoll, which is the simplest of all the
578 METABOLISM
amino acids, is entirely missing, and another, cystine, which is a sul-
phur-containing amino acid, is present only in small amount. The ab-
sence of glycocoll, however, is not of importance, because the organism
can manufacture it for itself (see page 630). In the case of cystine,
which the tissues can not manufacture themselves, the deficiency has to
be made up for by feeding an excess of casein so as to cover the needs
of the tissues for this amino acid. By so doing a superabundance of
most of the other units will be ingested, and this superabundance will
entail the destruction and excretion of the useless amino acids, a process,
however, which is conducted in such a way as to permit of the utilization,
by the organism, of a part of the energy which the cast-off amino acids
contain (see page 667). It is, therefore, not entirely a wasteful process.
When the supply of casein is limited, on the other hand, the curve
of growth becomes subnormal, because an insufficient supply of cystine
is thereby offered (Fig. 184). Similar results have been obtained in the
case of edestin, a protein from hempseed. This contains an insufficiency
of the diamino acid, lysine. Fed in abundance, edestin gave a normal
curve of growth, but when fed in insufficient amount the curve failed to
ascend properly, which, however, it could be made to do by adding some
lysine to the edestin.
There is a large group of proteins which fail to permit of any growth
no matter in what amounts they may be added to the basal ration. These
include: legumelin (soy bean), vignin (vetch), gliadin (wheat or rye),
legumin (pea), legumin (vetch), hordein (barley), conglutin (lupine),
gelatine (horn), zein (maize), phaseolin (kidney bean). The adequacy
to maintain growth of any of these pure proteins varies according to
the deficiency in their amino acids. In the case of gliadin of wheat or
rye, glycocoll is lacking, and lysine is present only in small amount (see
table). The absence of glycocoll can not, however, as we have already
seen in the case of casein, explain the inadequacy of gliadin as a foodstuff
for growth (Curve II in Fig. 183) . It must be the lysine that is at fault. A
still more deficient protein is the zein of maize. With this as the only
protein added to the basal diet, the curve of growth actually descends
(Curve III of Fig. 183), thus indicating that the animal is starving and
must soon succumb. The missing units in this protein are glycocoll,
lysine and tryptophane (see table on page 575), and it is very signifi-
cant that if the latter two amino acids are supplied along with zein, an
almost normal curve of growth will result. Some improvement can
even be brought about by giving tryptophane alone; that is to say, the
curve assumes a horizontal x line instead of descending, indicating that,
although inadequate for growth, the diet is now sufficient for the main-
tenance of life.
NUTRITION AND GROWTH
579
The important fact demonstrated by these experiments, is that cer-
tain diets are adequate for the maintenance of life although they are
inadequate for growth. In conformity with this conclusion, it was found
when young white rats were fed with gliadin alone for periods of time ex-
Fig. 185. — Photographs of rats of same brood on perfect diet (uppermost picture) ; on a main-
tenance diet but inadequate for growth (middle picture) ; and on a diet that was inadequate both
for maintenance and growth. (From Mendel and Osborne.)
ceeding those in which they should have become full grown, that
they remained in an ungrown stunted condition. The capacity to grow
had not, however, been lost, for when the gliadin was replaced by milk,
the animals resumed growth at a very great rate. The capacity to grow
580 METABOLISM
had only been inhibited by the inadequate diet, and there was nothing
really abnormal about the stunted animals. For example, the reproduc-
tive function developed normally, as was shown in the case of a young
female rat which, after being fed with gliadin as the sole protein sup-
ply for 154 days, was mated and produced four young. Although the
mother was still maintained on the gliadin diet, the young rats pre-
sented normal growth, for they were living on the milk supplied by the
mother, and this milk, because it contained either casein or some other
necessary accessory factor (vide infra), was an adequate food.
After removal from the mother, three of these rats were fed on an arti-
ficial diet of casein, edestin and the basal ration, and continued the nor-
mal course of growth, but when one of them was placed on the gliadin
food mixture it immediately failed to grow properly. It would appear
from these experiments that, of the two amino acids that are missing or
deficient in gliadin — namely, glycocoll and lysine — it must be the lysine
that is essential for growth. This very important conclusion was fully
corroborated by finding that, in young rats stunted by previous gliadin
feeding, growth immediately started when lysine was added to the diet
and ceased again when the lysine was removed, and so on, the experi-
ments being often repeated in various modifications. Mendel and. Os-
borne call attention to the relatively high percentage of lysine in all
those proteins that are concerned in nature with the growth of young
animals; thus, it is present in large amounts in casein, lactalbumin and
egg vitellin.
It is particularly in protein of vegetable origin that indispensable units
are likely to be missing, the best known of these units being the aromatic
amino acids, tyrosine and tryptophane; the diamino acid, lysine; and
the sulphur-containing acid, cystine. Some animal proteins, Such as
gelatine, also fail to contain aromatic groups, and are therefore utterly
inadequate as protein foods.
That the absence of one or two units should render a protein utterly
incapable of maintaining life suggests that a specific role may be taken
by certain amino acids in the maintenance of nutritional rhythm; thus,
they may be necessary for the elaboration of some hormone or other in-
ternal secretion essential to life, such as epinephrine, the active principle
of the suprarenal gland. This is an aromatic substance not far removed
in its chemical structure from tyrosine (see page 734). It is
therefore natural to suppose that the absence of the tryptophane unit
in zein is the reason that this protein is incapable of maintaining the in-
itial body weight. •
In attacking the problem from this viewpoint, Hopkins and Willcock10
made observations on the survival period of young mice; that is, the
period during which the animals survived when fed on a basal diet
NUTRITION AND GROWTH 581
mixed either with zein alone or with zein plus small quantities of tryp-
tophane. It was found that, with zein alone, the mice were unable to
maintain growth; they lost in weight and died in from about a week to
about a month. Other mice fed on the same amount of basal diet and
zein, but to which was also added some tryptophane, although they did
not grow, were capable of maintaining their body weight and lived in
some instances for nearly a month and a half. There were other indica-
tions of the difference in the efficiency of the two diets. The mice fed
on the zein alone were very inactive, and remained for a considerable
period of the time in a condition of torpor. The hair was ruffled, the
eyes were half closed, and the ears, feet and tail were cold. The ani-
mals, however, gave evidence of having a good appetite. On the other
hand, the mice to which tryptophane was also given manifested a strik-
ingly different behavior, being active and more or less normal until
just before death. That both groups of animals failed to live more than
forty-four or forty-eight days is probably to be accounted for by the
absence in the zein of the other unit, lysine. Had this been added along
with the tryptophane it is probable, in the light of Mendel and Osborne's
observations, that the animals would have survived much longer.
To supply the missing unit, besides using the pure amino acid, we
may employ other proteins which contain the required amino acid (Curve
III of Fig. 184). That mixtures of protein foodstuffs are desirable has long
been apparent to those who have studied practical dietetics. We must com-
bine the unsuitable protein with others which, although in themselves
perhaps also unsuitable, yet furnish us with a mixture which contains all
the essential units both for maintenance and growth. As Mendel points
out, these considerations suggest that we may be able to utilize certain
of the low priced protein by-products of the cereal, meat and milk in-
dustries. The test of the adequacy of the corrected diet must, however,
be determined by experiments of the type which we have just described.
It is probably in stock-raising rather than in connection with human nu-
trition that these facts will prove of practical value; for, not only is the diet
of man more varied, but it contains animal proteins in which the deficien-
cies are not so common.
Most important work of this character is being conducted by McCol-
lum and his collaborators.12 It would take us beyond the confines of
this book to discuss the results in detail, but it may be mentioned that
they have shown that, since the adequacy of the diet depends on a
multiplicity of factors besides the amino-acid make-up of proteins, —
some of which we shall discuss immediately, — very extensive observa-
tions with various food mixtures must be conducted over long periods
of time. The nutritive values of the common cereals added to a stand-
ard diet that had brought the animals (rats) to the threshold of death,
582 METABOLISM
were found to be as follows: With, cornmeal there was immediate recov-
ery and rapid growth, both of which were also secured in considerable
degree by wheat embryo and entire wheat kernel ; with rye and oats, on
the other hand, there was little if any improvement.
Much work is, of course, yet to be done before we can determine the
exact role which each unit plays in the physiologic development of
young animals. To sum up what we already know, it may be said that
glycocoll is not essential, since it can be manufactured by the animal
itself; that tryptophane is essential for maintenance, probably because
it is required for the production of certain essential hormones, for the
make-up of which in its absence other tissues must become disintegrated,
leading therefore to a diminution in body weight; and that lysine ap-
pears to be essential for growth. Tissues can be maintained without
lysine, but they can not grow, for the slight trace which most food con-
tains of this important amino acid may be sufficient for maintenance
purposes, but utterly inadequate for growth. That the young rats in
the experiments of Mendel grew normally while living on milk supplied
by the stunted mother indicates that the requisite lysine must have been
produced in the mother's body.
In the application of the foregoing principles to human dietetics, it
is undoubtedly safe to follow Bayliss's advice to take care of the calo-
ries and allow the proteins to take care of themselves.11 For example,
in the case of milk the deficiency of cystine in its chief protein, casein,
is corrected by the presence of lactalbumin, which, though present in
only small amounts, contains sufficient quantities of this amino acid to
meet the demands of the growing tissue. •
These observations on maintenance and growth suggest very interest-
ing applications in connection with the growth of tumors. Is it possible
that we might retard the growth of tumors by a diet that was insufficient
for growth while sufficient for maintenance. In an experiment devised
to test this proposition mice were fed on a diet of starch, lard, lactose
and gluten on which they could merely maintain existence but failed
to grow. Some of these rats were inoculated with a rapidly growing
tumor at the same time as another batch of mice kept on normal diet, and
it was found that the tumor grew much more slowly in the stunted mice
than in the others. One mouse, for example, on the restricted diet had
a scarcely visible tumor 52 days after the inoculation. When this mouse,
however, was placed on a normal diet of bread, milk, etc., the tumor
immediately began to grow at a very great rate.13 Too much importance
should not be placed on this experiment.
We shall now pass on to consider some of the factors besides the pro-
tein content which have an important bearing on dietetic efficiency.
CHAPTER LXV
NUTRITION AND GROWTH (Cont'd)
THE RELATIONSHIP OF OTHER FACTORS THAN PROTEINS
The Relationship of Carbohydrates. — As we have seen elsewhere, car-
bohydrates are almost certainly essential for normal metabolism. If they
are not given with the food, they must be manufactured out of protein by
the organism itself. It is not surprising, therefore, that their absence
from the diet of growing animals should lead to abnormality in the
rate of growth. Pediatrists have not infrequently insisted that one
form of carbohydrate is more advantageous for growth than another.
This no doubt in the main is true, but the whole question of adequacy
probably depends on the digestibility of the carbohydrate and not upon
its essential chemical nature. It is likely that the only carbohydrate
required by the tissues is glucose. The readiness with which the car-
bohydrate of the food becomes converted into this monosaccharide is
pr-obably the only determinant of its efficiency as food material.
The Relationship of Fats.— Although fats are an invariable constit-
uent of practically every diet, it is yet a debatable question as to
whether they are essential to the maintenance of a healthy normal
organism. Difficulties standing in the way of a solution of this problem
are that it is not only technically very difficult to remove fat entirely
from the common foodstuffs, but also that the simple fats are usually
associated with substances having similar solubilities and physical
properties: namely, the lipoids, phosphatides, cholesterol, pigments, etc.
Since these substances are present in practically every cell, it is almost
certain that they can be manufactured by living protoplasm. Indeed,
experimental evidence is not wanting to show that this is actually the
case. Although the cell can manufacture lipoids, a young animal can
apparently not grow when these substances, as well as simple fat, are
entirely absent from the diet. This has been shown by feeding young
mice on a diet from which all traces of fat and lipoids had been removed
by extraction with alcohol and ether (Stepp)14. On such a diet the mice
lived only a few weeks. They could be kept alive much longer when
some of the alcohol-ether extract was mixed with the diet, but not so
when neutral fat instead of the alcohol-ether extract was added. The
583
584 METABOLISM
addition of the ash of the lipoid extract failed to maintain the mice, so
that the lacking substance could not be inorganic in nature.
More recent and extended observations, however, have shown that neutral
fat is also necessary for the adequate and continued growth of the
animal. For a period of two months or so an animal may, as we have
seen from Osborne and Mendel's experiments, grow in apparently nor-
mal fashion on an artificial fat- and lipoid-free diet composed of casein,
carbohydrate and inorganic salts, but sooner or later the great majority
of these animals begin to show failure of adequate growth. The in-
adequately growing animals often manifest indications of malnutrition
other than the failure to increase in weight; for example, inflammation
of eyes, roughening of the fur, etc. When certain fats are added to
the inadequate diet, normal growth is immediately resumed. Fats pro-
ducing this normal growth are such as butter fat, or the fat extracted
from egg yolk, or cod-liver oil, added to the extent of 5 per cent of the
ration. On the other hand, vegetable oils, such as olive oil or almond
oil, are inefficient in promoting growth. That all oils or fats do not
suffice to produce growth, and that one dose of an adequate oil or fat may
be sufficient to stimulate it, indicate that something other than the mere
presence of the comparatively simple fat molecule — that is, some acces-
sory material — must be the agency responsible for the growth.
This conclusion is further supported by the interesting observation of
McCollum and Davis that vegetable oils can be rendered efficient for
growth by shaking them with a solution of soap prepared by com-
pletely saponifying butter fat with potassium hydroxide in the absence
of water.
ACCESSORY FOOD FACTORS, VITAMINES
In searching for the nature of the accessory food factors, the im-
portant observations which have been made in recent years concerning
the so-called vitamines must be considered. These are substances essential
in the diet for the proper maintenance of nutrition in adult animals.
The existence of such substances was suggested by observations on
the disease beriberi, which is caused by exclusive feeding on polished
rice; that is, on rice from which the pericarp had been removed by the
process of polishing. When patients suffering from this disease were
given unpolished rice, the symptoms immediately disappeared. Further
investigation of the exact nature of these substances was greatly facil-
itated by the discovery that a similar condition is readily induced by
feeding fowls on polished rice. The birds develop a polyneuritis, from
which, however, they very promptly recover if some rice polishings or,
NUTRITION AND GROWTH 585
better still, an extract of rice polishings, is added to the polished rice
diet. The extract is made by means of slightly acid 91 per cent alcohol,
and from it Funk has succeeded in separating a substance in crystal-
line form apparently related to the pyrimidines, which it will be re-
membered are a characteristic constituent of the nucleins. Doses as
small as 0.02 to 0.04 gm. of this material given by mouth were adequate
to cure the polyneuritis of fowls in from six to twelve hours; indeed, in
some cases the bird seemed quite well after three hours. A similar sub-
stance has also been extracted from yeast, milk, brain and lime juice,
and it has been called, for want of a better name, vitamine.15
It is quite likely that other diseases, such as scurvy, may also be due
to the absence of some vitamine in the diet — some substance, namely,
which in the case of this particular disease would seem to be absent in
preserved food, the continued taking of which is so frequently its cause.
Fresh fruit and other foods added even in small amounts to such a diet
would appear to supply the necessary vitamine.
It is not the higher animals alone that suffer from the want of some
such substance as vitamine. It has been shown, for example, that, when
a normal artificial culture medium is inoculated with yeast in very
small amounts, it fails to grow, whereas the same quantity will grow
luxuriantly in a medium to which sterilized beer wort has been added.
Vitamine is not of the nature of a ferment, since it withstands heating
to 120° C. for more than an hour. The addition of yeast to dietaries
that are deficient in vitamines is an excellent corrective.
Returning now to the accessory substances that seem to be adherent
to certain forms of fat, we see at once that they can not be exactly
the same as the so-called vitamine of Funk, for they contain no nitrogen.
There are, therefore, probably two accessory factors concerned in ade-
quate growth. One of these must be present in the protein-free milk
which serves as a constituent of the basal diet used in Osborne and
Mendel's experiments, for we have seen that animals will grow on this
for a certain period, provided the proper amino acids are present.
Later, however, they pass into a state where there is no growth but
adequate maintenance. If now the other accessory factor is added, as,
for example, butter fat or a small amount of milk itself (i. e., in place
of protein-free milk), then growth will be resumed at its normal rate.
"Either of the determinants may become curative. Both are essential
for growth when the body store of them becomes depleted." McCollum
suggests that these accessory factors should at present be called the
"fat-soluble A" and "water-soluble B." The latter is present in yeast
cells, in fat-free milk, and in many other animal foods, and is probably
the same as Funk's vitamine. The former is soluble in the fat solvents,
586 METABOLISM
being present in most animal fats, but not in all; for example, it is
absent from the fat surrounding the pig's heart. By using such a
nomenclature it is recognized that the subject 'is as yet only in an early
state of development.
"We may sum up the main facts of this chapter by stating that growth
and maintenance are more than a mere problem of energy supply.
Granted that this is sufficient, we must also have a suitable admixture of
building units of protein and the presence of extremely small quantities1
of some unknown accessory substances. These are present in some natural
foods but not in others, and some are soluble in water and others in fats.
They are found, for example, in animal fats but not in those of vegetable
origin. Both fat- and water-soluble factors are present in large quanti-
ties in milk.
Both accessory food factors are necessary, as is illustrated in the fol-
lowing summary of experiments from Lusk's "Science of Nutrition,"
(third edition).
Purified protein -f carbohydrate + vegetable fat + inorganic salts — no growth.
" -f " + butter fat 4- " " — no growth.
" " + " + vegetable fat + " " +vitamines (accessory
factor B) = no growth.
" " + " + butter fat + " " + vitamines = growth.
The Relationship of Inorganic Salts. — Inorganic salts ale also an es-
sential ingredient of the diet. McCollum found that young animals soon
ceased to grow when fed on a diet of corn and purified casein, but that
rapid growth returned when a suitable salt mixture was added. Oats,
wheat, and beans have also been shown to require some adjustment of their
ash content to make them adequate for growth. Most of the animal foods
contain in themselves sufficient inorganic material, as is evidenced
among other things by the adequacy of milk alone as diet for growing
animals and the abhorrence of salt that is shown by strictly carnivorous
animals. In the usual mixed diet of man there is almost always enough
inorganic material, the salt which he adds being largely for seasoning
purposes. When a preponderance of vegetable food is taken, however,
the salt comes to have a real dietetic value.
The practical application of the results of these numerous and at
present somewhat bewildering observations to the nutrition of man,
and particularly to the dietetic treatment of disease, is undoubtedly
very great. This is especially so in infants and growing children, in
whom the correction of some slight inadequacy in the diet may have
the most pronounced results, not only on growth and nourishment, but
also on the power of resistance against disease and infection. The bene-
ficial influence of cod-liver oil, for example, may depend on some fat-
NUTRITION AND GROWTH 587
soluble accessory food factors, while the miraculous benefit which
scorbutic children derive from the addition of the juice of limes, lemons,
etc., to the food is undoubtedly due to such influences. The accumu-
lating mass of evidence as to the faulty nutrition in animals fed on
single kinds of food that fail to contain both kinds of food factors
emphasizes the necessity in the dietetic treatment of such diseases as
diabetes, nephritis, etc., of seeing to it that the diet is sound, not only
in calories, protein content, and palatability, but also with regard to
the presence of accessory food factors.
CHAPTER LXVI
DIETETICS
THE CALORIE REQUIREMENT
In the application of the important facts that have been reviewed in
the preceding chapters to the science of dietetics, the question arises as
to how we may determine with scientific accuracy just exactly how much
food should ~be taken under varying conditions of bodily activity. In a
general way, we know that the amount of food that we require to take
is proportional to the nature and amount of bodily exercise that is
being performed at the time ; and that, if the food supply is inadequate,
the work before long will fall off not only in quantity but in quality as
well. "Horses (also men) work best when they are well fed, and feed
best when they are well worked," is an old adage and one the truth of
which can not be overestimated in the consideration of all questions of
dietary requirements. An ill-fed beggar will rather suffer from the
pain and misery of starvation than attempt to perform a piece of work
that the well-meaning housewife bargains should be done before she
gives him a meal. The spirit may be willing but the flesh is weak. If he
could be trusted, he should be fed first and worked afterwards. Besides
the amount of work, two other factors are well known to influence the
demand for food — namely, growth and climate. A young, growing boy
will often demand as much if not more food than would appear to be
his proper share, from a comparison of his body weight with that of
his seniors; and, other things being equal, it is well knoAvn that we are
inclined to eat much more heartily of food during the cold days of
winter than during the sultry days of July and August.
That we know these facts in a general way, indicates that the first
step to take in the exact determination of dietetic requirements is to
find out how mueh energy the body expends under varying conditions
of activity. This, as we have seen, may be done by having the person
live for some time in a respiration calorimeter, so that we may measure
the calorie output. To the conclusions drawn from results of observa-
tions made under such artificial and unusual conditions of living, the
objection might, however, quite justly be raised that they need not
apply to persons going about their ordinary routine of life. To meet
588
DIETETICS 589
this objection another method, which we may call the statistical, is avail-
able. It consists in taking the average diet of a large number of indi-
viduals and comparing the calorie value with the average amount and
type of work that they are meanwhile called upon to perform, and can
best be used where the diet is accurately known, as in public institu-
tions, the army, the navy, etc. The total food supplied is then divided
by the number of individuals, this giving the per capita consumption.
Obviously some get more than others, but when a sufficient number of
individuals is included, such errors become eliminated by the law of
averages.
The reliability of this method is testified to by the remarkable corre-
spondence in the calorie value of the food consumed by farmers in widely
different communities:
Calories
Farmers in Connecticut 3,410
" " Vermont 3,635
" ". New York 3,785
" " Italy 3,565
" " Finland 3,474
Average 3,551*
*Lusk: The Fundamental Basis of Nutrition.
The average inhabitant of various cities:
London 2,665
Paris 2,903
Munich 3,014
Konigsberg 2,394**
«*Rubner.
Individuals in different callings:
Farmers ' families (U.S.A.) 3.560
Mechanics' families (U.S.A.) 3,605
Professional men's families (U.S.A.) 3,530
Army (U.S.A.) 3,851
Navy (U.S.A.) 4,998t
tAtwa'er.
In general, it is usually computed that a man
weighing 70 kg. requires in calories:
2,500 for a sedentary life,
3,000 for light muscular work,
3,500 for medium muscular work,
4,000 and upwards for very hard toil.t
JMcKillop.
These figures apply to the average man, but in calculating the calorie
requirements of a family or a community we must make allowance for
the lesser requirements of women and children. Several dietitians have
compiled tables showing how many calories are expended according to
age and sex, and the German authorities have recently taken these figures
and from them calculated a generalized mean, which shows in comparison
590 METABOLISM
with men the percentage that should be allowed for women and children.
The figures are as follows:
Man 100
Woman 83
Boy over 16 92
Boy 14-16 81
Girl 14-16 74
Child 10-13 64
Child 6-9 49
Child 2-5 36
Child under 2 23
In calculating the calorie requirement of the population as a whole,
the necessity of making allowance for the varying needs of men, women,
and children would obviously make the calculations far too complicated
for practical purposes. It is necessary to have a factor by which we
may multiply the total population in order to determine its ' ' man value. ' '
This factor is based on the relative proportion of men to women and
children, and it amounts very nearly to 0.75, i. e., three-quarters of the
total population gives "the man value." Knowing the total population,
say, of a city, we must therefore multiply this by 0.75 in order to ascer-
tain for how many men doing moderate muscular work (3000 C.) food
has to be provided.
THE PROTEIN REQUIREMENT
The facts considered in the previous two chapters lead to the question:
To what extent may the proportion of protein in the diet be reduced
with safety? It is evident that there must be a minimum below which
every one of the necessary building materials of protein could not be
supplied in adequate amount to reconstruct the worn-out tissue protein.
The extent to which the protein content of the diet of man can be
lowered with safety depends on several factors, of which the most im-
portant are: first, the nature of the protein; second, the number of non-
protein calories ; and third, the extent of tissue activity. Where so many
factors must be taken into consideration, the only method by which the
actual minimum can be determined consists in what may be called "cut
and try experiments." Of the many investigations of such a nature,
probably the best one for us to consider, is that recently published from
the Nutrition Laboratory of Copenhagen. The subject, an intelligent
laboratory servant, lived a perfectly normal and active life for a period
of five months on a diet of potatoes cooked with margarine and a little
onion, and containing 4000 C., with a total protein content of 29 grams.
During another period he did outdoor work as a mason and laborer, and
took 5000 C. daily, and 35 grams of protein.
DIETETICS 591
It is important to contrast these results with the following based on
municipal statistics of gross consumption.
MUNICIPAL FOOD STATISTICS
PROTEIN
FAT
CARBOHYDRATES
CALORIES
Konigsberg
Munich
gm.
84
96
gm.
31
65
gm.
414
492
2394
3014
Paris
98
64
465
2903
London
98
60
416
2665
It is certain that man can lead a normal existence and remain in good
health on very much less protein than the 100 grams which statistical
studies show to be the amount he actually takes. This discrepancy be-
tween the amount which experiment demonstrates to be adequate and
that which habit and custom demand, raises the question as to whether,
after all, our instincts may not have erred and so made us unnecessarily
extravagant in our protein intake. It has been suggested that such pro-
tein extravagance will in various ways have a dele'terious effect on the
organism; thus, that the excretory organs, such as the kidneys, will be
overtaxed in eliminating the unused amino acids, that the constant pres-
ence of these bodies in excess in the blood will cause degeneration and
sluggish metabolism, and that the excess protein in the intestine will
lead to the production of ptomaines, whose subsequent absorption into
the blood will cause toxemic symptoms.
Important support to such views appeared to be supplied some dozen
years ago by Chittenden, who was able to show that he himself and many
other persons doing different kinds of work could be supported on daily
amounts of protein that were not more than from one-third to one-half
of the amount usually taken. Not only so, but it was averred that dis-
tinct improvement was experienced in the general sense of well-being
and of mental efficiency as a result of the lesser protein consumption.
Taking these results as a whole, it is quite clear that man can get
along under ordinary conditions with much less protein than he usually
takes; but that really proves nothing, for the question is not can he but
should he, so deprive himself? Are instinct and custom wrong and is
Chittenden right ? That is the question. To answer it many studies have
been made of the condition of peoples who for economic or other rea-
sons are compelled to live on less protein than the average. Are these
people healthier, less prone to infections and degenerative diseases, and
more efficient mentally than others? In such studies great care must be
exercised to see that conditions other than diet, such as climate, exercise,
etc., are properly allowed for. It would not be fair, for example, to
compare the mental and bodily condition of pepples living in the tropics
592 METABOLISM
and who take comparatively little protein, with those living in temperate
zones, who consume much more. After discounting all of these other
factors, it has been quite clearly shown that, when the protein allowance
is materially reduced, the people as a whole are less robust, mentally in-
ferior, and, instead of being less prone to the very diseases which are
usually supposed to be due to overloading of the organism with useless
excretory products, are more liable to suffer from them.
That a decided reduction in -protein weakens the defense of the organ-
ism against infection is probably due to the fact that the fluids of the
body normally contain a great variety of so-called antibodies — that is,
of highly complex substances that are largely protein in nature. When
bacteria, or the poisons produced by them, enter the body, they are met
by one or more of these defense substances and destroyed or neutralized.
Now it is clear that there should always be a surplus of protein-building
materials from which the antibodies may be constructed. Such an excess
will constitute a "factor of safety" against disease. And there are fac-
tors of safety of another nature to be provided for, two of which we are
in a position to appreciate. In the first place, there must always be an
adequate supply of tryptophane, of lysine, and of cystine, not only to
meet the bare necessities of the protein constructive processes that go on
under normal conditions, but also to make good the larger amount of
protein wear and tear that greater degrees of tissue activity will entail.
Although moderate muscular exercise does not appear to cause any im-
mediate consumption of protein (carbohydrate and, later, fat being the
fuel material used to produce it), yet it does throw a greater strain on
the tissues and causes a greater wear and tear of the machinery, and
hence a demand for more protein-building material. In the second place,
there are certain of the internal secretions of the body, such as epineph-
rine (adrenaline), that are essential for life, and as crude materials
for the manufacture of which certain amino acids are essential. Tyro-
sine is one of these, and since proteins, as we have seen, differ from one
another quite considerably in the amount of this amino acid which they
contain, it is advisable to provide an excess, so that an adequate supply
of tyrosine may always be available.
The answer to one of the most important practical questions in die-
tetics— namely, "What proportion of protein should the diet contain?"
depends on these scientific principles. The source of the protein is the
important thing. With animal protein there is no doubt that we could
get along with perfect safety by taking daily not more than 50 or 60
grams, which is about half of what we actually consume. If the protein
is of vegetable origin and part of it of the first quality, as wheat and
Indian corn preparations, more should be taken so as to allow for the
DIETETICS 593
deficiency of certain amino acids. When vegetable proteins of the sec-
ond quality, such as those of peas, beans, lentils, etc., are alone available,
much larger amounts are necessary. Such proteins are inadequate in the
case of growing children at least, and even in adults it is undoubtedly
advisable that other proteins should supplement them.
To insure safety, therefore, it is almost imperative that the diet should
contain proteins of various sources. If for economic reasons the main
source must be proteins of vegetable origin, then some animal protein, such
as is contained in milk or meat or eggs, should be added to at least one of
the daily meals. When peas and beans are mainly depended on for the
protein supply, they should be taken either with milk or one of its prep-
arations, or with a thick gravy or sauce made from meat and containing
the finely minced meat. This must not be strained off, for if it is, the
sauce will contain only the meat extractives but not any of the protein,
which is coagulated by the boiling water. Meat extract, in other words,
contains no proteins; it is not a food but merely a condiment of no greater
dietetic value than tea or coffee.
ACCESSORY FOOD FACTORS
Little need be added to what has already been said regarding this
subject. The practical point to be remembered is that there are at least
two accessory factors concerned, one of them soluble in fat and present
in adequate amount in butter and other animal fats but not in vegetable
oils, and the other soluble in water and present in wheat, vegetables,
fruits, etc. Milk contains both of these factors, so that its inclusion in
a diet is a safeguard not only against inadequacy in suitable protein, but
also against the absence of accessory food factors. There is little danger
of the diet being inadequate with regard to food factors if it contains
some fruits or green vegetables or unheated fresh milk. The food fac-
tors are destroyed by prolonged cooking.
DIGESTIBILITY AND PALATABILITY
We have seen that practical dietetics depends on several factors, the
exact relative importance of which can not perhaps be gauged in every
case, but preparation of the food so as to make it appetizing must un-
doubtedly rank high. The importance of good cooking will now be ap-
parent. It is the act of making food appetizing and therefore digestible.
It is really the first stage in digestion, the stage that we can control, and
one therefore to which much attention must be given, especially when it
becomes necessary to make attractive articles of diet ordinarily considered
common and cheap. Most people can cook a lamb chop so as to make it
594 METABOLISM
reasonably appetizing, but few can take the cheaper cuts of meat and con-
vert them into cooked dishes that are as popular and attractive. And there
are still fewer who can take the left-overs and trimmings and convert them
in the same way. This is the real art of cooking, and too much encourage-
ment can not be given to the effort which our cooking experts are making
to show people how these things can be done. The waste of good food in
a large city is truly appalling.
Cooking has other advantages than making the food appetizing. The
heat loosens the muscle fibers of the meat so that it is more readily
masticated; it destroys microorganisms and parasites in the meat; it de-
stroys antibodies which might interfere with the action of the digestive
ferments. Thus, untreated raw white of egg is not digested in the stom-
ach because it contains one of the antibodies which prevent the pepsin
from acting on it; but boiled egg white, if properly chewed, can be di-
gested, and whipping the egg white into a foam partly destroys the in-
hibiting substance.
Before concluding, something should be said about the laxative quali-
ties of food, for it is often in this particular alone that one food is more
satisfactory than another. A diet of meat, milk, eggs, and white bread is
apt to be unphysiologic because there is nothing in it to act as what has
been called intestinal ballast; that is, a material which will keep the
intestines sufficiently filled to stimulate their muscular movements. This
ballast is best furnished in the shape of cellulose, the most important
constituent of green food. Peas, beans, cabbage, salad, and many fruits,
especially apples, should always occupy a place in the daily menu. An-
other valuable food yielding this ballast is the outer grain of wheat, oats,
etc. So much must not be taken as to produce a constant intestinal
irritation, and each person must determine for himself where this limit
lies. The difference among various breads is almost entirely in the de-
gree to which they supply ballast.
The all-important subject of food economies can receive no attention
here, except to point out that it is one which must be most carefully con-
sidered in the solution of all problems of dietetics. An admirable ac-
count of the subject will be found in Graham Lusk's "Science of Nutri-
tion" (third edition) and in McKillop's "Food Values."16
. CHAPTER LXVII
THE METABOLISM OF PROTEIN
Introductory. — The older physiologists believed that the protein taken
with the food was brought into a soluble condition by the digestive en-
zymes, and that it was then absorbed into the blood and directly incor-
porated with the tissues. The discovery of the enzymes trypsin and
erepsin and of free amino acids in the gastrointestinal contents clearly
showed that this simple theory of Liebig could not be correct. It was,
furthermore, found that when an excess of proteins such as egg albumin
gains entry to the blood, part of the protein appears in an unchanged
condition in the urine ; and that enzymes capable of digesting this protein
but not other varieties make their appearance in the blood.
After the injection of foreign proteins into the blood, symptoms of
varying severity often develop, from the almost instantaneous death
produced by snake venom to the slowly developing anaphylactic reac-
tions which follow the injection into the blood of many proteins chemi-
cally indistinguishable from those of the blood serum itself. When pro-
tein is taken in the usual amounts by mouth, these poisonous reactions
do not supervene, — even snake venom is harmless when swallowed, — nor
is it "possible during digestion of a protein meal to detect food protein in
the blood by means of the precipitin reaction. Finally it was discovered
that the very slow intravenous injection of completely digested flesh did
not produce on the part of the body any of the reactions that injected
protein itself produces, indicating that perfect assimilation had occurred.
From these and similar observations it soon became clear that protein
can not be absorbed as such from the alimentary canal, but must first of
all ~be completely broken down into the amino acids, which are then rebuilt
into the protein of the organism. The direct evidence for this important
change in belief concerning protein metabolism has been gained by the
discoveries that: (1) nitrogen equilibrium can be maintained in animals
fed with completely digested protein mixtures; and (2) amino acids can
be isolated from the blood.
The experiments of the first group consist, in principle, in breaking
down protein until there is no longer the characteristic biuret test, and
then feeding this digestion mixture to animals and observing them from
day to day, using as criteria of their nutritional condition the body weight
595
596 METABOLISM
and the nitrogen equilibrium, (Page 571.) It has been shown that suc-
cess in maintaining nutritional efficiency depends partly on the nature
of the process used for digesting the protein, and partly on the presence
or absence of carbohydrate in the digestion mixture. It was found
that acid hydrolytic products failed to maintain equilibrium, and it was
believed that this was owing to the fact that the acid had more completely
disrupted the protein molecule, and had left no polypeptides, which, it
was imagined, remained intact during enzyme action and were essential
for proper protein metabolism. This view has now been considerably
altered, since it has been shown that the acid actually destroys certain
ammo acids which the enzyme leaves intact. The amino acid particu-
larly concerned is tryptophane. Thus, when animals were fed with three
diets, consisting of (1) fully digested casein, (2) fully digested casein
from which the tryptophane had been removed, and (3) fully digested
casein from which the tryptophane had been removed and then the
proper amount of pure tryptophane added, it was found that nitrogen
equilibrium could not be maintained on the second diet, which contained
no tryptophane, whereas it was maintained on the first and third diets.
That this explanation is correct is further supported by the fact that,
if the protein is only partly digested by acid — that is, not digested
enough so as to break up all the tryptophane — it can efficiently maintain
nitrogen equilibrium.
Eegarding the necessity for carbohydrates, it is possible that under
certain conditions these may be produced from the protein itself. At
least, it has been possible for Abderhalden, who has done a large share
of this work, to maintain an animal in nitrogen equilibrium with a diet
of digestion products and fat containing no carbohydrate.
These results obtained in different classes of animals have also been
confirmed for the human subject. A boy suffering from a stricture of the
esophagus, when fed by rectum for fifteen days with digestion products
resulting from the action of trypsin and erepsin on flesh, gave evidence
of nitrogen retention.
Concerning the second type of evidence, many investigators attempted
to separate the amino acids themselves from the blood, particularly dur-
ing the digestion of a large amount of protein, but the results were at
first entirely negative because of the lack of methods that were suffi-
ciently delicate to make it possible to detect the slight increase that
could be expected even when a maximum absorption of nitrogen had
occurred. The very large flow of blood through the portal vein causes
such extensive dilution of any substances added to it that the concentra-
tion of the substance in an isolated sample of the blood can be only
trivial.
THE METABOLISM OF PROTEIN 597
To account for the indisputable disappearance of the amino acids from
the intestine during protein digestion, coupled with the impossibility of
detecting any of them in the blood, two views were current for many
years. One of these was that the amino acids become deaminated (NH2
group split up as NH3) by the intestinal epithelium, and the other, that
these cells are endowed with the power of reconstructing the amino acids
into protein, which then passes into the blood. Justification for the de-
amination hypothesis seemed to be obtained by the observation that there
is more free ammonia in the blood of the portal vein than in that of the
systemic circulation. The falsity of this evidence was, however, defi-
nitely established by Folin and Denis,32 who found by means of delicate
quantitative methods for the estimation of ammonia and urea in the blood
that the amount of neither of these substances became increased in the
portal blood during absorption of amino acids from the intestine. They
made the further important discovery that the ammonia in the portal
blood is really very little in amount, and represents that absorbed as
such from the intestinal lumen, where it is produced chiefly by the action
of putrefactive bacteria.
Nor could any evidence be obtained in favor of the hypothesis that
the absorbed amino acids become built up in the intestinal epithelium
into proteins, which are then transformed or carried away by the blood.
This hypothesis was based entirely on negative findings, and had there-
fore to be dropped when discovery was made of the actual presence of
amino acid in the blood.
This brief historical survey of the subject brings us to a position where
we may proceed to discuss the present-day teaching regarding protein
metabolism. Briefly stated, this teaching is to the effect that the protein
molecule is broken down into its ultimate building stones, the amino acids,
by the digestive enzymes of the gastrointestinal tract, and that these amino
acids are absorbed into the blood, by which they are carried to the various
organs' and tissues, which sift out the amino acids and use those of themi
which they require for the reconstruction of their broken-down protein.
The amino acids not required for the process, along with those wliich may
be liberated in the tissues themselves by disintegration of tissue proteins,
are then split into two portions, one represented by ammonia and the other
by the remainder of the amino acid molecule. The former is excreted as
urea and the latter is oxidized to produce energy.
CHEMISTRY OF PROTEIN
Before proceeding to discuss the evidence upon which the above con-
clusions depend, it will be necessary to consider some of the most important
facts concerning the chemistry of the protein molecule. We shall require
598 METABOLISM
this information not only to understand the history of protein in the
animal body, but also to follow intelligently the important work that
has already been discussed concerning the relative value of different
proteins as food. A knowledge of protein chemistry has come to be
essential in practically all branches of medical science.
Proteins, like starches, are composed of numerous smaller molecules,
In the case of starch these molecules are the various monosaccharides —
glucose (dextrose), levulose and galactose; in the case of proteins they
are the amino acids. The breaking apart of the links that hold the mole-
cules together is effected in both cases by the process of hydrolysis, so
called because of the fact that the reaction consists in the taking up of a
molecule of water at each of the places where the chain falls apart. This
hydrolysis may be effected either by the action of mineral acids or alka-
lies, or by enzymes, the only difference' in the action of these reagents
being that in the former case the breaking apart takes place more or
less indiscriminately, whereas in the latter it proceeds according to a
definite plan, which varies somewhat with the type of enzyme employed.
Just as a chemical knowledge of the structure of sugar or monosac-
charides is the basis of carbohydrate chemistry, so is that of the amino
acids the basis of protein chemistry.
Amino Acids. — There are, so far as known, eighteen different amino
acids concerned in the constitution of protein, but they are all alike in
their characteristic structure. The most striking characteristic depends
on the presence in the molecule of: (1) an amino group with a basicity
comparable to that of ammonia, and (2) an acid group with an acidity
comparable to that of acetic acid. Let us take in illustration one of the
simplest fatty acids — namely, acetic. It has the formula CH3COOH.
The COOH group is called carboxyl, and on it depend the acid properties
of the compound. The CH3 group is known as methyl, and the amino
group (NH2) is attached to it in place of one of the hydrogen atoms, thus
giving the formula CH2NH2COOH, which is aminoacetic acid or- gly-
cocoll. If we take the next higher acid of the fatty acid series, having
the name propionic and the formula CH3CH2COOH, its amino acid, called
alanine, has the formula CH3CHNH2COOH. Now let us place the formu-
las of these two acids side by side in the following manner:
H CH3
NH2-C-COOH NH2-C-COOH
(amino group) H (acid group) (amino group) H (acid group)
Aminoacetic acid Aminopropionic acid
(glycocoll) (alanine)
THE METABOLISM OF PROTEIN 599
It will be observed that the only difference between the two acids is
dependent upon a change in the group that is attached to the upper verti-
cal valency bond of the central carbon atom, which therefore must be
considered as the center of the entire molecule. The various amino acids
produced from protein differ from one another solely with regard to the
chemical nature of the group that is attached to this vertical valency
bond. Evidently, then, the reactions that amino acids possess in common
must depend on the terminal groups containing the carboxyl and amino
radicles, whereas the characteristic reaction of each of the eighteen amino
acids must depend upon the differences in the radicles attached to the
upper vertical bond. This may be represented thus:
Any radicle
NH,-C-COOH
H
Any amino acid
The end groups endow the amino acids with the power to combine with
both acids and bases. With acids they behave like substituted ammonias
to form salts, which can ionize into the amino acid, as the cation, and the
acid group, as the anion. With bases the carboxyl group reacts to form
salts, which yield amino acid as the anion. A most important reaction con-
sists in the condensation of aldehydes with the amino group. This occurs
particularly readily with formaldehyde, water being eliminated in the re-
action, and the basic nature of the amino acid being thus destroyed.
Upon this reaction depends the method of Sorensen for determining the
amount of amino acid in a mixture (see page 606). The titration is per-
formed by rendering the solution of amino acids neutral, then adding
formaldehyde and titrating with standardized acid, using phenolphtha-
lein as the indicator, and thus finding to what degree the acidity of the
mixture has become increased as a result of adding the formaldehyde.
Since this increase in acidity must depend upon the number of amino
groups, it furnishes us with an indirect estimate of the concentration of
the amino acids. The reaction is illustrated by the equation:
radicle H radicle
NH.-C-COOH -1-11-0 = 0 = CH, = N-C-COOH + H2O
i A
(amino acid) (formaldehyde)
Another reaction of amino acid of physiologic interest is that known
as the carbamino reaction, consisting in a union of the amino acid with
calcium and carbonic acid. Finally, it is important to note that the amino
600 METABOLISM
group is very firmly attached; it remains intact in acids and alkalies and
is removable only by a process of oxidation. This can be accomplished by
treating the amino acid with such reagents as hydrogen peroxide or per-
manganate, when the amino group is displaced and a so-called ketonic acid
formed. The reaction will be evident from the accompanying equation:
CH3 CHS
O + NH,-C-COOH ?± O = 0-COOH+ NH3
H
( alanine ) ( pyruvic acid )
To illustrate this reaction we have chosen aminopropionic acid or ala-
nine, because the substance formed by its oxidation and known as pyruvic
acid is of very great importance in intermediary metabolism. It serves
as the common substance from which proteins, carbohydrates or fats may
be formed, and therefore as the intermediary substance through which
one of them may pass on being transformed into another (page 666). The
use of two arrows pointing in opposite directions in the above equation
indicates that the reaction may proceed readily in either direction.
The ammonia set free from amino acids may be oxidized to free nitrogen
by using nitrous acid according to the general equation: NH3-f-HONO=
2H20+N2. Upon this reaction depends another extremely important
quantitative method for measuring the number of amino groups present in
protein (Van Slyke). To make the estimation, nitrous acid is allowed
to act on the amino acids, and the volume of nitrogen gas set free by the
reaction is measured, the principle being similar to that used for the de-
termination of urea by the hypobromite method.
The apparatus employed for decomposing the substance and collecting and measuring
the evolved nitrogen consists essentially of a mixing bulb, connected below through stop-
cocks with two small burettes, one containing a solution of sodium nitrite and glacial
acetic acid, and the other a solution of the substance to be investigated. The upper end
of the mixing bulb is connected through a three-way cock with a graduated gas burette
and with another bulb containing potassium permanganate solution. By allowing some
nitrite and acid solution to run into it and shaking, the mixing bulb is first of all filled
to a certain mark with nitrous oxide gas. A measured quantity of the amino solution
is then allowed to mix with the nitrite; the apparatus is shaken for five minutes at 15
to 20° C., and the evolved nitrogen and nitric oxide are dviven over into the permanganate,
which absorbs the nitric oxide, leaving the nitrogen, which is then measured in the burette.
The apparatus has now been so perfected that numerous analyses may
be made with it in a very short time and with a degree of accuracy that
is scarcely surpassed in any other biochemical estimation.
From the point of view of protein chemistry, the most significant reac-
tion of the amino acids is their ability to link together to form compounds
THE METABOLISM OP PROTEIN 601
called peptides. This linking occurs between the amino group of one
amino acid and the carboxyl group of the other. When alanine and glyco-
coll, with which we are familiar, are thus linked together, the reaction
takes place according to the equation:
H CH H
/!H + HO: OC-C-NH, — HOOC - C -NH - CO- C- NIL + H2O
HOOC-C-N | . I ] |
^ \H H H H
(alanine) (glycocoll) (alanyl - glycocoll)
In this manner, then, a so-called dipeptide is formed, in which it will
be observed there still remain free carboxyl and amino groups, thus per-
mitting the linking on of other amino-acid groups to form tripeptides or
tetrapeptides or other polypeptides. Indeed, this process of condensa-
tion may go on practically indefinitely, a polypeptide containing eighteen
amino-acid groups — namely, three leucine and fifteen glycocoll groups — hav-
ing actually been synthesized. The resulting polypeptides have the proper-
ties of derived proteins like the proteoses; thus, they give the biuret
and other reactions characteristic of proteins and are precipitated by
such reagents as mercuric chloride and phosphotungstic acid. Some are
also digested by trypsin and erepsin. They have the same optical" activities
as proteins. One of them has been prepared which produces a typical
anaphylactic reaction. So far a polypeptide that can be coagulated by
heat or is in other ways identical with the naturally occurring proteins,
has not been synthesized ; but there is no doubt that it is only a matter
of time before this will be accomplished.
Eighteen distinctly different amino acids have been isolated from pro-
tein, and it may assist in the conception of our problem if we place these
amino acids side by side and link them together in the manner described
above. This is done in the accompanying chart compiled by D. D. Van
Slyke, in which also various other important facts concerning the chem-
istry of the amino acids are incidentally added.
At the lower part of each formula will be seen the characteristic car-
boxyl and amino groups of neighboring acids linking together the ter-
minal carbon atoms. The upper vertical bond of this carbon atom is con-
nected with the characteristic group of the amino acid, which may be very
simple and represented only by hydrogen, as in glycocoll, or highly com-
plex and including a ring formation, as in tryptophane. It will further
be observed that there may be other amino groups connected in various
positions in this radicle. This is particularly the case in the first three of
the amino acids in the table — namely, the basic amino acids. In lysine
the extra amino group reacts with nitrous acid, liberating free nitrogen
602
METABOLISM
s
P
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NEUTRAL AMINO ACIDS CONTAIN ONE AMINO OROUP AND ONE CARBOXYL GROUP.
WIIIOH NEUTRALIZE EACH OTHER
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Batio Amino Acidi
Two or more basic groups to one acid
OOOH group
The three "hexone bases." Resemble
X
g g
!E B ,o— o— B! i +
x
x" s" x" x" B" i „
SB— o—o— o o— o— X « i i
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a fc *
THE METABOLISM OF PROTEIN 603
by the Van Slyke method; but in other cases, as in arginine, it fails to
give this and the other characteristic reactions of the amino group. That
the extra amino group in lysine reacts directly with nitrous acid explains
why various proteins when examined for amino nitrogen yield an amount
that is equal to half of the lysine nitrogen.
It will further be observed that the amino acids are arranged in three
main groups: one basic, another neutral, and the third acid. The acids
of the basic group are three in number and have an alkalinity similar to
that of ammonia. They have been called the hexone bases, because each
contains six carbon atoms. They are alone present in certain forms of pro-
tein called protamines. The neutral amino. acids contain one amino group
and one carboxyl group, which exactly neutralize each other. This is
the largest group of amino acids, and is further subdivided into three:
one containing aromatic or benzene rings and including the very im-
portant amino acids, tyrosine and tryptophane; another containing the
so-called pyrrolidine ring; and the third, the largest of all, containing
the so-called aliphatic chains; that is, the chains characteristic of the
fatty acids and which may be either straight or branched. "When the chains
are branched, the substance is called an isosubstance, as in isoleucine.
The acid amino acids, including glutamic acid and aspartic acid, are
characterized by containing two carboxyl groups and only one amino
group. They therefore resemble acetic acid in acidity.
It may be of assistance to some if we restate these chemical facts
from a slightly different standpoint as follows:
Glycine, or glycocoll, is aminoaeetic acid, CH2NH2COOH.
NH2
/
Alanine is glycine plus a methyl group, CH3CH ; it is therefore amino-
COOH
OH
/
propionie acid and is closely related to lactic acid, which is CH3CH . Many of
COOH
the other amino adds may ~be considered as derivatives of alanvne,* thus :
1. Serine is alanine with an "OH" (hydroxyl) group in place of one of the "H"
NH2
/
atoms of the methyl group, CH2OH - CH
COOH
2. Cysteine is alanine with an "SH" (thio) group in this position,
NH2
/
CH2SH-CH
COOH
604
METABOLISM
Two cysteine molecules united at the " S " groups give cystine.
NH.,
OILS - CH
COOH
NH,
3. Phenylalanine has a C6H6 (phenyl) group, CH2C6H5-CH
\
COOH
NH
4. Tyrosine has a C6H4OH (phenol) group. CH,CCH4OH - CH
COOH
C
\
5. Tryptophane has a C6H4 CH (indole) group:
\ /
NH
C — CH2 - CH - NH2 - COOH.
/ \
C.H4 CH
\ /
NH
CH
/ \
N NH
I !
6. llistidine has a CH — C - (imidazole) group:
CH
/ \
N NH
I !
CH = C.CH2 . CH. NH3-COOH.
The last two are also called keterocyclic compounds, of which there
is another, viz.;
Proline (and oxyproline), which is a-pyrrolidine carboxylic acid:
CH2 — CH2
I ' I
CH2 CH.COOH
\ /
NH
Other amino acids are:
(1) Valine
Leucine
Isoleticine
thus :
CH. CH, CH, CH3 CH3 C2H5
\ / \ / \ /
CH CH CH
CH.NH2
COOH
(valine.)
CH2
I
CH.NH2
COOH
(leucine)
CH.NH2
COOH
(isoleucine)
THE METABOLISM OF PROTEIN 605
(2) The amino dibasic acids:
Aspartic, which is aminosuccinic acid,
CH2COOH
I
CHNH2COOH; and
Glutaminic, which is aminoglutaric acid,
CH2
CH2 - COOH
CHNH2 COOH.
Lastly there are the diamino acids, in which two groups exist :
Lysine a e-diaminocaproic- acid,
NH2
NH2CH2 - CH2 - CH2 - CH2 - CH2 - CH
COOH.
Arginine o-amino — 8-guanidine-valerianic acid,
NH2
HN — C NH2
NH.CH2 - CH2 - CH2 - CH
COOH
The guanidine group in this acid is of interest because of its close relationship to
NH2 rl~
/
urea, which is O = C
"NH,
CHAPTER LXVIII
THE METABOLISM OF PROTEIN (Cont'd)
AMINO ACIDS IN THE BLOOD AND TISSUES
In the Blood. — Furnished with the general facts concerning the chem-
istry of proteins, we may now proceed to consider the more precise
knowledge recently acquired concerning the history of this substance
in the animal economy. Although no one has succeeded in separating
ammo acids in pure condition from drawn blood even during the height
of digestion, it has nevertheless been possible to do so from circulating
blood by a method of dialysis, known as vividiffusion, elaborated by
Abel33 and his pupils. The method consists in connecting a long tube
of collodion with the two ends of a cut artery in an anesthetized animal.
The tube, coiled many times, is then immersed in a solution containing
approximately the same salt content as the blood plasma of the animal.
The diffusible constituents of the blood plasma dialyze into the saline
solution ; or any one of them may be prevented from dialyzing by adding
that particular substance to the saline in such amounts as will make its
concentration in plasma and saline alike. In some ways, it will be seen,
the apparatus may be considered as an artificial kidney. Its possible
clinical application, for the purpose of removing poisons from the blood
is under investigation. It has been possible in this way to isolate several
of the amino acids and other ammonia-yielding substances from blood.
Thus, alanine and valine have been obtained as crystalline salts, and
histidine and creatine (see page 622) shown to be -present by their reac-
tions. All of the amino substances, however, do not dialyze, and these
exceptions are further characterized by the fact that they do not readily
give up their ammonia on the addition of sodium carbonate, as do the
diffusible substances (Rohde). Although amino acids can thus be sepa-
rated in a pure state from circulating blood, their concentration in a
drawn specimen is too low to make direct quantitative estimation possible.
By the methods of Van Slyke and Sorensen, already described, however,
it has been shown among other things that the blood always contains a
certain concentration of amino acids ; thus, in that of fasting animals from
3 to 5 mg. per 100 c.c. of blood are usually found present. During the
absorption of a protein meal, the amino content of the blood undergoes
606
THE METABOLISM OF PROTEIN
607
a marked increase, becoming doubled or more; and a similar result has
been obtained by placing pure ammo acids in the small intestine. After
10 grams of alanine, for example, the amino nitrogen of the mesenteric
blood rose from 3.7 to 6.3 mg. per cent.*
In the Tissues. — After entering the circulation, the amino acid very
quickly disappear from it again. This has been demonstrated by ob-
serving the amount of amino acids in the blood after intravenously
injecting a solution of amino acid into an anesthetized animal. After
injecting 12 gm. of alanine into the vein of a dog, 90 per cent was found
Fig. 186. — Vividiffusion apparatus of J. J. Abel.
to have disappeared from the circulation within five minutes. The ques-
tion is, What becomes of the amino acids that rapidly disappear? Are
they decomposed in the blood, or do they become absorbed by the tis-
sues ? This problem has been attacked by analyzing portions of various
organs and tissues removed before and some "time after the injection
into an animal of amino acid solutions. In the case of the muscles it
has been found that the amino-acid content increases until from 60 to
80 mg. per cent of amino acid has accumulated. Beyond this point,
however, the muscles do not seem to be able to take up any more amino
acid. The capacity of the intestinal organs, however, is more elastic;
"This is a convenient way of stating per 100 c.c. of blood.
608
METABOLISM
for example, the amino nitrogen of the liver has been observed to become
increased to 125 or 150 mg. per cent of the original amount. Although
this absorption of amino acids by the tissues is extremely rapid, it never
proceeds to such a point that the blood becomes entirely free of them.
Even after many days' starvation the blood contains its normal quota
of from 3 to 10 mg. per 100 gm. of moist tissue (Fig. 188). This indicates
that a certain equilibrium must become established between the amino-acid
content of the blood and that of the tissues, the concentration in the tissues
being approximately from five to ten times greater than in the blood.
150
100
50
Injectio
Muscl e
z
Hours
Fig. 187. — Curves showing the amount of amino nitrogen taken up by different tissues after
the cutaneous injection of amino acids. The lowermost curve shows the urea concentration of the
blood. (From D. D. Van Slyke.)
The absorbed 'amino acids are very loosely combined with the tissues,
for they can be extracted by such feeble reagents as water or dilute al-
cohol. Their presence can not, however, be merely due to diffusion;
for if it were, the concentration could not become greater in the tis-
sues than in the blood. The further fate of the amino acids is difficult
to follow. We know that they do not remain in the body for a long time,
because most of the protein nitrogen in the food is excreted as urea
within twenty-four hours after ingestion; and when single amino acids
are fed, they quickly reappear in the urine as urea.
THE METABOLISM OF PROTEIN
609
The tissues can therefore be only a stopping-place for the amino
acids. When the latter are determined in blood collected from different
parts while absorption of protein from the intestine is in process, it
has been found, as shown in Fig. 188, that during the passage of the
blood through the liver there is a greater fall in the concentration of
amino acids than during its passage through the entire remainder of
the body.
It will be seen that the above conclusions are drawn from estima-
tions made on blood taken from the vena cava, portal vein, and hepatic
S
3
8
O
_j
OQ
o
8
I
O
Fig. 188. — Curves showing the concentration of amino-acid nitrogen in the blood during fasting
and protein digestion. (From D. D. Van Slyke.)
artery, the upper curves in the chart being from animals during digestion
and the lower from fasting animals. The results show that the liver must
be particularly greedy of amino acids, which, however, must Rapidly be-
come transformed into other substances, since no conspicuous varia-
tion has been found to occur in the amino-acid content of the tissues
according to whether the animal is fasting or is digesting protein food.
This result, it is to be noted, is quite different from that which is ob-
tained after the intravenous injection of amino acids, and the results of
610 METABOLISM
the two experiments taken together, indicate that the amino acids after
their absorption can not remain in the tissues in a free condition for a
long time. It means that the amino acids during natural digestion must
~be disposed of at a rate which is practically the same as that at wh'ich ab-
sorption is proceeding.
THE FATE OF THE AMINO ACIDS
To follow the metabolism of the amino acids further we must deter-
mine the end product into which they are converted. This is urea,
whose estimation can nowadays be made with considerable accuracy on
account of the discovery, by Marshall, of the action of urease in con-
verting its nitrogen into ammonia, which can then be estimated by com-
paratively simple methods (Folin).
When the viscera are compared before and at various periods after
the intravenous injection of amino acids, the immediate increase in
amino nitrogen remains undiminished in all of them except the liver, in
which a very rapid reduction is observed to occur. At the same time
the percentage of urea in the blood steadily rises. These facts are illus-
trated in Fig. 187.
The simplest interpretation of these results is that the liver converts
the amino acids into urea and discharges this urea into the blood. This
conclusion, however, it must be observed, is not inevitable; for it is pos-
sible that the amino acids may be condensed into polypeptides in the
liver, just as sugar is condensed by this organ into glycogen, and that
the increase in urea is merely coincident (Fiske).
It must not be imagined that the conversion of the amino acids into
urea is exclusively a function of the liver. On the contrary, it is well
known that this process may occur in animals from which the liver has
been entirely removed. It is probably safe to conclude, however, that
the liver is the most active center for amino-acid transformation and
urea formation.
When urea is estimated in samples of blood removed at short inter-
vals of time after the ingestion of a large amount of protein, it is found
that the increase becomes very early established. In one experiment,
before the food was taken the concentration of urea nitrogen in the blood
was a little over 10 mg. per cent; one hour after taking 500 grams of
meat, it had risen to about 18, and in two hours to nearly 25. Evidently
the increase had occurred about the same time as the passage of food
•from the stomach into the duodenum. These facts indicate that urea
formation in the liver becomes stimulated long before the other tissues,
such as the muscles, have had time to take up their full quota of amino
THE METABOLISM OF PROTEIN 611
acids. During digestion of protein the liver does not appear to wait
until the other tissues have become saturated with amino acids before it
begins to destroy the unnecessary excess by conversion into urea; on
the contrary, this process sets in with the very first installment of amino
acid that reaches the liver by the portal blood. This conclusion is in
harmony with the well-established fact that, when protein is given to a
starving animal, the greater part of its nitrogen is soon excreted as
urea, leaving only a small fraction to be used for rebuilding the wasted
tissues (see page 643).
The amino acids that are absorbed by the extrahepatic tissues become
very quickly converted into formed protein, as is evident from the fact
that the concentration of free amino acids in the tissues of an animal
during absorption of protein is not perceptibly greater than in those of
a fasting animal, and the question remains to be considered, What be-
comes of the protein thus formed? The answer is, that it is gradually
used up in the metabolic processes, so as to liberate again the amino
acids, which add themselves to those absorbed from the intestine and be-
come used again or excreted, according to the demands of the tissues at
the time for amino acid.
This process of liberation of amino acid from the breakdown of body
protein goes on of course irrespective of absorption of amino acid from
the intestine. It goes on, for example, during starvation; indeed, in
this condition the percentage of free amino acids in the muscles is, if
anything, somewhat higher than that observed in an ordinarily fed an-
imal. In starvation also the migration of amino acid is going on among
the various organs, of which those whose activity is essential to the
maintenance of life, such as the heart and the respiratory muscles, are
supplied with amino acids from tissues that are less vital, such as the
skeletal muscles (see page 568). These experiments further show that
free amino acids can not serve to any significant extent as food reserves
in the same way as glycogen and fat. If amino acids were of value as
food reserves, we should expect the store of them to be depleted
by starvation. As to how long a period of time elapses between the
incorporation of the absorbed amino acids into tissue protein and their
subsequent liberation again by autolysis, we are entirely ignorant.
The researches which we have just been considering do not throw any
light on the relative value of different proteins in tissue metabolism.
They do not inform us as to which of the amino acids must be absorbed
ready-made from the digested food, and which of them may be dispensed
with since the organism can manufacture them for itself. We know that
the higher animals can synthesize some amino acids, such as glycocoll,
but not others, such as tryptophane; but which amino acids belong to
612 METABOLISM
the glycocoll and which to the tryptophane groups, can not as yet
be definitely stated. The investigation of this problem, has to be under-
taken by experiments of an entirely different type — namely, by observing
the welfare and growth of animals fed on proteins of varying amino-
acid composition. A full discussion of these experiments is given in
the chapters on Nutrition and Growth.
CHAPTER LXIX
THE METABOLISM OF PROTEIN (Cont'd)
THE END PRODUCTS OF PROTEIN METABOLISM
Introductory. — So far we have approached the problem of protein
metabolism by studying the behavior of the absorbed products of pro-
tein breakdown, and we have seen that these become gradually assimilated
by the tissues and used by them in their metabolic processes. "We have
been unable, however, to offer any facts regarding the exact chemical
changes which each amino acid undergoes during this process of tissue
metabolism. At first sight it might appear an easy matter to collect
such information by direct examination of the tissues themselves, either
by searching in them for amino derivatives which might be derived from
absorbed amino acids, or by studying the changes which occur when
the amino acids are subjected to the action of the isolated tissue en-
zymes that must be responsible for the change. Such methods of in-
vestigation are, however, fraught with technical difficulties so great that
very little can be learned from them, and for the present at least we
must be content to piece our information together from facts derived
by less direct methods. Such a method is offered by investigating
the behavior of the end products of protein metabolism.
The main end product is urea along with traces of its precursor am-
monia, but these are not the only ones, for some amino acids after being
incorporated with the tissue proteins break down into products that
are no longer members of the amino-acid series, although they may be
closely related to certain amino ocids. Such substances are creatine and
its anhydrid creatinine. A part of the amino acids during their pres-
ence in a free state in the blood may also be excreted unchanged by
the kidney. Our list so far therefore includes urea, ammonia, creatine,
creatinine, and amino nitrogen, of which the last is usually included in
metabolism investigations in the fraction designated undetermined
nitrogen.
Another group of closely related substances coming, not from the
general protein metabolism of the tissues, but from the metabolism
which is peculiar to the nuclei, consists of the so-called purine bodies.
Furthermore, so as to serve as a check on results obtained by examining
these nitrogenous metabolites, it is important to observe the manner of
613
614 METABOLISM
excretion of the sulphur moiety of the protein molecule, for it will be
remembered that it is in protein alone that sulphur is usually taken into
the animal body. The excretion of sulphur therefore runs more or less
parallel with the intensity of protein metabolism.
After selecting the end products that are most likely to be of signif-
icance, the first question concerns the amount of each of them excreted
during twenty-four hours on diets that are either rich or poor in pro-
tein. The possibility of conducting such investigations obviously de-
pends on the use of quick and yet reliable methods for the estimation
of the nitrogenous metabolites. Such methods have been furnished by
the painstaking and careful work of Folin, an example of whose results
are given in the accompanying table.
NITROGEN-RICH
DIET
NITROGEN-POOR DIET
Volume of urine
Total nitrogen
Urea nitrogen
Ammonia nitrogen
Uric-acid nitrogen
Creatinine nitrogen
Undetermined nitrogen
Total SO3
Inorganic SO3
Ethereal SO3
Neutral SO3
1170 C.C.
16.8 grams
14.7 grams =
0.49 gram :=
0.18 gram =
0.58 gram =
0.85 gram =
3.64 grams
3.27 grams =
0.19 gram —
0.18 gram —
87.5%
3.0%
1.1%
3.6%
4.9% .
90.0%
5.2%
4.8%
385 c.c.
3.60 grams
2.20 grams
0.42 gram
0.09 gram
0.60 gram
0.27 gram
0.76 gram
0.46 gram
0.10 gram
0.20 gram
= 61.7%
= 11.3%
= '2.5%
= 17.2%
= 7.3%
= 60.5%
= 13.2%
= 26.3%
The general conclusions which may be drawn from these results are
as follows:
1. With a protein-rich diet much more urine is excreted in twenty-
four hours than with one that is protein-poor. Evidently the nitrogenous
metabolites act as diuretics.
2. The total or absolute amounts of nitrogen and of all the other
nitrogenous metabolites, save creatinine, become diminished during the
starvation period. The same is true of the sulphur derivatives, except
in the case of the neutral sulphur, which behaves like creatinine.
3. The decrease in the portion of nitrogen excreted as urea is relatively
greater than the decrease in total nitrogen, this fact being shown in the
table by the percentage figures, which were secured by calculating
the proportion of nitrogen in the various substances as a percentage
of the total nitrogen excreted during the periods. The inorganic sul-
phate behaves in a manner similar to the urea — that is, the percentage
of total sulphate excreted in the inorganic form becomes much less
during starvation.
4. The relative amount of all the other nitrogenous metabolites, as
well as that of the ethereal and neutral sulphates, becomes increased
during starvation.
THE METABOLISM OF PROTEIN 615
The most striking results of the above investigation are that ereatinine
remains unchanged during starvation, but that urea becomes relatively
increased. The former must be derived from metabolic processes going
on in the tissues independently of the supply of foodstuff carried to
them, whereas the latter must depend, if not entirely, yet very largely,
on the protein content of the food. Creatinine may therefore be called
an end product of endogenous metabolism, and urea an end product of
exogenous metabolism.
Other metabolites — namely, ammonia, uric acid and the undetermined
nitrogen, as well as the ethereal sulphates — must represent processes
of metabolism that are partly exogenous and partly endogenous.
. Having made ourselves acquainted with the general nature of the
changes that occur in the nitrogenous metabolites when protein metab-
olism is stimulated by the taking of food or depressed by starvation,
we may now proceed to take up each of the metabolites separately and
see what other information can be obtained regarding their source and
origin in the animal body.
UREA AND AMMONIA
For various reasons it is important to consider these two metabolites
together. During the intermediary metabolism of the majority of the
amino acids, the amino group becomes broken off as ammonia, which
immediately combines with the available acids to form neutral ammonium
salts. The most available acid for this purpose is carbonic acid; there-
fore ammonium carbonate is formed in large amounts. A small propor-
tion of the ammonia may combine with other acid radicles, such as
chlorine, to form ammonium chloride. The fate of these two types of
salt is very different. The ammonium carbonate becomes quickly trans-
formed into urea, whereas the ammonium chloride is excreted in the
urine. The process of urea formation may therefore be considered as
having the function of preventing the accumulation of ammonium car-
bonate in the animal body. It is the means by which a harmful substance
is converted into an innocuous substance— a detoxication process, in
other words.
Regarding the nature of the chemical process involved in this trans-
formation of ammonium carbonate into urea, reference to the formulas
below will show that the ammonium carbonate that is formed by the
union of carbonic acid with ammonia, by losing one molecule of water
becomes ammonium carbamate, -which by repetition of the process be-
comes transformed into urea ;
616 METABOLISM
OH ONH, ONH4 NH,
CO + 2NH3?±CO -H2O?±CO -H2O — CO
\ \ \ \
OH ONH4 NH2 NH2
(carbonic (ammo- (ammonium1 (ammonium (urea)
acid) nia) carbonate) carbamate)
Some of the urea may come from metabolic processes of an entirely
different type. One of these at least is known ; namely, the splitting-off
of urea from arginine, which it will be remembered is guanidine-amino-
valerianic acid (see page 605). An enzyme called arginase, having this
action, has been isolated from various organs and tissues. The diamino-
valerianic acid, or ornithine, which remains after the urea is split off,
may be further used in protein metabolism. The reaction is shown in
the following equation:
NH2 - C - NH - CH2 - CH2 - CH2 - CHNH2 - COOH + H2O '
I!
NH (arginine)
= NH2-CO
| + NH2-CH2-CH2-CH2-CHNH2-COOH
NH2
(urea)' (ornithine)
On an ordinary diet, as we have seen, a man excretes somewhat more
than 90 per cent of his total nitrogen as urea and about 3 per cent as
ammonia, the remainder of the nitrogen appearing in the other nitrog-
enous metabolites.
Influence of Acidosis on Ammonia-Urea Ratio. — It sometimes happens
that a large proportion of the ammonia is not converted into urea, but
is used for the purpose of neutralizing abnormal acids present in the
organism. "When mineral acids are given to an animal, or when acids
are produced in the organism itself by some faulty type of metabolism,
the ammonia excretion by the urine immediately rises. In diabetes, for
example, where considerable quantities of /3-oxybutyric acid are pro-
duced (see page 683), a decided increase in the ammonia excretion by
the urine is observed. A milder type of acidosis may also be induced
in normal persons by withholding carbohydrates from the diet, and
here again the ammonia excretion is relatively increased.
In such cases it is quite evident that ammonia is used as an alkaline
reserve of the body; that is, as a substance which is capable of prevent-
ing acidosis by neutralizing the acids. It does not appear, however,
that all types of acidosis entail the utilization of ammonia as reserve
alkali, and an increase in the relative amount of ammonia in the urine
does not necessarily indicate a condition of acidosis. In the pernicious
THE METABOLISM OF PROTEIN 617
vomiting of pregnancy, for example, a relatively high excretion of am-
monia has been found associated with no greater a degree of acidosis, as
determined by the power of the plasma to absorb carbonic acid, than in
normal cases of pregnancy.
Influence of Liver on Ammonia-Urea Ratio. — Experimental Observa-
tions: (1) REMOVAL OF LIVER. — There are several facts which indicate that
other causes than acid-production may interfere with the conversion of am-
monia into urea. What are these causes? Since, as we have seen,
the liver is the organ which most actively converts amino acids
into urea, it would be natural to expect that, when the functions of
this organ were interfered with, relatively more of the nitrogen excre-
tion would occur as ammonia and relatively less as urea. In order to
determine the exact significance of the liver as a urea-forming organ,
two types of investigation' have been used; namely, (1) observation of
the changes produced in- the ammonia-urea ratio in the urine by partial
or total removal of the liver; and (2) observation of the urea-forming
power of a liver perfused outside the body.
To remove the liver from the circulation the portal vein is brought
in apposition with the vena cava, the two are sewed together, and a
passage opened between them, after which the portal vein is ligated above
the anastomosis (forming the so-called Eck fistula). The portal blood
then passes directly into the vena cava, and the liver is now supplied
only by the hepatic artery. The animals live for a considerable time
after the operation, and the urine frequently contains relatively less
urea and more ammonia than normal. The results are, however, not
nearly so striking as would be expected if the liver were the main seat
of urea formation. The experiments have nevertheless brought to light
a fact of considerable clinical interest — namely, although the animals
may thrive if kept on a diet not containing an excess of flesh, they im-
mediately begin to develop peculiar symptoms, not unlike those of ec-
lampsia or uremia, when they are fed with large amounts of flesh food.
Most of the symptoms can be referred to abnormal stimulation of the
central nervous system, and examination of the urine has shown a large
increase in the excretion of ammonia and a change from the normal
acid reaction to an alkaline one.
At one time it was assumed that the toxic symptoms were caused by
the presence in the blood of ammonium carbamate, since large quantities
of the calcium salt of this substance could be- separated from the urine.
It is now known, however, that the ammonium carbamate is present only
because of the excess of ammonium carbonate, the two salts existing to-
gether in solution according to the laws of mass action. That the intox-
ication is not due to ammonium carbamate does not exclude the pos-
618 METABOLISM
sibility that it may be due to ammonia itself, although it is more likely
that other nitrogenous metabolites, produced when excess of flesh food
is taken, are the responsible agents.
If the liver is entirely removed by ligating the hepatic arteries in an
animal with an Eck fistula, a more pronounced decrease in urea and
increase in ammonia occur during the short period of time that the
animal survives the operation.
The results observed after the removal or diminution of liver function
fail to occur when other viscera are removed from the animal, which
would at least tend to indicate that the liver is very important in the
manufacture of urea out of ammonia. This does not, however, warrant
the conclusion that the liver is the only place in the animal body in which
such a process occurs.
In corroboration of these observations on mammals, it may be of in-
terest to note that when the liver is removed from ~birds, which is a com-
paratively simple operation on account of a natural anastomosis between
the portal and renal veins, there is a marked decrease in the excretion
of uric acid and, a corresponding increase in the excretion of ammonia
during the twelve hours or so that the birds survive. In birds and
reptiles urea is excreted as uric acid, being produced by a synthetic
process in the liver (see page 644). The changes in this experiment are
of considerable magnitude ; thus, before the operation the amount of
ammonia nitrogen relative to total nitrogen has been found to vary be-
tween 10 and 18 per cent; after the operation it may be increased to
between 45 and 60 per cent. The uric-acid nitrogen normally varies be-
tween 60 and 70 per cent of the total nitrogen; after the operation it may
fall to between 3 and 6 per cent.
In animals with an Eck fistula and with the hepatic artery ligated,
an increase in the urea output occurs when amino acids* are injected under
the skin. This result corroborates the conclusion that the liver can not
alone be responsible for the conversion of ammonia into urea.
(2) PERFUSION OF ORGANS. — This method consists in removing the or-
gan into a warm chamber or bath and perfusing it, through cannulse
inserted in its main artery and vein, with a solution of defibrinated blood
or of defibrinated blood mixed with saline solution. The perfusion
liquid is kept at body temperature and is saturated with oxygen. By
means of a pump it is made to circulate in a pulsatile flow, and the total
amount of urea or other metabolite in the circulating fluid is determined
before and after the fluid has been circulated several times through the
organ. When the liver is perfused, urea gradually accumulates in the
fluid, particularly after the addition of one of its known precursors —
for example, ammonium carbonate. When other organs or viscera are
THE METABOLISM OP PROTEIN 619
perfused, no urea is formed. The evidence shows that the liver is an
important seat of urea formation, but not necessarily that other organs
are unable to form it in the intact animal, for there are many sources
of inaccuracy in perfusion experiments. Even though we exercise the
greatest care, we can not hope to maintain the organ in other than a
slowly dying condition. It is certainly far removed from the normal
state, as is revealed not only by histologic examination, but by the fact
that edema almost invariably sets in and the blood vessels become ex-
tremely constricted, thus necessitating a gradual increase in the per-
fusion pressure as the perfusion goes on. Furthermore, the organ being
isolated from the nervous system, there can be no control of the rela-
tive blood supply of different parts. In the intact animal the circula-
tion is more or less distributed according to the particular needs of the
different viscera, and such conditions obviously can not be simulated in
a perfusion experiment. Another objection depends on the fact that
the well-being of the organs in the intact animal is largely dependent on
hormones conveyed to them from other organs. Such hormones are
frequently quite labile in nature, and soon disappear from the perfusion
fluid.
Notwithstanding these objections, there can be no doubt that many
of the functions of an organ are retained much longer than they would
be if the organ were not perfused ; for example, the contractility of the
muscle or the power of forming urea in the liver. Perfusion experiments
are of value therefore when they yield positive results. Negative re-
sults may indicate either that the organ does not perform the particular
function that is being investigated or that it has lost this function as a
result of partial death. That a perfused muscle retains its power of
contraction does not necessarily indicate that it maintains all of its
metabolic functions; neither does the fact that the liver forms urea
prove that it is capable of performing its other functions. It is easy to
show that the liver dies piecemeal; some functions, such as glycogen-
formation, die early, while others, such as urea-formation, remain for a
long time intact. The use of perfusion experiments for the settling of
questions of metabolism should therefore always be very carefully con-
trolled and never used as the sole line of evidence on which to base impor-
tant conclusions.
(3) Before leaving this subject it may be well to point out that the
method which at first sight might appear to be the simplest for settling such
questions — namely, the examination of the inflowing and outflowing blood
of different parts or organs — is not applicable in most cases. This is be-
cause of the extremely small changes in concentration which may occur
even although large amounts of the particular substance in question
620 METABOLISM
are being absorbed or produced. As we shall see later, this criticism is
particularly applicable in the case of sugar. Even during the injection
of considerable quantities of sugar into the portal vein, no difference
in percentage can be demonstrated between the blood of the two sides
of the liver, although we know that sugar is being retained to form
glycogen. For the same reasons, differences in the percentage amounts
of amino acids or of urea are often difficult to demonstrate in the blood
entering and leaving the liver even when we know that large quantities
of them are being added to or removed from it.
Clinical. — Since the liver is an important seat of urea formation, the
question arises as to whether the relative percentage of urea and am-
monia in the urine will become altered by disease of the liver. Many
observations with this point in view have been undertaken, but it can
not be said that the results are very striking. In extreme destruction,
such as that produced by phosphorus poisoning, there may indeed be
a great increase in the relative amount of ammonia and a decrease in
that of urea. The same is true in acute yellow atrophy of the liver, in
which disease the nitrogen excreted as ammonia may amount to as much
as 70 per cent of that excreted as urea. In milder forms of liver dis-
turbance, however, such as cirrhosis, the figures are much less striking.
When an increased ammonia excretion is observed in such cases, we
must be cautious in drawing the conclusion that it is due primarily to
abolition of the hepatic function. It may just as well be caused by the
development of acids in the organism that require the ammonia for
their neutralization. It is significant, for example, that considerable
quantities of acids are produced in phosphorus poisoning.
Although the urea and ammonia excretions become altered by exten-
sive destruction of liver tissue, it is a remarkable fact that very little if
any change occurs in the amino nitrogen, either of the urine or of the
blood. In experimental necrosis of the liver produced by chloroform
or by phosphorus, it is only in the latest stages of the condition and
when it is of the very severest type that an amino-acid increase has been
found to occur in the blood and urine. The conditions seem to be some-
what different in man, abnormally high amounts of amino nitrogen hav-
ing been observed in the blood in a considerable proportion of patients
with impaired liver function. In very severe cases of diabetes, for ex-
ample, figures that are distinctly higher than normal have been observed
(Van Slyke, etc.). In eclampsia the marked pathologic changes in the
liver might be expected to be associated with an upset in the metabo-
lism of amino acids. Losee and Van Slyke35 have, however, recently
shown by the most accurate methods that neither in the blood nor in the
urine is any excess of amino acids to be found in this condition, although
THE METABOLISM OF PROTEIN 621
in cases of pernicious vomiting of pregnancy, there was a relative in-
crease in the ammonia excretion. We have already seen that this
increase did not bear any relationship to the acid-absorbing power of
the blood plasma (see page 617).
The importance of the kidneys in removing the urea from the ~bldod
is readily seen from the change in the percentage of urea in this fluid
after the partial or complete removal of the kidneys. Animals sur-
vive nephrectomy for about three days, and during this time urea rapidly
accumulates in the blood and begins to make its appearance in the
saliva and the intestinal secretions. In man also where the kidneys
are extensively diseased, a similar accumulation of urea occurs in the
blood, some of the excess being got rid of through the sweat and to a
certain extent through the intestine. The importance of encouraging
perspiration and a free movement of the bowels in cases of nephritis is
thus indicated. It must not be concluded that the accumulation of
urea in the organism is the direct cause of the symptoms. Urea itself
is comparatively inert, and it is generally believed that other metabolic
products with which the urea runs parallel in amount are the toxic
agents. Hewlett has found, however, that very large injections of urea
do produce symptoms in animals.34
CHAPTER LXX
THE METABOLISM OF PROTEIN (Cont'd)
CREATINE AND CREATININE
Creatine and creatinine are very largely products of endogenous metab-
olism ; they are mainly derived from chemical processes occurring in
the tissues although some of the creatine and creatinine present in the
food may appear as creatine in the urine.
Essential Chemical Facts
Before we proceed further with a discussion of the metabolism of
these important substances, it will be necessary to refer briefly to some
points in their chemistry. The simpler of the two bodies is creatine,
which is methyl-guanidine-acetic acid; creatinine is its anhydrid, being
formed from creatine by the removal of a molecule of water, so that the
NH2 groups become joined together in the same way as they do in the
formation of pep tides from amino acids (page 599). The relationships
are illustrated in the following formulas:
(methyl)
CH3— N
/ \
/ CH..COOH
NH — C - H20 —
\ (acetic acid)
(guanidine) NH2
(creatine)
CH3-N-CH-CO
I
NH = C
\
\ I
NH
(creatinine)
It should be noted that guanidine is closely related to urea
NH2
/
(0=0 ), and that when creatinine is formed from creatine a ring
NH,
THE METABOLISM OF "pROTEUST 623
formation occurs, giving what may be regarded as an imidazole deriva-
tive (see page 604). Creatine is also related to one of the important
diamino acids, arginine, since both contain guanidine radicles,
NH2
(NH=C ), and to histidine and the purines (see page 634), both
NH2
of which contain the imidazole ring. The close relationship which
'creatine bears to urea is illustrated by the fact that urea is formed
when creatine is subjected to- the action of boiling barium hydrate. When
it is oxidized by means of potassium permanganate, urea is also formed,
the remainder of the molecule, more or less intact, being split off as
NH-CH3
/
methyl-ammo-acetic acid (CH2 ), also known as sarcosine.
COOH
The conversion of creatine to creatinine goes on slowly in aqueous
solutions, but is much accelerated by heating with acid. Heated in an
autoclave at a temperature of 117° C. for thirty minutes, with half nor-
mal hydrochloric acid, the creatine goes over almost quantitatively into
creatinine. It will be noted that the creatinine ring is partly oxidized.
This renders it unstable, so that creatinine in the presence of alkalies
has the power of reducing metallic oxides. Like glucose it can reduce
alkaline solutions of copper, silver and mercuric salts; it also reduces
picric acid in weakly alkaline solution to picramic acid, which, being red,
furnishes us with a solution the strength of which can be estimated
colorimetrically.
Quantitative Estimation. — Although the presence of creatinine in the
urine has been known for many years, there being from 1 to 2 grams of
it in the twenty-four-hour urine, little progress was made in the study
of its metabolism because of the absence of a reliable method for its
estimation. The elaboration by Folin of a colorimetric quantitative
method for creatinine, depending on the reduction of picric acid, has
furnished the starting point for the modern work which has been done.
To estimate the creatine by this method, it is usual to proceed as fol-
lows: The creatinine content is first of all determined, another portion
of urine being then heated with acid in the autoclave until all of its
creatine has been converted into creatinine. A second determination of
creatinine is then made, and the difference between the two is calculated
as creatine.
624 ' METABOLISM
It should be pointed out that, since the creatine is estimated by an
indirect method, there are considerable chances for inaccuracy. Indeed,
it has been shown that errors may have been incurred in some of the
recent work on account of the fact that when acetoacetic acid is present
in the urine it prevents the creatinine from developing its full reducing
power on picric acid in the cold, so that when subsequently the urine is
heated with acid for the purpose of converting the creatine into creati-
nine, the destruction of acetoacetic acid allows the reducing power of the
creatinine to develop to full intensity. It is obvious that this would make
it appear as if creatine had been converted into creatinine. It is par-
ticularly in the urine of diabetic patients, in which acetoacetic acid is
present that mistakes are likely to be made.
Metabolism
When we come to consider the metabolism of creatine and creatinine,
we find that there are remarkably few facts definitely known concerning
it. The average amount excreted daily, expressed as the number of milli-
grams of creatinine in twenty-four hours per kilogram body weight,
is known as the creatinine coefficient (Shaffer).36 For a lean person this
is about 25 mg. ; for a corpulent person, about 20 mg., the difference in-
dicating that muscle mass, and not body weight, is the important factor
determining the coefficient. Further evidence that this relationship ex-
ists is furnished by the fact that in the muscular atrophies creatine ex-
cretion is distinctly below normal. It must be the mass of the muscles
rather than their activities that is the determining factor, for the creatine
excretion does not become increased by muscular exercise.
Influence of Food, Age, and Sex. — Although creatine and creatinine are
endogenous metabolites, it must be remembered that, under ordinary
dietetic conditions, a part of each is derived from these substances pres:
ent in the food. It is important therefore to consider the conditions
under which the creatine and creatinine in the food appear in the urine.
Regarding creatinine, it is pretty well established that practically all
that is taken with the food reappears as creatinine in the urine. Shaffer
has, for example, succeeded in recovering 76 per cent of ingested creat-
inine in the urine excreted during twenty-one hours following the in-
gestion of 0.7 gm. creatinine.
The conditions for the excretion of creatine are more complex. It is
present in the urine of children in considerable amount, but in that of
adults only as traces. In the first years of life the creatine in boys'
urine may amount to one-half of the total creatine and creatinine, but
it becomes gradually less and practically disappears at about seven
THE METABOLISM OF PROTEIN 625
years of age. Girls, on the other hand, continue to excrete creatine until
about puberty, after which, although ordinarily absent, it reappears in
the urine at each monthly sexual cycle, and is present during pregnancy
and for some days after delivery. Feeding creatine to children causes
it to appear in the urine, accompanied usually by a slight increase in
the creatinine. The same results can be observed in women during the
monthly periods, when as much as 0.1 gm. may be present, and during
pregnancy. Creatine is also present in the urine of most if not all of
the other mammalia. Some of these facts are shown in the following
table :
AGE
CREATININE-N
CREATINE-N EXCRETED
IN 24-HR. URINE
2
0.025
0.023
-3
0.057
0.022
Boys
5
8
0.112
0.163
0.025
0.0
11
0.157
0.0
15
0.378
0.0
5
0.069
0.005
6
0.032
0.003
Girls
7
0.157
0.066
10
0.147
0.020
12
0.201
0.011
(From Mathews.)
When creatine is given to an animal that has been kept in a starved
condition, most of it seems to disappear. It can not be recovered in the
urine either as creatine or as any other nitrogenous metabolite. It seems
to functionate more as a food than as a useless substance. The possi-
bility that some of it can be destroyed by the intestinal bacteria being
admitted, there is nevertheless some justification for the view that the
creatine finds a useful function in the anabolic process of the muscles.
Influence of Complete and Partial Starvation. — Although, as we have
seen, the creatinine excretion remains constant when the amount of pro-
tein in the diet is greatly reduced, yet it does not remain constant during
complete fasting or when carbohydrates are entirely withheld from the
diet. In fasting it has been found that creatine appears in place of the
creatinine which has disappeared, so that if both creatine and creatinine
are determined, very little if any diminution will be found to have oc-
curred. Fasting, therefore, causes the adult creatine and creatinine
metabolism to become like the juvenile metabolism. As pointed out by
Mathews, it would be interesting in the light of this observation to see
whether other substances, passed in the urine of young animals but ab-
sent in that of the adult, would reappear in the urine when the animals
were made to fast. In the case of man, for instance, allantoin would be
worth investigating in this regard (page 641).
626 METABOLISM
A similar replacement of some of the creatinine by creatine appears
when carbohydrate is entirely withheld from the diet, or in diabetic
animals, either in the disease diabetes mellitus in man or in the experi-
mental condition induced in animals by giving phlorhizin. Unfortu-
nately, in a considerable part of the work that has been done on this
phase of the subject a method of estimation was employed which did not
take sufficiently into account the influence of acetoacetic acid on the
creatine estimation; but even after allowing for this possible source of
error, there can be no doubt that creatine appears in the urine when
carbohydrates are improperly metabolized. If carbohydrates are given
to a starving animal, for example, the creatine is replaced in its urine by
creatinine, although this will not occur when either protein or fat is fed.
The general conclusion which may be drawn from these observations is
that carbohydrates in some way are required for the proper conversion
of creatine into creatinine in the animal body (Cathcart)37.
Origin of Creatine and Creatinine
Notwithstanding the amount of excellent work that has recently been
done on the metabolism of creatine and creatinine, w,e know very little
indeed regarding the origin of these bodies in the animal organism. It
would be profitless to discuss this problem to any great extent, but a
few of the most important facts so far established may be of interest and
of value. The first step in attacking such a problem is to compare the
amounts present in the various organs and tissues, in the blood, and in
the excreta. Of the approximately 12D grams of creatine and creatinine
in the body of an average adult, a very large proportion is in the muscles,
the voluntary muscles containing the largest percentage, 'the heart con-
taining a medium percentage, and the involuntary (intestinal) muscles
containing relatively a small amount (Myers and Fine)38. Next to the
skeletal muscles, and containing more than the involuntary mus-
cles, come the testis and brain. The liver, pancreas, thyroid, kidneys,
spleen, etc., contain traces, the smallest amount of all being found in the
blood.
In all these places by far the greatest proportion of the total creatine-
creatinine exists as creatine, which is exactly the reverse of the condi-
tion obtaining in the urine of adults, where practically all is excreted as
creatinine. The close chemical relationship between creatine and creat-
inine, considered along with the above facts regarding their quantitative
distribution in the body, indicates that the creatinine of the urine is de-
rived from the creatine of the tissues. The question is, How does the
creatine come to be converted into creatinine? Such a transformation is
THE METABOLISM OF PROTEIN 627
probably effected by many of the tissues of the body and certainly by
the blood, the active agency in all cases being no doubt an enzyme. That
the blood contains such an enzyme is indicated by the fact that creatine
is transformed to creatinine by blood serum more quickly than it is
when merely dissolved in water. Even heated blood serum possesses
some of this power. The liver also probably brings about the transfor-
mation, as has been shown by perfusion experiments, and by the fact
that in cases of phosphorus or hydrazine poisoning creatine displaces
creatinine in the urine.
The problem therefore narrows itself down to the question of the
origin of creatine. In the light of chemical knowledge there are several
precursors from which creatine might be formed. One, for example, is
arginine, which it will be remembered is guanidine-amino-valerianic acid
(see page 605). By oxidation this might become changed into guani-
dine-amino-acetic acid, which by methylation would then be changed into
creatine. That such a process of methylation may actually occur in the
animal body is definitely known, for it happens when such substances as
pyridine or naphthalene are given with the food. They appear in the
urine as methyl derivatives. The possibility of the derivation of creatine
from arginine is not, however, borne out by the result of the injection of
arginine, for such injection does not increase the creatinine in the urine.
The closely related substance, guanidine-acetic acid, when fed to animals
(rabbits) does cause a slight increase in the excretion of creatine (Jaffe),
and also, it is said, an increase in the creatine content of the muscle.
Even in this case, however, by far the largest proportion of the admin-
istered guanidine-acetic acid is excreted in the urine unchanged.
The large percentage of creatine in muscle tissue leads one to expect
that some relationship must exist between muscular metabolism and the
amount of creatine present either as such in the muscles or as creatinine
in the urine. Regarding the latter point it is definitely established that
muscular exercise leads to no increase in the creatinine excretion, al-
though it is said that an increase occurs following a tonic contraction
of the muscles. With regard to the creatinine in the muscles, no definite
results indicating that muscular metabolism changes its amount are on
record. In the light of the fact already stated regarding the presence
of creatine in other organs than the muscles, it seems probable that the
substance has really little to do with muscular contraction as such, but
rather is concerned in some way in the formative metabolism of the cell,
with its general growth or maintenance. Indeed, it is a question whether
creatine is an actual constituent of the living tissue. It may rather, as
has been suggested by Folin, be a postmortem product, represented dur-
ing life by creatinine.
628 METABOLISM
Creatine appears in the urine in phosphorus poisoning, in carcinoma of
the liver and during postpartum involution of the uterus. It is not de-
rived from the disappearing uterine muscle, however, for creatinuria also
occurs after cesarean section with removal of the uterus. Creatine
elimination is not an index of cellular destruction, for it has been found
large in a dog injected with phlorhizin and maintained in constant weight
by feeding with washed meat (S. E. Benedict). Muscular fatigue also
leaves the creatine content of muscle unchanged. In late stages of
nephritis, creatinine accumulates in the blood and serves as an index of
the gravity of the condition (page 651).
CHAPTER LXXI
THE METABOLISM OF PROTEIN (Cont'd)
UNDETERMINED NITROGEN AND DETOXICATION
COMPOUNDS
In the present chapter we shall refer briefly to the groups of urinary
substances styled undetermined nitrogenous compounds and to the com-
pounds that are excreted in the urine as the result of the combination in
the body of certain toxic bodies with chemical substances that render
them harmless (detoxication compounds).
Undetermined Nitrogen
Included under undetermined nitrogen are amino acids, peptides and
basic substances. The amount of amino acids and peptides in normal
urine is very small but may become considerable in disease, especially
of the liver, when leucine and tyrosine may appear. The presence of
traces of amino acid and peptone in normal urine is to be expected,
for although the actual concentration of amino acids .in the blood is
never very great, a certain leakage of amino acids must occur into the
urine.
The peptide is sometimes known as oxyproteic acid. It becomes dis-
tinctly increased in phosphorus poisoning and in such conditions as are
accompanied by excessive protein metabolism. The basic constituents
include such substances as trimethylamine, ethylamine, putrescine and
cadaverine (page 502), and there are probably many more of a similar
nature. Many of these substances are similar to the so-called ptomaines
found in meat, etc., and they have been called the ptomaines of urine,
from which they can be isolated by rendering the urine alkaline and
shaking out with ether. It is probably to the presence of these sub-
stances that urine mainly owes its toxic action.
The Detoxication Compounds
Certain nocuous substances are produced in the intestine during the
digestive process (see page 501), and others may result from the meta-
bolic processes in the tissues. To guard against the harmful action of
these substances on the organism, they become detoxicated in various
629
630 METABOLISM
ways, mainly by forming inert compounds with other substances, par-
ticularly with glycocoll, sulphuric acid or glycuronic acid. The com-
pound thus formed is then excreted in the urine.
Hippuric Acid. — Glycocoll is used mainly to detoxicate the benzoic
acid which results from the oxidation of the aromatic substances pres-
ent in large quantities in vegetable food and fruit (particularly in cran-
berries). Some benzoic acid may also be produced by the breakdown
of the aromatic group of the protein molecule; phenylalanine, for ex-
ample, gives rise to benzoic acid by bacterial decomposition. The com-
pound formed is hippuric acid, this name indicating that it is present in
large quantities in the urine of the horse, as it is also in the urine of
all herbivorous animals.
Hippuric acid is benzoyl-glycine (C(iH5.CO.NH.CH2COOH), and it
can readily be produced in the laboratory by bringing together benzoyl
chloride with glycocoll, thus:
C6H5 . CO i Cl + H~ i HN . CH2COOH = CtH5CO ..NH . CH2COOH + HC1.
(benzoyl chloride) (glycocoll) (hippuric acid)
Under ordinary dietetic conditions only a trace of hippuric acid is
present in the urine of man, but much larger quantities, 2 grams a day
for example, may appear when the diet contains a large proportion of
fruit or vegetables. It is not known to undergo any characteristic varia-
tions in disease. The benzoic acid which is contained in certain canned
foods as preservative also combines in the body with glycocoll, so that
any toxic effect which it might produce is practically negligible. There
is certainly no very evident reason why canned foods containing benzoic
acid should be tabooed, for in so far as the berzoic acid is concerned, they
can be no more toxic than a diet composed largely of vegetables and
fruit.
This detoxication of benzoic acid requires the presence in the organ-
ism of a constant supply of glycocoll, which, it will be recalled,
is the lowest in the series of amino acids, being aminoacetic acid
(CH,NH2COOH). It is present in greatest amount in the protein of the
connective tissues. It is said, however, that not more than from 2 to
3.5 per cent of glycocoll is available in the proteins of the body. Al-
though this amount of glycocoll would amply suffice to detoxicate the
benzoic acid produced by the metabolism of the food in carnivora, it
is quite inadequate for this purpose in the case of herbivora, and the
question naturally presents itself as to where the glycocoll in these
animals comes from. It is said, for example, that of the total nitrogen
excretion in herbivora 50 per cent may appear as glycocoll under cer-
tain conditions. These facts indicate that the organism is capable of
THE METABOLISM OF PROTEIN 631
producing new glycocoll for itself, and it is interesting to consider how
this glycocoll may be derived. A very probable source is by synthesis
between ammonia and glyoxylic acid (CHO. COOH). That glyoxylic acid
or. its aldehyde, glyoxal, is readily produced during metabolism from car-
bohydrates and that ammonia is always available would seem to lend
some support to this view (see page 665). 'The synthesis of glycocoll
from glyoxal and ammonia occurs thus:
H.COCHO + NH3 = CH2NH.,COOH.
(glyoxal) (glycocoll)
The linking up of glycocoll with benzoic acid occurs in the kidney.
If the kidney is removed from the circulation in the majority of animals
that produce hippuric acid in large amount — the rabbit being a.n excep-
tion— no hippuric acid will accumulate in the blood. On the other hand,
an isolated perfused preparation of the kidney produces hippuric acid
provided benzoic acid is added to the perfusion fluid, and the latter also
contains an abundance of oxygen, which is best secured by using de-
fibrinated arterialized blood instead of artificial serum (Locke's solu-
tion). The necessity of a plentiful supply of oxygen is further shown
by the fact that, if the hemoglobin of the blood is rendered incapable
of carrying 02 by bubbling carbon monoxide gas through it, no synthe-
sis of hippuric acid will result from perfusing the blood through the
kidney. The actual chemical process by which the synthesis occurs (de-
.hydration) is similar to that by which polypeptides are formed by the
union of amino acids, or ereatinine from creatine.
(C6H5CO IOH + H; HNCH2COOH).
Glycocoll may be used for detoxicating other substances than benzoic
acid, particularly cholic acid, forming the glycocholic acid of the bile
(see page 494) and phenylaeetic acid. In birds the benzoic acid be-
comes combined with diamino-valerianic acid or ornithine (NH2-CH2-
CH2 - CH2 - CH - NH2 - COOH) in place of glycocoll, so that in the urine
of these animals in place of hippuric acid a compound called ornithuric
acid occurs.
It is of importance to point out here that this pairing of aromatic toxic
substances with certain of the metabolic products of the organism has
frequently been found an excellent experimental method for demon-
strating the presence of intermediary metabolic substances that other-
wise \vould not have appeared in the excreta. These substances are
thus diverted from their normal course in metabolism so as to form
neutralization or detoxication compounds. Glycuronic acid is an example.
632 METABOLISM
Ethereal Sulphates and Glycuronates. — The other substances used for
detoxication purposes are sulphuric and glycuronic acids. Phenol, and
its derivative cresol, after being absorbed from the intestine, in the
contents of which they are produced by the bacterial decomposition. of
protein (see page 501) become combined in the body, probably in the
liver, with sulphuric acid or with glycuronic acid to form the sulphate
or glycuronate. The aromatic sulphate further combines with potassium
to form the so-called ethereal sulphates, ias which the substance is excreted
in the urine. A small amount of phenol may however appear in the
urine unchanged. As we have already seen, the sources of the phenol
in the intestine are tyrosine and phenylalanine (see page 530), and
since these amino acids are also present in the tissues, it might be sup-
posed that some of the phenol sulphate of potassium present in the
urine could come from the tissues. It is usually assumed that, however,
derivation from the tissues does not occur.
Another ethereal sulphate is indoxyl sulphate of potassium, which re-
sults from the absorption into the blood of the indole and skatole pro-
duced by intestinal putrefaction from tryptophane (see page 502).
Immediately after absorption indole is- oxidized to indoxyl, which then
combines with sulphuric acid and with potassium to form indoxyl sul-
phate of potassium, which is the well-known indican of the urine. As
in the case of phenol sulphate of potassium, none of the urinary indican
seems to come from the normal metabolism (of the tryptophane) of the
tissue proteins. It is a much more reliable indicator than phenol sul-
phate of potassium of the extent of intestinal putrefaction, but it also
becomes increased in amount during putrefaction in the body itself,
as for example in abscess formation.
The amount of indican in the urine may be roughly gauged by oxi-
dizing the urine by means of hypochlorite and then shaking out with
chloroform. If the resulting extract is more than light blue in color,
it indicates excessive putrefaction. A negative test does not neces-
sarily mean that intestinal putrefaction is absent, but a marked positive
test always indicates that it is occurring. Skatole, the methyl deriva-
tive of indole, may undergo similar processes and appear in the urine
during excessive intestinal putrefaction. Its presence in the blood some-
times confers on the breath a distinct fecal odor, for this body, as its
name indicates, is that to which the odor of the feces is due.
Glycuronic acid, the other substance used for detoxication processes,
is of the nature of a dextrose molecule with the one end-group oxidized
to carboxyl (CHO - (CHOH)4 - COOH). It is probably produced under
normal processes of metabolism in the animal body, but is destroyed
unless when such poisonous substances as camphor, chloral hydrate or
THE METABOLISM OF PROTEIN 633
certain aromatic alcohols are given, when it is used for the purpose of
detoxicating them. The resulting glycuronates have reducing powers
and may be confused with glucose when present in large amount. Gly-
curonates may be distinguished from glucose in the urine (1) because
they are levorotatory, and (2) because they do not ferment. The free
acid itself, however, is dextrorotatory.
CHAPTER LXXII
Introductory. — The participation by highly trained organic chemists
in the investigation of biochemical problems has brought our knowledge
of the history of the purine substances in the animal body from a state
of chaos and guesswork to one of system and scientific accuracy. The
peculiar solubility reactions of uric acid and its salts and the discovery
of urates in gouty deposits served to make uric acid metabolism one of
the earliest research problems in both the medical clinic and the bio-
chemical laboratory, but the earlier results were practically valueless,
partly because they were inaccurate and partly because their interpretation
was impossible in the absence of even the most elementary facts concerning
the chemistry of uric acid.
Before any real progress was possible, a clean sweep had to be made
of all the old speculations and hypotheses, such as that dignified by the
high-sounding name of "uric-acid diathesis," and a foundation of ac-
curate chemical knowledge established. This foundation is now wonder-
fully complete, and a superstructure of biochemical fact is already
beginning to grow upon it. In the present chapter we shall examine
some of the most important contributions that have made this progress
possible.
As in the study of any other problem of metabolism, we must, however,
make ourselves familiar with the main facts concerning the chemistry
of the purine bodies and of the tissue constituents into the composition
of which they enter, before proceeding to the more strictly biological
aspect of the subject.
The Chemical Nature of the Purines
By an examination of the empiric formulas of the purines of biochem-
ical interest, it will be observed that they are all derivatives of a sub-
stance purine, which although in itself of no importance is interesting,
since it serves as the basic substance from which the others are derived.
The list is as follows:
Purine . C5H4N4
Hypoxanthine C5H4N4O Monoxypurine 1
Adenine . C5H3N4.NH2 Amino-purine I Purine
Xanthine . C5H4N4O2 Dioxypurine I bases.
Guanine . C5H3N4O.NH2 Amino-oxypurine J
Uric acid . C5H4N4O3 Trioxypurine
634
URIC ACID AND THE PURINE BODIES 635
The first oxidation product of purine is hypoxanthine, which has
long been known as a constituent of meat extract. Adenine, the amino
derivative of hypoxanthine, occurs in combination with other substances
in the nuclear material. The second oxidation product is xanthine and
its amino derivative, guanine. They occur in the same places as hypo-
xanthine and adenine. The highest oxidation product of all is the well-
known urinary constituent, uric acid, which may therefore be chemically
designated as trioxypurine. In addition to the purines of animal origin,
there are also certain ones of vegetable origin — the methyl purines, which
exist as the alkaloids of tea and coffee — namely, caffeine, theobromine,
and theine.
To understand the chemical structure of this group of substances,
it is perhaps simplest to start with that of uric acid. This consists
essentially of two urea molecules linked together by a central chain of
three carbon atoms, as will be evident from the accompanying structural
formula :
HN-CO
OC C-NH
! I! \
. co
I II /
HN-C-NH
(urea) (urea)
\
(central chain)
This structure can be shown by methods both of decomposition and
of synthesis. When uric acid is decomposed by oxidizing it with nitric
acid, it yields urea and a residue called alloxan ; or it can be synthesized
from urea and trichlorlactamide, a derivative of lactic acid, which it
will be remembered contains three carbon atoms. The changes involved
in this synthesis will be made clear by examination of the accompanying
structural formula, in which the manner of production of the by-
products of the reaction (NH3, H20 and HC1) are shown by dotted lines:
NH. ! H NH.. ; - C = O
/ I
CO
\
H J-C ! OH H ! NH
Cl i || CO
\ C- i~~Cl~H~~i NH . (urea)
(urea) NH. j ~ II Cl j
( trichlorlactamide )
636
METABOLISM
By milder oxidation by means of potassium permanganate in the
cold, uric acid becomes quantitatively converted to allantoin:
C5H4N403 + H20 + 0 = C4H6N403 + C02.
(uric acid) (allantoin)
The importance of this transformation lies in the fact that in most
animals, man and the higher apes being exceptions, uric acid is thus
decomposed in the animal body. The structural formulas for the other
purine bodies in relationship with those of purine and uric acid are given
below.
Purine itself has the following structural formula:
IN Ce H
H2 -
» - NH7
•\
08- H
sN — C4 - NS
(For convenience of description the atoms in purine are numbered as shown.)
HN-C=O
H- C C-NH
HN-C=O
O = C C-NH
\
/
C-H
N - C - N
(hypoxanthine) (6-oxypurine)
N = C-NH,
\
HN - C - N
(xan thine)
C-H
(2-6-oxypurine)
H- C C-NH
\
H,N =
HN-C=O
I-
NH
C-H
-C- N
\
N-C- N
C-H
(adenine) (6-amino-purine) (guanine) (2-amino-6-oxypurine)
HN-CO
Jc '
C-NH
CO
HN- C -NH
Uric acid (2-6-8-trioxypurine)
The substances with which the purine bases are most closely related
are the pyrimidine bases. Three of these are known:
thymine (NH-CQ cytosine ( N = C-NH2 and uracil (NH-CO
I ! I II
CO C.CH8 CO CH CO CH
NH-CH ); NH-CH); NH-CH).
URIC ACID AND THE PURINE BODIES 637
From an examination of the structural formulas, it will be seen that
they are more or less related to purine (having one of the urea radicles
omitted), although it can scarcely be doubted that they exist as separate
constituents of the nucleic acid group in the animal body, and are not
derived from purine. They are primary products.
The Chemical Nature of the Substances in Which Purine and
Pyrimidine Bases Exist in the Animal Body. — In general it may be said
that the amino purines — adenine and guanine — together with the
pyrimidine bases — thymine and cytosine — occur combined with phos-
phoric acid and a carbohydrate in the various nucleic acids, each of which
is again combined with some simple protein to form nuclein, the essen-
tial constituent of the chromatin of the nucleus. One of the oxypurines,
hypoxanthine, may also exist combined with phosphoric acid and carbo-
hydrate to form a substance present in muscle and known as inosinic
acid. The general scheme of construction of a nucleic acid of animal
origin is illustrated in the following formula suggested by Levene and
Jacobs :39
HO
O = PO — C6H]004 — C5H4N50
/ (hexose) (guanine group)
O
HO |
\
O = PO — C6H8O2 — C5H5N202
/ | (hexose) (thymine group)
HO
O
HO
\
O — PO — C6HSO2 — C4H4N3O
/ | (hexose) (cytosine group)
HO
O
Phosphoric acid \
groups O = PO — C6H10O4 — C5H4N5
/ (hexose) (adenine group)
HC/
According to this formula nucleic acid may be considered as a com-
pound of polyphosphoric acid, containing carbohydrate groups, which
serve to link the phosphoric acid molecules to those of purine or pyrimi-
dine. In nucleic acids of animal origin, such as the example given
above, the carbohydrate is a hexose, (i.e., contains 6 C-atoms), whereas
638 METABOLISM
in those of plants (e. g., yeast), it is a pentose (5 0-atoms). It has been
found necessary to introduce some terms to designate the different parts
of the nucleic acid molecule ; thus, the whole molecule is called a tetra-
nucleotide, each mononucleotide molecule of which is composed of a
phosphoric acid molecule plus a nucleoside, which again is composed of
a purine or pyrimidine nucleus attached to pentose or hexose. The
nucleoside is so named because it is similar in structure to a glucoside.
Apart from differences in the carbohydrate group, it appears that
there is a close similarity in the structures of nucleic acids from dif-
ferent cells. This would indicate a common function for them all, which
may be either of a skeletal or of a physiologic nature; that is, nucleic
acid may have to do with the sustentacular material that builds the
nucleus, or it may have to do with some physiologic function common
to all cells, such as irritability, or growth, or respiration. If nucleic
acid is merely a sustentacular material, then the study of the behavior
of chromosomes and chromatine in cells can not have the significance
that it would have were nucleic acid concerned in the more vital activ-
ities of the nucleus. All the so-called nuclear stains owe their specific
staining properties to the fact that they are of a basic nature and com-
bine with nucleic acid. Until we know more definitely what the exact
function of nucleic acid may be, it is unwise to place too much weight
on the behavior of the chromosomes in cytologic researches.
The History of Nucleic Acid in the Animal Body. — We shall first
of all study the manner in which nucleic acid may be broken down. As
is to be expected from its complex structure, various types of enzymes
are concerned in this process. The first to act are known as the nucle-
ases. They split the tetranucleotide molecule into two dinucleotides,
which immediately afterward split further into mononucleotides. Four
nucleotides, two of purine and two of pyrimidine, are thus formed from
each molecule of nucleic acid. Each nucleotide molecule may now un-
dergo decomposition in one of two ways: (1) either by the splitting off
of phosphoric acid, leaving a nucleoside (guanosine or adenosine), or
(2) by the splitting off of both phosphoric acid and carbohydrate, leaving
free purine bases. Nucleuses have been found which specifically effect
either of these decompositions, and they have been called phospho-
nucleases* (1), and purine-nucleases (2), respectively. In the decompo-
sition of nucleic acid all of the four purine compounds — guanine, guano-
sine, adenosine and adenine — may be formed. This is illustrated in the
accompanying schema, in which the nucleic acid is represented as a
purine nucleotide:
•The numbers refer to the enzymes indicated in the schema.
URIC ACID AND THE PURINE BODIES 639
NueteicAcid (without the pyrimidine group)
/
tf
(Action of nucleuses)
d ' \
Guanine<L-(7) Guanosine A d en o sine (8)— >Adenine
euses) \
\ \
d en o sine 8— >Ad
(4) (5) (6)
(Action of deamvnizing enzymes')
>Jr <V
Xauthosine Inosine
(9) (Action of hydrolysing enzymes) (10)
-
Uric Acidf- ( 11 ) Xanthine < - • ( 11 ) - » Hypoxanthine
(Action of xanthine oxidase)
(Jones.)
The next step in the disintegration process is that the amino group
is removed and the corresponding oxypurine is produced. To bring this
about, there exists a specific deaminizing enzyme for each of the above
amino compounds, and each enzyme is named according to the exact
amino purine upon which it acts; thus, guanase (3), guanosine-deaminase
(4), adenosine-deaminase (5), and adenase (6) have all been identified.
The free base may then be split off from the nucleosides by specific
hydrolyzing enzymes (1} (8) (9) (10).
The joint action of these enzymes leads to the formation of oxypurines,
xanthine and hypoxanthine, which are oxidized to uric acid by xanthine-
oxidase (11).
In man and the anthropoid apes uric acid is the end product of the
above changes, but in other mammals most of the uric acid is further
oxidized into allantoine. It has also been found, except in man and the
chimpanzee, that extracts of organs such as the liver, are capable of
decomposing uric acid into allantoine. The identification of these specific
enzymes is sought by a determination of the free amino-purine bases
and the phosphoric acid produced by allowing an aqueous extract of
the tissue in question to act on nucleic acid (of yeast)* at body tempera-
ture. Another portion of the digested mixture is then hydrolyzed by
means of boiling sulphuric acid and the constituents again determined.
From the results it is often possible to draw conclusions as to the exact
nature of the enzymes present.
The most remarkable outcome of this work has been to show that
the distribution of the enzymes is not the same in the tissues and organs
of different animals. Very briefly, some of the most important results
that have so far been obtained are as follows: Gastric and pancreatic
juices do not contain a trace of any of the enzymes. Intestinal juice,
'Yeast nucleic acid is used because it is less resistant to disintegration than thymic nucleic acid.
640 METABOLISM
on the other hand, contains a nuclease capable of splitting the poly-
nucleotides into monomicleotides. The two pyrimidine nucleotides split
off do not undergo further change, but the purine nucleotides are con-
verted into nucleosides (the enzyme being designated "nucleotidase")-
Extract of the intestinal mucosa, besides having the same action as the
intestinal juice, can also decompose the purine, but not the pyrimidine
nucleosides, into carbohydrate and purine groups (specific action of
"nucleosidase"). A similar action is produced by extracts of kidney,
heart muscle, and liver. Blood serum, hemolyzed blood, and extract of
pancreas, on the other hand, are capable only of carrying the decompo-
sition as far as the mononucleotides.
Eegarding the other enzymes mentioned in the above list, it is im-
portant to note that they appear at different stages in embryonic develop-
ment, and that their distribution varies considerably in different species
of adult animal, the spleen, liver, thymus, and pancreas containing them
most abundantly. The distribution of enzymes in the organs of the
monkey resembles that in the lower animals considerably more than it
does that in man. Some remarkable facts have come to light regarding
guanase and adenase, particularly that guanase is deficient in the organs
of the pig, in the urine of which animal it has also been found that the
purine bases are in excess of the uric acid. This absence of guanase
no doubt accounts for the fact that deposits of guanine may occur in the
muscles, and that these may be so large as to constitute the condition
known as guanine gout found in this animal. Adenase, on the other
hand, is absent from the organs of the rat, which again corresponds with
the fact that, when adenine is injected subcutaneously into these ani-
mals, it undergoes oxidation without the removal of its amino group.
In the human organism, adenase appears to be absent from all of the
organs, whereas guanase is present in the kidney, lung and liver, but
not in the pancreas or spleen. Xanthine-oxidase exists only iri the liver.
The distribution of uricase is perhaps the most interesting. It is pres-
ent in most of the lower animals. On account of its presence extracts
of the liver, spleen, etc., in all breeds of dogs, with the exception of
Dalmatians, rapidly destroy uric acid; and practically no uric acid
when injected subcutaneously can be recovered unchanged in the urine,
but appears as allantoine. Uricase, however, is absent in man. This has
been demonstrated by finding (1) that when uric acid is injected sub-
cutaneously, nearly all of it appears in the urine, and (2) that uric acid
is not destroyed when extracts of the organs are incubated at body
temperature with uric acid or its precursors. It must of course be kept
in mind that, although the uric acid is thus shown not to be destroyed
in vitro, it may nevertheless be destroyed in the living animal.
URIC ACID AND THE PURINE BODIES 641
The importance of the above described results rests in the fact that
from them we may hope to be able, ultimately, to state exactly in what
organs and tissues the intermediary metabolic processes concerned in
nucleic acid metabolism occur. The work at the present time is of spe-
cial significance, since it represents one type of evidence which we must
have before we can trace exactly every step in the metabolism of any
other biochemical substance.
The absence of uricase from the tissues of man places him in a unique
position with regard to the metabolism of nucleic acid, and renders the
investigation of the problem particularly difficult, since animal experi-
mentation is useless. Recently, however, S. R. Benedict has discovered
that the Dalmatian breed of dog — also known as the carriage dog, and
having a spotted or mottled skin — has a purine metabolism like that of
man.4 When fed on food containing no purine substances, he excretes
large quantities of uric acid, and when the latter substance is injected
subcutaneously, it is eliminated quantitatively as such in the urine. We
shall see later how experiments on this animal have been made use of
in the investigation of problems of purine metabolism as applied to man.
In all other animals most of the uric acid is oxidized to allantoine before
being excreted. The degree to which this occurs varies between 79
and 98 per cent of the uric acid in different species. This has been
called the uricolytic index (Hunter and Givens).
The Balance between Intake and Output of Purine Substances under
Various Physiologic and Pathologic Conditions. — The main purine ex-
cretory product in man is uric acid, but there is also a certain amount
of purine bases. The presence of uric acid has attracted attention for
a great many decades in medical investigation, because of the relative
ease with which it can approximately be determined quantitatively, and
because of the well-known fact that it may be responsible for certain
diseases, such as gout, when it accumulates in the tissues in an insoluble
form. On a diet containing meat, or more particularly on one con-
taining glandular substances, the total daily excretion of uric .acid is
very considerably greater than when the diet contains no such food
stuffs. The conclusion which Burian and Schur43 drew from this ob-
servation is that purine must be partly of exogenous and partly of
endogenous origin. In other words, some* of it is derived more or less
directly from performed purine substances in the food, and the remain-
der from the purine constituents of the animal's own tissues.
Endogenous Purines. — It was thought that a definite proportion of
each of the administered purines could be invariably recovered from
the urine. Although this has not been found to be exactly true, there
is nevertheless a certain constancy in the proportion of administered
642 METABOLISM
purine that is excreted. Thus, Mendel and Lyman have found recently
that about 60 per cent of injected hypoxanthine, 50 per cent of xan-
thine, 19-30 per cent of guanosine, and 30-87 per cent of adenine were
eliminated as uric acid. When combined purines — i. e., nuclear mate-
rial— are given, only a small proportion of the purine reappears as uric
acid in the urine. There is, therefore, a general parallelism between
the purine content of the food and that of the urine, which indicates that
purine-rich food should be eliminated from the diet of patients who are
suffering from deposition of insoluble urate in the tissues, as in gout.
The fate of the purine that disappears in the body is unknown ; some of
it may be decomposed in the intestine, but why so much of the remainder,
after absorption by the blood, should disappear is a mystery, since no
uricase can be discovered in any of the organs or tissues. The destroyed
purines can not be shown to influence any of the other well-known
nitrogenous metabolites of the urine.
The following table of experiments by Taylor and Rose45 may serve
to illustrate these points. The subject was placed on a purine-free diet
consisting of milk, eggs, starch and sugar, for three days. After this
period a part of the total nitrogen (3 grams) was supplied as sweet-
breads— thymus gland, etc. — containing a high percentage (0.482) of
purine nitrogen; for another period of four days still more of the nitro-
gen (6 grams) was replaced by sweetbread nitrogen; and this was fol-
lowed by a final period in which the original diet of milk, etc., without
purine substances, was restored. The following table gives the results:
1ST PERIOD 4TH PERIOD
PURINE-FREE 2ND PERIOD 3RD PERIOD PURINE-FREE
DIET DIET
Total urinary N
8.9
8.7
9.1
8.8
Urea N and NH2
7.3
7.1
7.1
7.05
Creatinine
0.58
0.55
0.56
0.47
Purine N (total)
0.11
0.17
0.26
0.10
Urie acid N
0.09
0.14
0.24
0.07
Remainder N
0.91
0.88
1.18
1.18
The increase of uric acid accounted for less than half of the purine
nitrogen ingested. This appeared as uric acid, the excretion of purine
bases being practically unchanged.
CHAPTER LXXIII
URIC ACID AND THE PURINE BODIES (Cont'd)
SOURCE OF ENDOGENOUS PITRINES
Even after the entire elimination of all purine substances from the
food in the case of man, purine continues to be excreted in the urine
as uric acid. This, as above remarked, is called endogenous excretion.
At first it was thought by Burian and Schur that the total nitrogen of
the purine-free diet could be considerably varied without causing any
alteration in the amount of the endogenous purine excretion, but a rep-
etition of the work has shown that, when these changes are of consider-
able magnitude, the endogenous moiety does not remain constant. This
has already been demonstrated in the table on Folin's results (see page
614), and is still better illustrated in the accompanying table, which
shows the excretion of uric acid and coincidently of urea from hour to
hour in the urine after taking food which is free from nuclein or purine
substances. After a fast of six hours, a diet consisting of bread and
potatoes was taken at 1:30, and the urea and uric acid measured in the
urine each hour thereafter.*
TIME
UREA
GM.
URIC ACID
MG.
AMOUNT OF URINE
C.C.
10-11
1.07
26
175
11-12
1.13
27
118
12-1 P.M.
1.07
24
164
1-2 (meal)
0.64
21
•60
2-3
1.12
22
43
3-4
1.16
38
41
4-5
0.84
40
53
5-6
1.16
56
59
6-7
1.20
39
56
7-8
1.37
30
95
8-9
1.47
33
183
9-10
1.33
24
155
10-11
1.33
23
180
(Hopkins and Hope.)46
A postprandial increase of endogenous purine excretion is very dis-
tinct, and it indicates that during the process of assimilation something
must be occurring in the organism which entails the production of purine
*These investigations should be repeated, since there is some question as to whether the method
of analysis employed (Folin-Shaffer) is suitable for determining hourly uric-acid excretion.
643
644 METABOLISM
from the organism itself. As to what this may be, it is impossible to
say. It may be associated with the work of the gastric and intestinal
glands, which recalls the interesting suggestion, originally made. by
Horbaczewski, that ingested substances increase the excretion of uric
acid by causing a leucocytosis, the purine being derived from the nucleic
acid set free when the leucocytes become broken down. That this is
not the correct explanation, however, is indicated by the fact that in-
gested substances that give rise to an increased number of leucocytes
affect the excretion of uric acid during the period the leucocytes are
present in the blood, and not after they have disappeared, which would
have to be the case were the uric acid a product of purine substances
liberated by their breakdown. This would indicate that the purine sub-
stance is a metabolic product of the living leucocytes and not a break-
down product of those that are dead. It should be noted that the increase
in the postprandial uric-acid excretion occurs earlier than that of urea.
The most pressing question concerns the origin of the endogenous
purines. Uric acid is the purine with which we are most concerned in
the case of man, and chemistry shows us that it may be produced either
by the oxidation of the lower purines — namely, of those which are the
constituent parts of the nucleic-acid molecule — or by a synthesis of two
urea molecules with a carbon residue containing three carbon atoms.
There are consequently two sources from which the endogenous purine
excretion in man may be derived : (1) synthesis of two urea molecules,
and (2) oxidation of the lower purines.
We will consider first the possibility of synthesis. In birds and
reptiles practically all the nitrogen is excreted in the form of uric acid,
and it is easy to show that this has been produced in the organism by
the synthesis of urea with carbon-rich residues, occurring mainly in the
liver. Minkowski found that by removing the liver from geese, which
is a comparatively simple operation on account of an anastomotic vein
between the portal and. the renal veins, the uric acid in the urine became
very markedly decreased and ammonium lactate took its place (page
618). Since we know that ammonium in the animal body is ordinarily
converted into urea, we may conclude from this observation that some-
thing has occurred to prevent the synthesis of urea into uric acid. In
confirmation of this conclusion it was subsequently found that, if am-
monium lactate was added to the blood perfused through the isolated
liver of the goose, uric acid was produced in the perfusion fluid.* Fur-
thermore, when birds and reptiles are fed with ammonium salts or
with the degradation products of protein, there is an increase in the ex-
*The reason for the formation of this relatively insoluble metabolite in place of the soluble urea
is connected in some way with the fact that birds and reptiles do not take such large quantities
of water with their food as other animals.
URIC ACID AND THE PURINE BODIES 645
cretion of uric acid instead of urea. Everything which in a mammal
tends to cause an increase in urea excretion causes in birds and reptiles
a similar increase in the excretion of uric acid.
In the early days of research in the uric-acid problem, not inconsid-
erable mistakes were made on account of failure to recognize the essen-
tial difference in the metabolism of uric acid in birds and mammals,
and the tendency for some time after the exact state of affairs was
discovered was to consider that in mammals none of this synthetic proc-
ess occurs. The latter view, however, is surely incorrect, for a cer-
tain amount not only of .uric acid itself ~but of the, lower purine bodies
can ~be produced ~by synthesis in the mammalian body. Thus, Ascoli and
Izar47 discovered that uric acid could be made either to disappear or
to be formed when a minced preparation of liver was incubated, depend-
ing upon whether oxygen or carbon dioxide was bubbled through it.
With oxygen uric acid disappeared, whereas with carbon dioxide uric
acid accumulated, indicating that in the presence of this gas the destroyed
uric acid became reformed from the disintegration products of the oxy-
genation process. As similar results were obtained from the livers of
birds, it is clear that no essential difference exists between the purine
metabolic processes occurring in the livers of birds and of mammals.
The difference is a quantitative not a qualitative one.
Regarding the chemical nature of the product into which uric acid is
broken down and from which it may be resynthesized, it has been pos-
sible so far to identify but one substance — namely, dialuric acid. This
is a perplexing result, for from all other investigations it would appear
that in mammals, with the exception of man and the anthropoid apes,
uricase splits uric acid into allantoine (see page 640), which substance,
however, when added to liver extract did not cause any uric acid to be
formed ; nor did any of the other known decomposition products of uric
acid have such a result. The chemical reaction involved in the produc-
tion of uric acid from dialuric acid and urea is indicated as follows:
NH
/
/
'
NH
— C
J
J
-*
J
!
= O
NH
C
NH
= O
(urea)
— H.OH H
= O H
(dialuric acid)
The synthesis of uric acid is brought about by the combined action
of a thermolabile enzyme in the blood and a thermostable body in the
tissues. An aqueous extract of blood-free liver of the dog can destroy
646 METABOLISM
uric acid only in the presence of oxygen; it can not reform it even in
the presence of carbon dioxide. On the other hand, blood serum can
not reform uric acid, whereas a mixture of the bloodless liver extract
and blood serum produces uric acid readily under suitable conditions.
Boiling of the liver extract does not affect the result, but boiling of the
blood serum renders it incapable of exerting its joint action with the
bloodless liver extract.
These experiments with dog's liver serve only as circumstantial evi-
dence that uric-acid synthesis occurs in mammals as well as in birds.
More direct proof that purine synthesis occurs in mammals is as follows:
(1) It was discovered long ago by Miescher that salmon, after leaving
the sea to ascend the rivers, have a well-developed muscular system, but
that in the upper reaches of the stream the muscular system becomes
considerably atrophied and the testes enormously developed. As the
fish takes no food during the migration, there must be conversion of
the protein of the muscles into the cellular tissue of the sexual glands,
and nucleic acid must be produced. (2) A hen's egg before its incuba-
tion contains practically no nucleic acid, whereas after development has
well started nucleic acid increases by leaps and bounds. Similarly the
eggs of insects increase in purine content very markedly as development
proceeds. (3) Milk contains practically no purine derivative, and yet
when it is fed to young growing animals, the organs lay on purine sub-
stances abundantly. In general, indeed, it may be said that the combined
purine increase is in proportion to the increase in body weight on the
milk diet. (4) In Osborne and Mendel's experiments already alluded
to, it has been shown that adequate growth depends primarily on the
nature of the protein building stones, and not upon the purine content
of the food. (5) An objection might be raised to these results on the
score that they do not apply to the adult mammal. Investigation of
the problem has hitherto been seriously impeded by the fact that 'no or-
dinary laboratory animals were known in which uric acid is excreted in
the urine. The discovery that this occurs in the Dalmatian dog has,
however, made it possible for S. R. Benedict*1 to show, not only that
after increasing the amount of nonpurine food there was a very distinct
increase in the uric-acid excretion, but also that when the animal was
kept for a year on such foods there was excreted a total amount of uric .
acid at least ten times greater than could have come from the traces
unavoidably included in the food.
Regarding the chemical nature of the substance from which the purine
is synthesized, we know at present practically nothing. No doubt some
of the protein building stones functionate in this capacity, pyrimidine
being probably the product that is first formed. Thus, pyrimidine may
URIC ACID AND THE PURINE BODIES 647
be produced as a result of the combination of amino-malonic acid with
urea, the amino-malonic acid being produced by condensation of hydro-
cyanic-acid molecules:
3 HCN -» H2N- CH(CN)2 + CO(NH)2 -^ NH - CO
I !
CO CNH2
! I!
NH - CNH2
(hydrocyanic (amino-malonic (urea) (oxy-diamino-pyrimidine)
acid) nitrile)
Another possible source of pyrimidine is the oxidation of arginine to
guanidine-propionic acid, which then condenses to form ammo pyrimi-
dine.
Purine synthesis undoubtedly occurs in the mammalian body, but it
is difficult to recognize in metabolism investigations because it is a slow,
continuous process. The probability of its occurrence, however, is indi-
cated by such results as those described on page 614, in which increase
in purine excretion is observed after varying the intake of food, even
when this is itself entirely free from purine substances. Whether or not
changes in the activity of purine synthesis occur in conditions of disease
is a question which awaits investigation.
The Influence of Various Physiologic Conditions, of Drugs, and of
Disease on the Endogenous Uric-acid Excretion. — Muscular exercise was
thought by Burian to cause an increased excretion of uric acid, from
which he drew the conclusion that the hypoxanthine present in compara-
tively large amount in muscular extract, or its precursor, inosinic acid,
must be an important source of endogenous uric acid. Other observers
(Leathes, etc.) have found that strenuous exercise causes a distinct in-
crease in uric-acid excretion, which, however, is much less marked on
repetition of the same kind of exercise on the next day. If some new
kind of muscular work is performed, another increase in uric acid will
result. There are still other investigators who deny that muscular work
has. any influence on uric-acid excretion.
It has been observed by several investigators that the endogenous
purine excretion is distinctly higher during the waking h'ours than during
sleep. This can not be shown to depend on variations in the urinary
function, and since it is decidedly doubtful whether ordinary muscular
activity has any influence, the diurnal variation is most difficult to
account for. The endogenous excretion in man is not the same for
different individuals, even when calculated for the same body weight; it
varies between 0.12 and 0.20 per cent purine nitrogen in an adult man.
It remains remarkably constant for a given individual from time to
time, being unaffected by moderate degrees of variation in the amount
648 METABOLISM
of food taken provided this be purine-free; when, however, the amounts
are extremely variable, changes are produced (see page 614).
In disease, fever causes an increased excretion. This has been most
clearly shown by Leathes, who took a large enough dose of antityphoid
serum to produce a distinct degree of fever (103° F.), and found that
an increase in uric-acid excretion occurred. That increased combustion
processes occurring in the tissues were responsible for the uric acid,
was shown by the same author, who caused a similar increase by sub-
jecting himself to cold baths for a considerable period of time. The in-
creased loss of heat thus induced stimulated the combustion processes in
the body so as to maintain the body temperature, and as a result there
was an increase in uric-acid excretion. It has long been known that an
excessive amount of uric acid is excreted in leucocythemia. The nuclein
of disintegrated leucocytes is commonly held responsible for the increase.
Naturally, much work has been done on the endogenous and exogenous
purine excretion in gout. No very striking anomalies of excretion have,
however, been brought to light, except perhaps that after the ingestion
of purine-rich foodstuffs it takes longer for the resulting exogenous ex-
cretion to develop and pass away.
Certain drugs affect the excretion of uric acid. Salicylic acid is said
to cause an increased excretion, and citrates certainly have this effect.
In both cases the increase is followed by a compensatory fall, which
indicates that these drugs act by facilitating the excretion rather than
by influencing the metabolic processes that are the source of the uric
acid. The effect of caffeine has been very carefully investigated. Given
to the Dalmatian dog, referred to above, S. R. Benedict found that a
small dose caused a slight decrease, but that a larger dose had practically
no effect, although there was a notable retention of nitrogen. On man,
however, different results were secured, for it was found that when 1
gram of caffeine was given daily for several days, a slight but definite
progressive increase in the endogenous uric-acid excretion occurred, and
it lasted for 10 days after the caffeine administration was discontinued.
Liberal allowance of this alkaloid may, therefore, not be quite so innocu-
ous as it is assumed to be.
Uric Acid of Blood. — In all of the investigations considered above,
the behavior of uric acid is judged from the amount of it excreted in
the urine. Valuable though such results must be, their interpretation is
always difficult, since two factors that are quite independent of each
other have to be kept in mind — namely, the production of the uric acid
in the organs and tissues and its excretion by the kidneys. In connection
with the latter factor, we must also consider the method of transporta-
tion of uric acid by the blood from its place of production (or absorp-
URIC ACID AND THE PURINE BODIES 649
tion) to the kidneys. These problems have recently been very consider-
ably simplified by the elaboration of an accurate method for the estima-
tion of the uric-acid content of Hood.
By observing changes in the amount of uric acid in the blood rather
than in the urine, the excretory factor is partly controlled, and 'it can
be completely so if urine and blood are both investigated. Thanks to
the work of Folin, it is now possible to determine with an extreme de-
gree of accuracy the uric acid in as little as 10 c.c. of blood. The impor-
tance of this achievement ivill be appreciated when we state that prior
to Folin 's work no method existed by which uric acid could be approx-
imately measured even when large quantities of blood were available.
Much of the work that has been done by the use of this new method
has so far applied to the amount of uric acid in the blood of man in
various diseases. We shall refer to these results immediately, but
meanwhile it is important to call attention to some very suggestive
observations concerning the condition of uric acid in the blood. For
many years there have been investigators who have thought that uric
acid can not be simply dissolved in the blood plasma, like sugar or some
inorganic salt. It is believed by many that at least a portion of the uric
acid circulates in combination with nucleic (thymic) acid (see page 637),
which would account for the fact that some purines are catabolized in
the body when they are given in a combined state, as thymic acid, but
are excreted unchanged when ingested in a free state. When given freely,
certain purines — adenine, for example — may moreover cause inflamma-
tion and calculus formation in the kidneys of dogs, a result not obtained
when thymic acid is fed.
Other observers have concluded that uric acid exists as two isomeric
varieties, lactam and lactim, the monosodium salts of which are of un-
equal stability. The less stable a-salt is much more soluble in blood
serum than the stable /3-salt. It is the a-salt that becomes increased in
the blood in gout, the deposition of urates in the tissues, which is the
most characteristic symptom of this disease, being caused by conversion
of the a-salts into /?-salts. The structural formulas of the two isomers
are as follows:
H.N - C : O N - C.OH
! I II II
O : C C - NH HO.O C - NH
\
CO
\
C.OH
H.N-C-NH N = C-N
[lactam modification forming [lactim modification forming
unstable a-urates] stable /3-urates]
(relatively soluble) (relatively .insoluble)
650 METABOLISM
The most recent work of S. R. Benedict has shown that uric acid ex-
ists, chiefly in combination in the blood of most mammals but not in
that of the bird. It was found, for example, that fresh ox-blood exam-
ined by the Folin method contains only 0.0005 gm. free uric acid per 100
gm. of blood; after boiling the protein-free blood filtrate with hydro-
chloric acid, however, the uric acid increased by about ten times. This
larger amount was also found present in whole blood that had been
allowed to stand for some time, indicating that the uric-acid compound
can be split by means of an enzyme. The compound exists in the cor-
puscles and not in the plasma. It is of some significance that after thus
setting free the uric acid, there should be about 50 per cent more of it
present in the blood of the ox than in that of the bird, where most exists
in a free state in the serum, although the urine of the ox contains only
the smallest trace of uric acid, and that of the bird is loaded with it.
Investigation of the condition of uric acid in human blood is at present
in progress.
Uriceinia in Gout and Nephritis
The practical application of these observations is particularly impor-
tant in connection with the etiology of gout. In typical cases of this dis-
ease, the uric acid of the blood increases from its normal value of 1 to
3 mg. per cent to nearly 10 mg., indicating a considerable degree of
renal insufficiency. This uricemia can not in itself, however, be the cause
of the deposition of urates in the joints, because it also occurs in other
diseases with renal retention, such as nephritis. Moreover, the blood
serum is capable of dissolving much larger quantities of uric acid than
are ever found present in it in gout. The real cause for the gouty deposits
must depend on some change affecting the blood so as to alter the form
in which uric acid exists therein, with the result that it is excreted into
the joints and deposited there.
Other diseases showing uricemia are lead poisoning and nephritis. In
the latter disease the damaged excretory function of the kidney is
manifested first of all by an increase in the uric-acid content of the
blood, accompanied later by a retention of urea and later still by one
of creatinine. The severity of the renal involvement may therefore be
gauged by determining the percentage of these three metabolites. On
account of the importance of these facts from a clinical standpoint, we
append a table containing results secured by Myers and Fine, in which
the behavior of the metabolites in the blood is shown in relationship
to the severity of the case as gauged by the blood pressure.
URIC ACID AND THE PURINE BODIES 651
URIC ACID, UREA N AND CREATININE OF BLOOD IN GOUT AND EARLY AND LATE NEPHRITIS
URIO
UREAN CREATININE SYSTOLIC
DIAGNOSIS
ACI1)
BLOOD
MG. TO 100 C.C.
BLOOD PRESSURE
Typical Cases of Gout
9.5
13
1.1
230
8.4
12
2.2
164
7.2
17
2.4
200
6.8
14
1.7
Typical Early Interstitial Nephritis
9.5
25
2.5
185
8.0
37
2.7
150
• .
5.0
37
3.9
130
7.1
16
2.0
6.6
24
3.3
185
6.3
18
2.1
8.7
20
3.6
100
7.0
33
2.6
117
6.3
31
2.1
6.3
23
2.4
150
Chronic Diffuse and Chronic Inter-
8.0
80
4.8
240
stitial Nephritis
4.9
17
2.9
170
8.3
72
3.2
238
5.3
21
1.9
145
9.5
44
3.5
210
2.5
19
1.9
120
7.7
67
3.1
6.7
17
1.6
165
8.3
39
2.9
6.5
24
3.0
200
.Typical Fatal Chronic Interstitial
22.4
236
16.7
210
Nephritis
15.0
240
20.5
225
14.3
263
22.2
220
13.0
90
11.1
265
8.7
144
11.0
225
(Myers and Fine: Arch. Int. Med., 1916.)
Lastly, regarding the influence of drugs on the blood uric acid in dis-
ease, it has been found by Fine that both atophan and salicylates cause
a pronounced decrease in the amount, but that it gradually rises to the
old level even while administration of the drugs is being continued.
Important contributions to the. behavior of uric acid in blood are
constantly appearing at present, mainly from the laboratories of Folin
in Boston, of S. R. Benedict, and of Myers and Fine in New York.
CHAPTER LXXIV
THE METABOLISM OF THE CARBOHYDRATES
The healthy animal organism is capable of rapidly oxidizing large
quantities of carbohydrate, as is evident from the following facts: If
carbohydrate is given to a starving animal, (1) the energy output very
shortly afterward increases; (2) the respiratory quotient also increases,
indicating that, relatively to oxygen intake, more carbon dioxide is being
excreted (see page 647) ; and (3) none of the ingested carbohydrate
makes its appearance in the excreta. Indeed, of the three proximate
principles of food, carbohydrate is the most available for combustion,
in the animal body. It may therefore be considered as the quickly
available fuel for the body furnaces.
CAPACITY OF THE BODY TO ASSIMILATE CARBOHYDRATES
Assimilation Limits. — When the limit to the amount of carbohydrate
that the organism can metabolize is overstepped, some of it appears in
the urine. The amount that can be tolerated without causing glycosuria
is commonly called the assimilation or saturation limit. The use of the
term "limit" is, however, very unfortunate, for it implies that beyond
this point the organism is capable of dealing with no more carbohy-
drate, which is far from being the case, for if a larger amount is taken,
only a small trace of the excess will appear in the urine. When the
urine is allowed to collect for twenty-four hours, the mixed specimen
shows no trace of glucose in the majority of healthy individuals after
the ingestion of 200 gm. ; after 300 gm. a somewhat higher percentage
of cases develop a mild glycosuria, but frequently none is evident even
after 500 gm. Beyond the last mentioned amounts the limit of ingestion
is reached, on account of nausea, etc., and it is improbable that, even
if larger amounts could be tolerated, any more of the dextrose would
be absorbed than with 300 or 400 gm. The testing of the so-called
assimilation limit has been considered an important aid in the diagnosis
of early cases of diabetes, the characteristic feature of such cases being
the inability of the organism to assimilate properly the usual quantity
of carbohydrate contained in the diet.
It has been found that to make the results of any value, certain
conditions must be fulfilled in applying the assimilation test. The most
652
THE METABOLISM OF THE CARBOHYDRATES 653
important of these concerns the activities of the gastrointestinal appa-
ratus at the time the sugar is given, for it has been found that if other
foodstuffs are being absorbed at the same time as the sugar, more of
the latter can be tolerated than when the sugar alone is being absorbed.
It has therefore been customary to give the sugar dissolved in water,
or in weak coffee, the first thing in the morning after the patient awakes;
i. e., at least twelve to sixteen hours after the last meal was taken. In
making these tests the urine voided before the sugar is estimated should
of course itself be thoroughly examined for reducing substances, and
the urine should be collected every ninety minutes and examined by a
reliable test (Benedict's or Nylander's).*
Although a limit is set to the ability of the organism for retaining
sugar (mono- or di-saccharides), this does not seem to apply, in healthy
individuals at least, when starches (polysaccharides) are ingested. Thus,
it is a well-known fact that people can eat enormous quantities of pota-
toes or of bread without the appearance of any trace of reducing sub-
stances in the twenty-four-hour urine. On the other hand, urine collected
and examined at short intervals (every half hour) after taking large
quantities of polysaccharicle-rich food will frequently be found to contain
traces of reducing substances.
For practical purposes it has been considered that an individual who
develops glycosuria after; taking 100 gm. of glucose must be considered
as at least a potential diabetic. In the light of the above results and
for many other reasons, there is, however, considerable doubt as to the
value of the assimilation test. Thus, when a solution of glucose is
given orally, its rate of absorption will depend very largely on the
motility of the stomach. If this is normal, the solution will very quickly
find its way past the pyloric sphincter into the intestine, where it will
be rapidly absorbed. If, on the other hand, the pyloric sphincter does
not open freely, the passage of the glucose into the intestine may be
so delayed that no more is present in this place at one time than would
be the case after an ordinary diet of polysaccharide. And even after
the sugar solution enters the small intestine, differences in the amount
of the intestinal contents with which it becomes mixed, in the extent of
bacterial growth, and in the absorption process, may very materially
affect the rate at which the glucose gains entry to the blood.
Although often of doubtful diagnostic value, determination of the
assimilation limit is of considerable aid in controlling the treatment of
'Examination of normal individuals has shown that the assimilation limit for different sugars
varies somewhat; for glucose it appears to be from about 150 to 250 gm. ; for levulose, which, it
will be remembered, is the monosaccharide associated with glucose in the construction of the cane-
sugar molecule, the assimilation limit is from 100 to 150 gm. ; for cane sugar or saccharose itself
the figures seem to vary considerably, but are given as between 50 and 200 gm.; for lactose, another
disaccharide, and the sugar present in milk, the assimilation limit is distinctly lower — namely, 100 gm.
654 METABOLISM
diabetes. For this purpose the patient should first of all be instructed
to follow his usual diet, so that, by examination of the amount of sugar
excreted in the urine, an opinion may be formed of the severity of the
case. The diet should then be changed so as to consist of a part that
contains no carbohydrates and another composed entirely of starchy
food. The former is made up of eggs, fish, green vegetables, fat, etc.,
and the latter, to start with, should consist of 100 grams of bread, dis-
tributed between the two main meals of the day, one of which is break-
fast. This diet should be continued until the glycosuria either disappears
or attains a constant level. If it disappears, "the case is classified as a
mild one of diabetes, and the daily allowance of bread may be increased,
by 50 grams a day, until the sugar again makes its appearance in the
urine, indicating that the assimilation limit has been reached. For
therapeutic purposes, the patient should now be instructed to take about
three fourths of this amount of carbohydrate in his daily rations, and
he should be supplied with explicit instructions in the shape of diet
tables as to what variety and quantities of the various carbohydrate
materials his food may contain. His urine should be examined at fre-
quent intervals — once a week — and he should be instructed as to the
nature of his disease and the importance of his remaining aglycosuric.
By further treatment such so-called latent cases of diabetes may be
kept in perfect health for many years.
When, on the other hand, the glycosuria exists with 100 grams of
bread in the daily ration, this must be reduced to 50 grams, and if after
some days the first reduction does not suffice to render the urine free
from sugar, carbohydrates must be withheld entirely from the diet.
If the glycosuria does not now disappear, the case is to be considered
severe, and it may be necessary to undertake the starvation treatment,
which has recently been developed in this country by Allen18 and Joslin19
with apparent success. By the reduction of carbohydrate, or by the
starvation treatment, it is usually possible to make even the severest
cases of diabetes aglycosuric, and when this has been attained, then
gradually to increase the amount of protein or carbohydrate food until
the assimilation limit has been reached.
Saturation Limits. — To avoid error caused by irregular absorption from
the intestines, some investigators have recommended the determination
of the assimilation limit after intravenous or subcutaneous injections
of sugar. But even this refinement in technic has not, as a rule, had the
effect of rendering the results of any very evident value as a criterion
of the utilization of glucose in the animal body. The reason for this
unreliability of the method is mainly that the period of injection of the
glucose solution usually oc.cupies only a few minutes, so that it causes
THE METABOLISM OP THE CARBOHYDRATES 655
a sudden instead of a very gradual increase in the sugar concentration
of the blood, the conditions being quite unlike those which exist during
the normal absorption of glucose from the intestine. The mechanism
by which the body ordinarily disposes of excessive amounts of glucose
absorbed into the portal blood, is not adjusted to operate when the sys-
temic blood is suddenly overcharged' with this substance. In the one
case the glucose is a foodstuff; in the other, because of its excessive
concentration in the blood, it is more or less of a poison. Such results, in
other words, merely show us how much glucose can be added at one
time to the organism without any overflow into the urine, but they
furnish us with no information regarding the power of the organism to
utilize a constant though moderate excess of this substance. In the one
case it is the "saturation limit," in the other the "utilization limit" of
the organism for glucose, that we are really considering.
Consideration of these principles has led Woodyatt, Sansum and Wil-
der20 to undertake a thorough reinvestigation of the whole problem of
the utilization or, as they prefer to call it, the tolerance of the ~body for
glucose. They emphasize the obvious fact that the ability of the organism
to utilize glucose "must depend on the rate at which the tissues are
able to abstract it from the blood by their combined powers, to burn it,
to reduce it into fat or to polymerize it into glycogen. " To form any
estimate of the combined effect of these processes, we must take into
account not only the amount of glucose per unit of body weight (grams
per kilogram), but also the rate of injection, for "tolerance must be
regarded as a velocity, not as a weight."
Briefly summarized, the conclusions which Woodyatt, etc., have so far
drawn from their investigations are as follows: In a normal rabbit, dog,
or man, 0.8-0.9 gm. of glucose per kilogram body weight and per hour can
be utilized by the organism for an indefinite time without causing gly-
cosuria. When between 0.8^ and 2 gm. are injected, a part of the excess
appears in the urine, steadily increasing until a maximum is reached,
after which the excreted fraction remains constant (at about one-tenth).
If more than about 2 grams per kilogram an hour are injected, "a large
percentage of all glucose in excess of the 2 gm. per kilogram an hour
appears in the urine when constant conditions are once established."
The fact that so much glucose injected intravenously can be used
without the appearance of any of it in the urine, indicates a method by
which foodstuffs may be supplied to the tissues in cases where, on account
of gastrointestinal disturbances, it is impossible to have food absorbed
by the usual pathways. The possible value of such a method of treat-
ment in cases of extreme weakness has been tested on laboratory animals
by Allen, who states that such injection seems to have a valuable nutri-
656 METABOLISM
tive and strengthening effect. He found, for example, that in cats
starved to extreme weakness the injection of a fraction of a gram per
kilogram of glucose had an unmistakable strengthening effect, and
sometimes appeared to save life. Such results would seem to indicate that
in certain cases where blood transfusion is impracticable, glucose in-
fusions should be tried. Subcutaneous injection of sugar, either for the
purpose of determining the assimilation limit or with the object of sup-
plying foodstuffs parenterally, is impracticable because of the pain and
sometimes sloughing produced at the point of injection.
We have devoted no inconsiderable space to a discussion of assimila-
tion limits because of the great interest in diabetic therapy which this
procedure has aroused during recent years. We may now turn our
attention to a closer analysis of the changes that take place in carbohy-
drates during their passage through the animal body.
DIGESTION AND ABSORPTION
Digestion. — All digestible -carbohydrate taken with the food is con-
verted by the digestive agencies into the monosaccharides, glucose and
levulose, as which it is absorbed into the blood of the portal system.
To bring about this resolution of carbohydrate into monosaccharides,
several enzymes are employed. The first of these is the ptyalin of saliva.
It is not a very powerful enzyme, being capable of acting only on starches
that are in a free state, i. e., not surrounded by a cellulose envelope ;
but even on free starch, ptyalin displays little of its activity during the
time the food is in the mouth. After the food is swallowed and becomes
deposited in the fundus of the stomach, there is an interval of time —
lasting until hydrochloric acid has been secreted to such an extent as to
permit some of the acid to exist in a free state — during which the ptyalin
acts on the starch of the swallowed food. During this time the activity
of the ptyalin is actually assisted on account of the fact that a slight
increase in hydrogen-ion concentration of the digestive mixture accel-
erates the action of ptyalin.
The product of ptyalin digestion is maltose, a disaccharide composed
of two molecules of glucose. On entering the intestine, the carbohydrates
therefore exist partly as undigested starch, partly as glucose, and partly
as maltose. In the favorable environment of the duodenum a much
stronger diastatic enzyme called amylopsin very quickly hydrolyzes the
starch through dextrine into maltose. The maltose derived from the
starch and the unchanged sugars, such as cane sugar, maltose and lac-
tose, which have been taken with the food, unless they are present in very
high concentration in the intestinal contents, are not immediately ab-
THE METABOLISM OF THE CARBOHYDRATES 657
sorbed into the blood, but become subject to the action of other enzymes
contributed by the intestinal juice — namely, the inverting enzymes, one
of which exists for each of the disaccharides. By their action maltose
is converted into two molecules of glucose by the enzyme maltase; lac-
tose, into galactose and glucose by lactase; and cane sugar, into levu-
lose and glucose by invertase. It is interesting to note that in animals
whose food does not contain one of those disaccharides, the correspond-
ing inverting enzyme is absent from the intestinal juice. The herbivo-
rous animals, for example, do not take any lactose in their food, and the
intestinal juice contains therefore no lactase, although it is present in
that of the young animals while still suckling.
A certain amount of carbohydrate becomes attacked by the intestinal
bacteria. These split the monosaccharides into lower fatty acids and
gases, such as methane and carbon dioxide. Besides this obviously de-
structive process, bacteria also perform a useful function in the digestion
of carbohydrates, in that certain strains of them are able to digest cellu-
lose, for which no special enzyme is provided. Bacterial digestion is con-
sequently essential in herbivorous animals; it takes place in the eecum,
which is enormously developed for this purpose (page 463).
Absorption. — The glucose and levulose produced by digestion are
absorbed into the blood of the portal system. "When a very large quan-
tity of a disaccharide, such as cane sugar, is present in the food, a certain
amount of the sugar is absorbed unchanged — that is to say, as cane sugar
— and appears in the blood, from which, since it is an abnormal con-
stituent, it is excreted unchanged in the urine. This alimentary glyco-
suria is particularly evident when the sugar is taken without any other
food; thus, after taking cane sugar in an amount corresponding to 5
grams per kilogram body weight, it was found in one and a half hours
afterward that the urine of ten out of seventeen healthy individuals con-
tained cane sugar. The urine of three of these men, however, also con-
tained invert sugar — that is, dextrose and levulose. Cane sugar con-
tinued to be excreted for from six to seven hours.
The Sugar Level in the Blood. — While no absorption of sugar is going
on, the percentage of this substance in the blood of the portal vein is the
same as that in the systemic circulation. During absorption the former
becomes perceptibly raised — to what extent we can not say — and in the
latter a less marked increase of sugar concentration is usually detectable.
Evidently, then, between the point at which the sugar is absorbed and
the blood of the systemic circulation, some barrier exists which holds
back some of the excess of absorbed sugar. We have very inaccurate
information as to how efficiently these barriers hold back the excess of
absorbed glucose because of the technical difficulty in collecting blood
658 METABOLISM
from the portal vein without serious disturbance to the animal. Indeed,
the only way by which the problem has been accurately studied is by
comparing the blood of the portal circulation with that of the systemic
circulation during the injection of a solution of dextrose into one of the
smaller branches of the portal vein.21 In such experiments it has been
found that the percentage of sugar is a little less in the blood of the
abdominal vena cava than in that of the portal vein, and is still less in
the blood of the systemic veins, such as the femoral. — results which justify
the conclusion that the barriers responsible for taking out some of the
absorbed sugar from the blood exist in the liver and in the muscles. The
curve in Fig. 189 will illustrate to what extent the mechanism operates.
/n //omirt /*
/ .-"^
/
/•* JM9K
/ jf
,y in/0gn*i'r\
/ -•'*
/ •*
po' .-' 1
1 "»
r .^^
in 'Of mi'i.
/IS /
* •• y
./
.~
Cans tint i*/ectSen of /gXOextnst $• I ut/'ow.
/» <cr to sf 30 if to y*~ &> ss- 60 tr TO rs so tr to is- /oo
Fig. 189. — Curves showing the percentage of glucose in blood after a constant injection of
an 18 per cent solution into a mesenteric vein. V.C., vena cava, continuous line; P.O., pan-
creaticoduodenal vein, broken line; /, iliac, dotted line.
It will be observed that, so far as can be judged from changes in the
concentration of sugar in the blood, the sugar-retaining power of the
liver is about equal to that of the muscles — a conclusion which is, how-
ever, contrary to the usually accepted one that the liver has such pro-
nounced sugar-retaining powers that under ordinary circumstances it
removes from the portal blood all the excess of sugar added to it by
absorption and which is not required by the organism.
One objection which may properly be made to these observations is
that the animals on which they were made were under anesthesia, and
that the anesthetic had a paralyzing effect on the sugar-retaining power
of the liver. In view of this criticism it is important to examine the
results obtained on animals that are not under the influence of anes-
THE METABOLISM OF THE CARBOHYDRATES 659
thesia. Such observations have been made on rabbits, and a few on man
himself. By collecting blood from the ear veins of rabbits, it has been
found that, after giving from two to ten grams of glucose by stomach,
the glucose concentration of the systemic blood begins to rise in fifteen
minutes, attaining a maximum in about an hour and then returning to
the normal level in about three hours.
Similar results have been obtained by examination of the venous blood
in man. After giving 100 grams of glucose by mouth, for example, there
is commonly an increase in blood sugar amounting to from 30 to 34 per
cent of the normal and lasting for from one to four hours. The existence
of this postprandial hyperglycemia, as we may call it, indicates that the
sugar-retaining powers of the liver and muscles are not sufficiently de-
veloped to prevent the accumulation of some of the absorbed sugar in the
systemic blood. Whenever this increase exceeds a certain limit, some of
the sugar begins to escape through the kidney into the urine, producing
glycosuria — postprandial glycosuria. The percentage of blood sugar above
which glycosuria occurs is, in the case of man, probably about 0.10 to 0.11
gm. per cent. After damage to the kidney, as in nephritis, or in long-stand-
ing cases of mild diabetes, the percentage may probably rise considerably
higher in the blood without evidence of glycosuria.
Value of Blood Examination in Diagnosis of Diabetes. — The determina-
tion of the amount of ingested carbohydrate required to bring about post-
prandial glycosuria constitutes, as we have already seen, the so-called
assimilation limit for sugar, which is often taken as an index of the sugar-
metabolizing power of the organism. It is evident, however, that the time
of onset, and the extent and duration of postprandial hyperglycemia must
serve as a more certain index of the sugar-retaining power of the liver
and muscles; and now that a simple and rapid clinical method exists
(Lewis-Benedict method) for the accurate determination of sugar in small
quantities of blood, there is no reason why this index should not be used
for the detection of failing powers to metabolize carbohydrate.
In no disease, probably not even in tuberculosis, is it more important
than in diabetes that an early diagnosis should be made. Thus, if we find
that the postprandial hyperglycemia after a certain amount of carbo-
hydrate develops to an unusually high degree and persists for an unusual
length of time, we are justified in curtailing the carbohydrate supply so as
to hold these values down to their level in normal individuals. It is almost
certain that the first sign of diabetes is an unusual degree and duration
of postprandial hyperglycemia. At first the excess of sugar leads to no
damage and it is insufficient to cause any evident glycosuria, although it is
quite likely that if the urine in such individuals were collected at very
frequent intervals after eating carbohydrate-rich food, glucose would be
660 METABOLISM
found present in at least some of the specimens. In incipient diabetes,
however, the condition progresses, until the postprandial hyperglycemia
after one meal has not become entirely replaced before the next is taken,
so that the increase in sugar produced by the second meal becomes super-
added on that following the first meal. The curve of blood sugar rises
ever higher and higher, until at last permanent hyperglycemia is estab-
lished, or rather the normal level from which the postprandial rise occurs
has become permanently raised, so that in blood collected at any time a
higher percentage of sugar is found.
The Relationship Between the Sugar Concentration of the Blood and
the Occurrence of Glycosuria. — Claude Bernard first pointed out that the
percentage of sugar in the blood may rise considerably above its normal
level without the appearance of any of the sugar in the urine, or at least
without a sufficient amount to give the usual tests for sugar. Even when
this limit is reached, as we have seen, the sugar which appears is not all of
the excess but only a small part of it. This overflow hypothesis, as it is
called, has not been universally accepted because of the many results
which are not in conformity with it. Many of these exceptional results
have been explained as due to alterations in the permeability of the kidney
for sugar, and in general it is probably safe to accept Claude Bernard's
hypothesis with certain reservations.
Strong support has been lent to a modified form of the hypothesis by
the recent work of Woodyatt and his collaborators, who have shown by
continuous intravenous glucose injections that as much as 0.8 gm. of
glucose per kilo body weight can be injected during an hour into an
animal without any glycosuria, although under such conditions a very
distinct increase occurs in the percentage of sugar in the blood.
To explain the failure of glucose to pass into the urine under normal
conditions, it has been supposed by several investigators that the glucose
exists in some form of chemical combination in the blood. This compound
is believed to behave like a colloid. One of the recent supporters of this
view is Allen, who has observed that, when glucose is injected intrave-
nously, it causes diuresis as well as glycosuria; whereas glucose injected
subcutaneously or taken by mouth causes neither of these conditions to
become developed ; indeed it causes for some time after the administration
of the sugar a distinct anuria. To explain these differences in behavior
between glucose administered intravenously and that taken in other ways,
it is supposed that the glucose molecule in passing through the intervening
wall of the capillaries combines with some substance to form a compound
which becomes available for incorporation into and utilization by the
tissues, glucose in a free state being incapable of utilization. This com-
pound is supposed to be of a colloidal nature, and the substance which
THE METABOLISM OF THE CARBOHYDRATES 661
combines with glucose to form it is believed to be related to the internal
secretion of the pancreas (see page 676).
The difficulty in explaining why the glucose of the blood does not con-
stantly leak into the kidney is, however, the only evidence upon which the
hypothesis of a blood sugar compound rests. No chemical evidence can
be offered in support of such a view. On the contrary, all experimental
work indicates that the sugar exists in a free state; but unfortunately even
this evidence is not convincing. Thus, it has been found that, when speci-
mens of perfectly fresh blood are placed in a series of dialyzer sacs sus-
pended in isotonic saline solutions, each solution containing a slightly dif-
ferent percentage of glucose, diffusion of glucose, in one or other direction,
occurs in all of them save one — namely, that in which the percentage of
glucose in the fluid outside the dialyzer is exactly equal to the total sugar
content of the blood. Such a result can be explained only by assuming that
all of the sugar in the blood exists in a freely diffusible state. In its general
nature this experiment is analogous to that by which the tension or partial
pressure of C02 is determined in blood (see page 338).
It has been assumed by many clinicians that glycosuria may sometimes
become developed because the kidney fails to hold back the blood sugar
even when the percentage is not above the normal — so-called renal dia-
betes. For the diagnosis of this condition a comparison must be made be-
tween the sugar concentration of the blood and that of the urine. In order
to do this at least two samples of blood must be taken, one of them at the
beginning and the other at the end of a period during which urine is being
collected. Merely to find that one sample of blood collected before or after
or during the period of urine collection contains a normal percentage of
sugar, does not necessarily indicate that at some other period while the
urine was being produced a temporary hyperglycemia may not have ex-
isted.
CHAPTER LXXV
THE METABOLISM OF THE CARBOHYDRATES (Cont'd)
FATE OF ABSORBED GLUCOSE. GLUCONEOGENESIS
We may now consider what becomes of the sugar that is retained by
the liver and muscles. Two things may happen to it: It may become
stored, or it may become oxidized or split up. Of these processes, storage
occurs in both the liver and muscles, whereas oxidation occurs mainly if
not entirely in the muscles, although a certain amount of splitting of the
glucose molecule may also occur in the liver.
Storage of Sugar. — For the present we shall consider the process of
storage of sugar and defer a consideration of its utilization until after we
have studied, not only the nature of the process by which the storage
occurs, but also the immediate destiny of the stored sugar. The storage
of sugar by the liver is brought about by its conversion into a polysac-
charide called glycogen. After an animal has been absorbing large quan-
tities of glucose, an acidified watery extract of a portion of liver mad?
immediately after death will be found to contain no more sugar than that
of a normal liver. On the other hand, it will be observed that the extract
is highly opalescent and yields on the addition of alcohol a copious precip-
itate, which on further purification can readily be shown to consist of a
polysaccharide— that is to say, of a starch-like substance which on hydrol-
ysis with mineral acid becomes entirely converted into sugar. If instead
of removing the liver immediately after death, it is allowed to stand for
some time, the yield of glycogen will greatly diminish, and in its place
will appear large quantities of glucose, indicating that some enzyme must
exist which attacks the glycogen after death and converts it into sugar
This enzyme is called glycogenase. The existence of postmortem glyco-
genolysis, as it is called, would seem to indicate that during life also there
is a constant tendency for the glycogen in the liver to be attacked by
glycogenase, but that this is prevented by conditions which depend on the
vital integrity of the liver cell. It is evident that if anything should
happen during life to interfere with this inhibiting influence, the glycogen
will become converted into glucose, which on escaping into the blood
will produce hyperglycemia and glycosuria.
Sources of Glycogen. — In studying the source of sugar in the animal
body it is important therefore that we should first of all know exactly the
662
THE METABOLISM OP THE CARBOHYDRATES 663
conditions under which glycogen may be formed in the liver; that is,
whether it is formed from absorbed sugar alone, or from other substances
also, such as protein and fat. The importance of such knowledge rests
in the fact that in severe diabetes, sugar still continues to be added to
the blood, although no sugar is being taken with the food. To check the
hyperglycemia in such cases it becomes necessary, therefore, to curtail
the diet not only with regard to its carbohydrate content, but also with
regard to whatever other foodstuff may be capable of causing glycogen
formation. The practical question therefore is, "What are these foodstuffs?
There are two methods by which the problem may be investigated. The
first, which we may call the direct method, consists in rendering the liver
free of glycogen and then some time afterward feeding the animal with
the foodstuff in question, afterward killing it and examining the liver
for glycogen. The other, which we may call the indirect method, con-
sists in first of all rendering the animal incapable of oxidizing glucose —
that is, making it diabetic — and then proceeding to see whether the in-
gestion of a given foodstuff causes an increase in the sugar excretion in
the urine. The methods for rendering an animal experimentally diabetic
will be considered later ; for the present it is important to note that, if
a diabetic animal excretes more glucose while fed on a given foodstuff,
we may infer that the normal animal would convert it into glycogen.
The results of the direct method are much less reliable than those of
the indirect for the reason that it is extremely difficult to remove all
traces of glycogen from the liver. The methods employed for this pur-
pose have consisted in: (1) starvation of the animal; (2) muscular ex-
ercise; (3) exercise and starvation combined; and (4) the production of
certain forms of experimental diabetes — for example, that produced by
phlorhizin. Starvation alone is unsatisfactory, for it has been found
that, although at certain stages of this condition the liver may become al-
most entirely free from any trace of glycogen, at a later stage glycogen
may again make its appearance. It is therefore most difficult to decide
at what stage in starvation the animal should be considered as glycogen-
free.
If the starving animal is made to perform muscular exercise, complete
removal of glycogen from the liver can be depended upon. The exercise
may be produced by the administration of strychnine in such dosage as
just to produce convulsions of the voluntary muscles without permanent
contraction of those of respiration. The most useful method, however,
consists in starving the animal for a few days and then placing it in a
cold, damp room, after giving it a cold bath. The evaporation of mois-
ture from the surface so cools the body down that the glycogen store all
becomes used up in the attempt to supply fuel for the production of
664 METABOLISM
sufficient heat to maintain the body temperature. This method can he
rendered still more certain in effecting a removal of all carbohydrate
from the body by giving the animal phlorhizin every eight hours. This
drug, as we shall see, renders the animal diabetic.
After removing the glycogen, further deposition in the liver can be
readily shown to occur when any of the ordinary sugars or starches are
given as food. It does not occur, however, when chemical substances
closely related to ordinary sugar, such as the wood sugars (pentoses)
or the alcohols and acids corresponding to dextrose, are contained in the
diet. Nor does it occur with cellulose or with inulin, a polysaccharide
built up from pentose sugar. When proteins are fed the results are not
so definite, although many observers have claimed that glycogen is
formed. With fat, on the other hand, no glycogen formation can be
shown to occur, although we know that a trace of carbohydrate must be
formed out of the glycerine of the fat molecule.
The results of the direct method, even when the conditions are per-
fectly controlled, are very unreliable, because any new sugar produced
by the ingested substance instead of being stored as glycogen may be
directly used by the tissues as it is formed. Where only a slight degree
of gluconeogenesis is occurring, it is not likely that any of the glucose
will be retained in the body as glycogen.
The methods employed for producing experimental diabetes in investi-
gation of these problems by the indirect method are (1) the entire removal
of the pancreas, and (2) the continuous administration of the drug
phlorhizin. The animal rendered diabetic by either of these methods is
first of all observed for several days to determine the normal daily ex-
cretion of sugar. At the same time the nitrogen excretion for the day
is determined, the ratio between the total nitrogen and the glucose being
known as G to N ratio, and being about 1 to 3.65 when complete diabetes
has become established. The foodstuff in question is then fed to the
animal, and the amount of extra glucose excreted thereby is taken to
represent that which has been derived from the ingested food. By this
method it has been possible to show that, not only the above mentioned
carbohydrates, but protein as well produce a very considerable quan-
tity of glucose in the animal body. Fats, however, yield only negative
results.
The indirect method has another great advantage over the direct in
that the results are much more quantitative in character; for example,
Lusk and his pupils have been able to determine the amount of glucose
which can be produced by feeding certain of the building stones of the
protein molecule. The great practical importance of such results in
THE METABOLISM OP THE CARBOHYDRATES 665
the therapy of diabetes makes it advisable for us to go into the subject
a little more in detail here.
Dogs are rendered diabetic by phlorhizin followed by a cold bath
and exposure in a cold room. When all of the original glycogen in the
body has been got rid of, as evidenced by the constancy of the G to N
ratio in the daily quantities of urine excreted, the substance under in-
vestigation is fed. If this substance contains no nitrogen and causes no
change in the nitrogen excretion, any increase in that of glucose must
obviously represent the extent to -which the substance has become con-
verted into this sugar. On the other hand, if the substance itself con-
tains nitrogen, or if it causes a change in the excretion of nitrogen, it
becomes necessary to calculate how much of the excreted glucose might
have been derived from the body protein, assuming that this can form
glucose, and how much from the administered substance.*
From the results of this method it has been an easy matter to show
that the following substances are converted in the animal body into
glucose: (1) Glycol aldehyde (CH2OH-CHO). By placing three mol-
ecules of this substance together, a hexose molecule will be produced,
a . synthesis which can be accomplished in the chemical laboratory.
Glycol aldehyde may be formed in normal metabolism out of glycocoll
(CH2NH2COOH).
(2) Glycerol (CH2OH - CHOH - CH2OH) may also readily be con-
verted into hexose in the laboratory, the possible intermediary products
being dioxyacetone (CH2OH - CO - CH2OH) and gly eerie aldehyde
(CH2OH-CHOH-CHO). Two molecules of either of these may be
polymerized to form a hexose molecule, and when this process occurs
in the animal body, the hexose formed is glucose.
(3) Lactic acid (CH3CHOH - COOH) is completely converted to dex-
trose in the diabetic animal, and the process must involve both a re-
arrangement of the molecule and subsequent polymerization. The related
substance, propyl alcohol (CH3- CH2-CH2OH) is also converted into
glucose in the phlorhizinized dog. As to the exact nature of the chemical
changes which occur as intermediary steps in the conversion of these
substances into glucose, we are not as yet certain, but a step has been
made in the discovery that a substance called methylglyoxal (CH3COGHO)
may be obtained from lactic acid and also from glucose, and that this
substance is converted into glucose when it is administered to phlorhi-
zinized dogs. We shall find later an important role for this substance
'This calculation is made as follows: The amount of nitrogen in the administered substance is
deducted from the nitrogen excretion, and the difference, which must represent the nitrogen of the
body protein, is multiplied by the G to N ratio which prevailed on the day previous to that on
which the substance was fed. We obtain in this way the glucose derived from the body. The
glucose coming from the administered substance can then be ascertained by deducting that derived
from the body protein from the total glucose excretion.
666 METABOLISM
in the case of fat metabolism, and it can also readily be produced during
the intermediary breakdown of certain of the protein building-stones,
such for example as alanine (CH3CHNH2COOH).
These chemical possibilities regarding the nature of the substances
that serve as stepping stones between the above sugar-forming sub-
stances and sugar itself may be translated later into probabilities on
account of the discovery that the enzymes exist in various tissues which
are involved in converting methylglyoxal into lactic acid:
CH3 CH3
! I
CO + H2 -+ HCOH
I O <- |
CHO COOH
(methylglyoxal) (lactic acid)
These enzymes are called glyoxalases, and since the reactions which
they mediate are undoubtedly reversible in character, it is probable that
the conversion into sugar of lactic acid and alanine — to take those two
as among the commonest of the sugar precursors of the animal body —
occurs according to the following equation:
CH3CHNH2COOH v
(alarinc) CILCOCHO -H» C6H]2O6
CH,CHOHCOOH /*
(lactic acid) (methylglyoxal) (hexose)
The unique position of methylglyoxal, besides explaining the known
resolutions of protein and fat and carbohydrate in intermediary metab-
olism, is also of importance in explaining the synthetic production of
glucose from fructose (or levulose). Fructose will first of all become
converted into methylglyoxal radicles, and these will then become syn-
thesized into glucose.
The theory of the conversion of glucose into lactic acid as a stepping
stone in the metabolism of carbohydrate meets with one objection;
namely, that the lactic acid is not produced from carbohydrate in the
organism, except in cases where there is oxygen deficiency or excess of
alkali in the tissue fluids.
Coming now to the amino acids, which, it will be remembered repre-
sent the building stones of the protein molecule, it has been found that
glycocoll, alanine, and aspartic and glutamic acids. increase the glucose
excretion when given to phlorhizinized dogs, whereas leucine and tyro-
sine have no such action. By the method described above, it is possible
to determine the exact proportion of the carbon of each of those amino
acids which becomes converted to glucose. This is shown in the accom-
panying table.
THE METABOLISM OF THE CARBOHYDRATES
667
TWENTY GRAMS OF THE VARIOUS AMINO BODIES WERE GIVEN TO
PHLORHIZIN-DIABETIC DOGS
ACID AND FORMULA
AVERAGE AMOUNT
OF GLUCOSE PRO-
DUCED IN BODY
PROBABLE GLUCOSE THAT
CHANGE WOULD BE PRO-
DUCED BY CHANGE
Glycocoll
CH2NH2COOH
i. alanine
CH3CHNH2COOH
Aspartic acid
COOH— CH2—CHNH,— COOH
13.43
one
18.77
12.42
(five dogs,
gave 15.77)
(tw.o dogs)
(four dogs)
All C converted
to glucose
it
Three of the four
C atoms converted
16.00
20.22
13.52
Glutamic acid
COOH
CH,
13.31
to glucose
Three of the five
C atoms converted
to glucose
CH2—
CHNH2
COOH
12.24
It is of further interest to point out that these four amino acids
constitute about 26 per cent of all the amino acids in flesh protein, and
that the total yield of glucose from them could be 26.3 grams; thus
accounting for nearly one half of the 66 grams which a diabetic animal
produces from 100 grams of flesh.
Gluconeogenesis in Normal Animals. — Although it has been clearly
shown by the indirect method that not only protein but its decomposi-
tion products as well, can be readily converted into glucose, yet this does
not necessarily indicate that a similar conversion occurs in the nondia-
betic animal. That such is the case, however, can be shown in various
ways. Thus, at the end of a period of long starvation considerable
quantities of glycogen are quite commonly found in the body, and the
blood sugar, although lower than normal, never entirely disappears.
Now, since no carbohydrate is being ingested, and the body stores of this
foodstuff become exhausted early during starvation (cf. page 663), it
is evident that the carbohydrate must be produced from the protein of
the animal's body. A still more convincing experiment can be con-
ducted by producing strychnine convulsions in a starving animal. If
the animal is killed after the convulsions have lasted for a certain time,
the tissues will be found almost if not entirely free of glycogen,
but if the convulsions are made to disappear by giving chloral and the
animal allowed to sleep for some time before killing it, glycogen again
accumulates in the body. This glycogen must have been manufactured
out of noncarbohydrate material.
Corroborative evidence of a somewhat different nature is furnished by
668 METABOLISM
an examination of the respiratory quotient, which, it will be remem-
bered (page 54.7), varies according to the nature of the foodstuff or body
constituent that is undergoing metabolism at the time, being about 1
with carbohydrate and about 0.7 with protein. If the quotient is
observed during starvation, it will often be found to fall below 0.7, a
figure which can be explained only by assuming that oxygen has been
retained in the body beyond the quantity which is necessary for imme-
diate purposes of oxidation (cf. equations on page 548).
Since it is known that this retained oxygen can not exist in the body
in a free state it must be concluded that it has become incorporated
into substances having a high oxygen content. Such would be the case
if protein or fat, which contains only from 12 to 20 per cent of oxygen,
were converted to carbohydrate, ' which contains about 53 per cent.
Utilization of inhaled oxygen for this purpose, as we have seen, becomes
very striking in the case of hibernating animals during the winter sleep.
CHAPTER LXXVI
THE METABOLISM OF THE CARBOHYDRATES (Cont'd)
FATE OF GLYCOGEN
Having become familiar with the sources from which glycogen may
be derived, we may now proceed to study the fate of the glycogen found
in the liver cells and in the muscles. For the present we shall confine our
attention to the glycogen of the liver. If a portion of liver removed
from 'a well-fed animal is examined microscopically after staining either
with iodine or with carmine by Best's method, it will be found that the
cells of the lobules are filled with glycogen except for the nuclei, which
are free from this substance. If, on the other hand, the liver is from an
animal that has not been recently fed, the lobules will contain no glyco-
gen except for an area bordering on the central vein and perhaps a
narrow strip at the periphery of the lobule. When it is present the rela-
tive amount of glycogen in different lobules, as determined chemically,
is the same over the entire liver— that is to say, no one lobe is richer in
this substance than another. Nothing definite is known as to how the
glycogen is held in the protoplasm of the cells, although some histolo-
gists suggest that it is combined with a sustentacular material especially
provided for this purpose.
The glycogen stored in the liver is gradually given up to the blood of
the hepatic vein at such a rate as to maintain in the blood of the sys-
temic circulation a more or less constant percentage of glucose. Under
ordinary conditions this process of glycogenolysis is relatively slow, but
when the requirements of the organism for fuel become increased, as
during muscular exercise, it becomes very rapid. The glycogenic func-
tion of the liver appears therefore to exist, in part at least, for the
purpose of preventing the flooding of the blood of the systemic circu-
lation with excess of sugar during absorption from the intestine and of
maintaining the normal percentage at other times. This function is
analogous to that occurring in plants, in which the sugar produced in
the leaves, if not immediately required, is transported to various parts
of the plant and there converted into starch, which, when the plant
requires it, as during new growth, may again become transformed into
glucose.
The agency converting the glycogen into glucose is the diastatic
669
670 METABOLISM
enzyme glycogenase, which is present, not only in the liver cell, but
also in the blood and lymph. If is a difficult matter to explain why
glycogen should be able to exist at all in the liver cells in the presence
of this powerful enzyme. The following possibilities may be considered:
(1) That glycogenase does not really exist in the living liver cells, but
is a postmortem product; (2) that, although present, glycogenase is pre-
vented from acting on the glycogen in the living liver cell on account of
the latter being protected from its influence by combination with the
sustentacular substance; or (3) that some chemical substance in the liver
cell prevents the glycogenase from acting on the glycogen — an anti-
glycogenase. Since the removal of any one of these inhibiting influ-
ences would cause glycogenolysis to become excessive, and so bring
about hyperglycemia, it is important, in searching for the possible
causes of this condition, to examine the evidence that has been brought
forward in support of each of these views.
Against the view that glycogenase is a postmortem product may be
cited the very rapid conversion into glucose that occurs when glycogen
is added to living blood, as by injecting some into a vein. On account of
the active glycogenolytic action of blood, it has been suggested that
during life glycogen does not become transformed into glucose until
after it has been discharged into the blood from the liver cell. When
increased sugar must be mobilized, glycogen passes unchanged, or per-
haps as seme dextrine, into the blood and lymph of the liver capillaries
and lymphatics, the glycogenase of which converts it into glucose, the
conversion being so rapid that, by the time the blood has traveled from
the liver through the heart and pulmonary vessels to the arteries, all
the glycogen has already become transformed into glucose. Postmortem
glycogenolysis, according to this view, is due to the opposite occur-
rence— the transference of glycogenase from the blood into the liver
cell. Some facts supporting this view are as follows: (1) It has been
found that the amount of free glucose in the blood of the vena cava
is sometimes less than in that collected simultaneously from the carotid
artery. (2) After giving certain substances, such as phosphorus or
peptone, there is distinct diminution in the amount of glycogen in the
liver, accompanied, however, by no increase in the amount of glucose
in the blood. And (3) if the liver of an animal that has been rendered
diabetic by stimulation of the splanchnic nerve or by puncture of the
floor of the fourth ventricle is examined microscopically, after staining
by the carmine method, masses of stained glycogen can be found present
in the capillaries (sinusoids) that lie among the liver cells.
According to the second view, the glycogen is removed from the
influence of the intrahepatic glycogenase on account of its combination
THE METABOLISM OF THE CARBOHYDRATES 671
with a sustentacular material. By disrupting this combination and thus
exposing the glycogen to the action of glycogenase, glycogenolysis will
occur. We may call this the mechanical hypothesis and it deserves
serious consideration, for it has been shown that very little postmortem
glycogenolysis occurs in the intact liver of frogs in winter, — even though
at this time the organ contains an excess of glycogen, — but becomes
marked when the liver is broken down by mechanical means.
The third view depends on the well-known fact that enzyme activities
become most markedly altered by slight changes in the chemical nature
of the environment in which they act. Diastatic enzymes are partic-
ularly susceptible to the reaction (CH) of their environment, a very
slight degree of acidity favoring and a trace of alkalinity markedly
depressing their activities. That a tendency to increasing acidity in
the liver cells may retard the formation of glycogen is suggested by
the depressing effect produced on the assimilation limit of sugars by
administering acids, and by the observation that postmortem glycogen-
olysis becomes marked in proportion as the dying liver becomes acid in
reaction. It might be thought then that glycogenolysis in the liver cell
could be set up by the local production of a certain amount of acid.
Such a liberation of free acid could be brought about by a curtailment
in the arterial blood supply of the hepatic cell, producing a local accu-
mulation either of carbonic or of other less completely oxidized acids
(e.g., lactic). It may be that asphyxia causes hyperglycemia by such
a mechanism. Vasoconstriction and consequent curtailment of arterial
blood supply occurs in the liver when the hepatic nerves are stimulated,
and it is possible that the glycogenolysis which is also set up by such
stimulation is due to the appearance of acids. The accelerating effect
of epinephrine on glycogenolysis might also be explained as due to
limitation of blood supply on account of vasoconstriction.
THE REGULATION OF THE BLOOD SUGAR LEVEL
The level at which the concentration of .sugar in the systemic blood
is maintained represents the balance between two opposing factors: (1)
the consumption of glucose bv the tissues, and (2) the production of
glucose by the liver. Since this is the most readily oxidizable of all
the proximate principles of food (page 652), muscular activity causes
large quantities of it to be consumed, so that its concentration m the
blood tends to fall below the physiologic level, a tendency which is
immediately met by an increased discharge of glucose from the liver.
The question therefore arises as to Iww the muscles or other tissues
transmit their requirements for glucose to the liver. There are two
672 METABOLISM
possible ways by which this could be done: (1) by means of a nervous
reflex, or (2) by changes in the composition of the blood, either with
regard to the percentage of sugar itself or because of the appearance in
it of decomposition products of glucose or of some special exciting
agent or hormone.
In order to ascertain the relative importance of these methods of
correlation between the places of supply and demand of glucose in the
normal animal, it is necessary to investigate the conditions under which
an excessive discharge of glucose occurs as a result of overstimulation
of the nervous control, or because of the presence of exciting substances
(hormones) in the blood. The glycogenic function can be excited through
the nervous system in a variety of ways so as to produce hyperglycemia
and glycosuria. This constitutes one form of experimental diabetes. In
laboratory animals mechanical irritation of the medulla oblongata and
stimulation of the great splanchnic nerves act in this way. Similar stimula-
tion may also occur under certain conditions in man. Excitation as a result
of changes in the composition of the blood can be produced experimen-
tally by certain drugs (phlorhizin), or by the removal of certain of the
ductless glands or the injection of extracts prepared from them, such
as epinephrine.
Nerve Control and the Nervous Forms of Experimental Diabetes. —
The simplest experimental condition which illustrates the relationship
between the nervous system and the blood sugar is electrical stimulation
of the great splanchnic nerve in animals in which, by previous feeding
with carbohydrates, a large amount of glycogen has been deposited in
the liver. By examination of the blood as it is discharged into the vena
cava from the hepatic veins, the increase in blood sugar is very evident
in from five to ten minutes after the first application of the stimulus;
but it is not until later that a general hyperglycemia becomes estab-
lished. The conclusion which we may draw from these results is that
the splanchnic nerve contains efferent fibers controlling the rate at
which glycogen becomes converted to glucose in the liver. The center
from which these fibers originate is situated somewhere in the medulla
oblongata, for the irritation that is set up by puncturing this portion of
the nervous system with a needle yields results similar to those which
follow splanchnic stimulation. This "glycogenic" or diabetic center, as
it has been called, must be provided with afferent impulses. Such im-
pulses have indeed been described in the vagus nerves, but their dem-
onstration is by no means an easy matter on account of the disturbance
in the respiratory movements coincidently produced by the stimulation.
The changes that such disturbances bring about in the aeration of the
THE METABOLISM OP THE CARBOHYDRATES 673
blood may in themselves be responsible for the hyperglycemia (see page
332). It can at least be said that when the respiratory disturbances are
guarded against, as by intratracheal insufflation of oxygen, vagal hyper-
glycemia is much less marked, if not entirely absent. But this question
awaits more thorough investigation.
The increased glycogenolysis which results from stimulation of the
efferent fibers in the splanchnic nerves may depend either on a direct
control exercised over the glycogenic functions of the hepatic cells, or
on the discharge into the blood of some hormone which excites the
glycogenolytic process. It must furthermore not be lost sight of that
the glycogenolysis may be secondary to local asphyxial conditions in
the liver cells resulting from vasoconstriction. From their anatomic
position, the adrenals are to be thought of as the source of the hormone,
and evidence that splanchnic hyperglycemia isi due to hypersecretion
from these glands has seemed to be furnished by the fact that after they
are extirpated splanchnic stimulation no longer produces hyperglycemia,
neither, indeed, does puncture of the medulla. There is also no doubt
that the nervous system, acting by way of the splanchnic nerves, does
exercise a control over the discharge of the internal secretion of the
adrenal glands and that extracts of the gland, which we must suppose
act in the same way as the internal secretion, cause hyperglycemia when
injected intravenously (epinephine hyperglycemia and glycosuria).
But on theoretical grounds alone, certain difficulties immediately pre-
sent themselves in accepting this as the mechanism by which the nervous
system controls the sugar output of the liver, for if increased sugar
formation in the liver is dependent on a discharge of epinephrine, the
question may be asked why this secretion should be caused to traverse
the entire circulation before reaching the liver.
There are, besides, certain experimental facts which do not conform
with such a view. Thus, after complete severance of the hepatic plexus
of nerves, stimulation of the splanchnic nerve does not cause the usual
degree of hyperglycemia, whereas electric stimulation of the peripheral
en'd of the cut plexus does cause it. On the one hand, therefore, there
is evidence that stimulation of the efferent nerve path above the level of
the adrenals has no effect on the sugar production of the liver in the
absence of these glands; and on the other, we see that when they are
present, stimulation of the nerve supply of the liver is effective, even
though the point of stimulation is beyond them. There is but one con-
clusion that we may draw — namely, that the functional integrity of the
efferent nerve-fibers that control the glycogenolytic process of the liver
depends on the presence of the adrenals, very probably because of the
hormone which the glands secrete into the blood. This conclusion is
674 METABOLISM
corroborated by the fact that stimulation of the hepatic plexus, even
with a strong electric current, some time after complete removal of
both adrenals, is not followed by the usual degree of excitement of the
glycogenolytic process.
These experiments demonstrate an important relationship between
the nervous control, and at least one form of hormone control, of the
sugar output of the liver. They indicate that when a sudden increase
of blood sugar is required, the glycogenic center sends out impulses
which not only directly excite the breakdown of glycogen in the he-
patic cells, but also simultaneously influence the adrenals in such a man-
ner as to produce more epinephrine in the blood and so augment the ac-
tion of the nerve impulse.
We are as yet quite in the dark as to the mechanism by which the
nerve impulses or the hormone brings about increased glycogenolysis.
It must consist of a removal of the influence that prevents glycogenolysis
from occurring in the normal liver, for it has been shown by direct ob-
servation that there is no increase in the amount of glycogenase present
in extracts of the liver removed from diabetic animals over that present
in extracts of the liver of normal animals. The possible nature of this
influence has already been discussed (page 669). The change may con-
sist either in a loosening of the combination between the glycogen and
the protoplasm of the liver cell, or in a removal of the chemical influence
that ordinarily prevents the glycogenase from attacking the glycogen.
In the former case the glycogen liberated from its union with the sus-
tentacular substances would either become attacked by the glycogenase
present in the liver cell itself or it would first of all migrate, as glyco-
gen, into the blood capillaries and there be attacked by the blood
glycogenase. Evidence for the possibility of the occurrence of such a
process has already been given (page 670). The chemical change re-
ferred to under the second possibility might consist in an alteration in
the hydrogen-ion concentration of the liver cell, a change, however,
which for obvious reasons it is impossible to investigate.
Nervous Diabetes in Man. — The main interest attaching to the inves-
tigation- of these nervous forms of experimental diabetes depends on the
insight which they afford us into the nature of the mechanism by which
a prompt mobilization of glucose may be brought about in the normal
animal. There is also some evidence that a relationship may exist be-
tween certain of the clinical varieties of the disease in man and repeated
excitation of glycogenolysis brought about by nerve stimulation. In-
creased glucose output from the liver as a result of nerve excitation
may be a normal process, but there is reason to believe that frequent
repetition of this process tends to induce a permanent rise in the glucose
THE METABOLISM OF THE CARBOHYDRATES 675
level of the blood and therefore a tendency to diabetes. There have
recently been collected several facts which lend some support to this
view. The frequent occurrence of diabetes in those predisposed by
inheritance to neurotic conditions, or in those whose daily habits entail
much nerve strain, and the aggravation of the symptoms which is likely
to follow when a diabetic patient experiences some nervous shock, all
point in this direction.
Diabetes is common in locomotive engineers and in the captains of
ocean liners — that is, in men who in the performance of their daily duties
are frequently put under a severe nerve strain. It is apparently in-
creasing in men engaged in occupations that demand mental concentra-
tion and strain, such as in professional and business work. Cannon23
found glycosuria in four out of nine students after a severe examination,
but only in one of them after an easier examination.* In the urine of
twenty-four members of a famous football squad, sugar was found pres-
ent in twelve immediately after a keenly contested game. Anxiety and
excitement were responsible for its appearance, for five of the twelve
were substitutes who did not get into the game.
Although these nervous conditions, by excitement of hepatic glyco-
genolysis, produce at first nothing more than an excessive discharge of
sugar into the blood — a condition which is exactly duplicated in our
laboratory experiments by stimulation of the nerve supply of the liver —
their repetition may gradually lead to the development of a permanent
form of hyperglycemia. To prevent the repetition of these transient
hyperglycemias must be one of our aims in the treatment of early stages
of the disease.
Although there can be no doubt that the glycogenic function of the
liver is subject to nerve control, it is probable that its control by hor-
mones is of equal if not greater importance. This dual control of a
glandular mechanism is by no means unique for the glycogenic function,
for we have already seen it to exist in the case of the gastric glands
and the pancreas, and it is probable that it also exists in the case of
the thyroid. It may well be that the nerve control of the glycogenic
function has to do only with those transitory changes in sugar produc-
tion that would be demanded by sudden activities of muscle, and that
the hormone control has to do with the more permanent process of build-
ing up and breaking down of glycogen to meet the general metabolic
requirements of the tissues.
*We have been unable to confirm this observation even though the examinations were made
unusually "nerve-racking."
676 ' METABOLISM
HORMONE CONTROL AND PERMANENT DIABETES
Nervous excitation can explain only transitory increases in blood sugar,
the more permanent hyperglycemias being dependent upon some dis-
turbance in the hormone control of carbohydrate utilization. This dis-
turbance is a much more serious affair than that produced by nervous
excitation. In the latter case the hyperglycemia ceases whenever all
of the glycogen stores of the liver have been exhausted; whereas a dis-
turbance in the hormone control, besides causing as its first step a
breakdown of all the available glycogen, goes on to cause a production
of sugar out of protein. A process of gluconeogenesis (new formation
of glucose) becomes superadded on one of glycogenolysis.
To ascertain the nature of this hormone and the mechanism of its
action has been the object of most of the researches on those forms of
diabetes produced by changes in certain of the ductless glands. The
following possibilities may be considered: (1) that it is the concentration
of glucose in the blood ; (2) that it is the presence in the blood of decom-
position products of glucose; (3) that it is due to a hormone produced
from some ductless gland. Regarding the first of these possibilities, it
is supposed that the mechanism involved in the adjustment between the
blood sugar and the glycogenic function is one explicable on the basis
of the law of mass action; namely, that glycogen becomes converted
into glucose whenever the blood flowing to the liver contains less than
its normal concentration of glucose, and conversely, when this blood
contains an excess of glucose, as during absorption, a glycogen-building
process takes place. Although there can be little doubt that the process
of glycogen formation or destruction will depend to a certain extent
upon the amount of; glucose present in the blood flowing to the liver
cells, yet it is impossible that this can be an important means in the
control that exists between sugar production by the liver and sugar
consumption by the tissues, because the sugar that is added to the portal
blood during absorption would mask any depletion caused by sugar
consumption in the -tissues.
The second possibility — that the hormone is some decomposition prod-
uct of glucose — would appear to have some support, if we consider this
hormone to be an acid product (carbon dioxide or lactic) produced by
sugar metabolism, for it is known that an increase in the hydrogen-ion
concentration of the blood flowing to the liver cells excites a glycogen-
olysis.. As we have already seen, however, it is difficult to secure ex-
perimental evidence, in anesthetized animals at least, that glycogen-
olytic activity is readily excited in this way.
The third possibility — that some specific hormone may exist in the
THE METABOLISM OF THE CARBOHYDRATES 677
blood exciting the glycogenolytic process — is investigated by producing
disturbances involving various of the ductless glands, particularly the
pancreas, the adrenals, the parathyroids and the pituitary. The influ-
ence of certain, of these glands may be closely bound up with that
exercised through the nervous control, as we have seen to be the case
with the adrenal gland. Whether it is by the production of hormones
directly necessary for proper carbohydrate metabolism, or by the re-
moval from the blood of such substances as interfere with this process,
that the ductless glands functionate, is one of the main problems we
have to consider.
Utilization of Glucose in Tissues. — Although the experimental diabetes
induced by disturbances in the function of the ductless glands is at
first dependent on an upset of the glycogenic function and later of glu-
coneogenesis, the utilization of glucose in the tissues ultimately becomes
interfered with. It is therefore important that we should digress for a
moment to consider briefly what is knoAvn regarding the process by
which sugar becomes utilized in the organism. That glucose becomes
used up by active muscle there can be no doubt. Thus, if the muscles
of one leg in the frog are tetanized, the glycogen content, compared with
that of the other leg, will be found to be diminished.
At first sight it migh.t appear that the easiest way to study the utiliza-
tion of glucose in the muscles Avould be to compare its concentrations
in the blood flowing to and coming from the muscle. The muscle that
has been most successfully employed in studies of this kind has been the
heart. Some years ago Starling and Knowlton24 examined the consump-
tion of sugar by the excised mammalian heart, and in their earlier
experiments seemed to be able to show that the extent to which this
consumption occurred was 4 milligrams per 100 grams heart muscle
per hour. A more thorough repetition of these experiments later by Pat-
terson and Starling25 showed, however, that the results can furnish no
criterion of the actual consumption of glucose by the tissue on account
of the fact that the tissue itself may store away large quantities of
carbohydrate in an unused state — i. e., as glycogen.
Other investigators have thought to study the utilization of glucose
by observing the rate at which it disappears from drawn blood kept in
a sterile condition at body temperature for some hours after death.
This process is called glycolysis, and it has been assumed that the process
is similar to that which occurs in the tissues themselves — an assumption,
however, for Avhich there is no warranty. Indeed, it may readily be
shown that the glycolysis occurring in blood has very little if anything
to do with the utilization of sugar in the tissues, for it has been found
that glucose disappears from drawn blood very slowly indeed when
678 METABOLISM
compared with the rate at which it disappears from the blood of animals
in which the addition of glucose from the liver has been prevented by
removal of this viscus (Macleod).26
A third method for studying the utilization of glucose consists in
observing the respiratory exchange of animals. In normal animals the
injection of glucose causes an increase in the carbon-dioxide excretion
and a rise in the respiratory quotient, which it will be remembered is
a ratio expressing the relationship between the amount of carbon dioxide
excreted and of the oxygen retained in the organism. When carbohy-
drate is undergoing combustion, the quotient is nearly 1, whereas with
that of protein it is about 0.7 (see page 547). By observing the quotient
under given conditions one can compute the proportions of carbohydrate
and of fat and protein that are undergoing metabolism. In the hands
of Murlin and others,27 this method has proved of some value in settling
certain questions concerning the utilization of glucose in normal and
diabetic animals ; but the results must be interpreted with great care on
account of the fact that temporary changes in the blood may cause a
greater or a less expulsion of carbon dioxide from it. Thus, if acids
appear in the blood, they will dislodge carbon dioxide from it, and
apparently cause the respiratory quotient to rise. Alkalies, on the other
hand, apparently cause it temporarily to fall, and unless the observa-
tions are done over a long period of time and with great care, faulty
conclusions are very apt to be drawn from the results.
Diabetes and the Ductless Glands
We are now in a position to consider the forms of experimental dia-
betes produced by disturbances in the ductless glands.
Relationship of the Pancreas to Sugar Metabolism. — In no other of
the many causes of diabetes has greater interest been shown than in
that due to disturbance in the pancreatic function. Many of the earlier
clinicians that followed cases of diabetes mellitus into the postmortem
room, noted that definite morbid changes in the pancreas were a fre-
quent accompaniment of the disease. Prompted by these observations,
several investigators attempted experimental extirpation of the gland,
but did not succeed in producing glycosuria in the few animals that
survived the operation. Their failure, no doubt, resulted from incom-
plete extirpation. To reduce the severity of the operation, Claude Ber-
nard injected oil into the pancreatic duct, and tied it; but he succeeded
in keeping only two dogs alive for any length of time, and these did
not exhibit glycosuria. Neither were other investigators that adopted
similar methods any more successful. It looked as if the pancreas had
very little to do with the cause of diabetes. In the year 1889 Minkowski
THE METABOLISM OF THE CARBOHYDRATES 679
and von Mering in Germany, and de Dominicis in Italy, by thorough
extirpation of the gland, succeeded in producing in dogs a marked and
persistent glycosuria, accompanied by many of the other symptoms of
diabetes. The first two authors attributed the condition to removal of
an internal secretion.
The course of the diabetes thus produced is, however, somewhat differ-
ent from that usually observed in man. It is extremely acute from the
start, the G: N ratio being 1 :3.6 (see page 664), and it is unaccompanied
by any of the classical symptoms seen in the clinical condition. Experi-
mental pancreatic diabetes can, however, be made to simulate very closely
the disease in man. This was first of all demonstrated by Sandemeyer,
who found that if the greater part of the pancreas was removed, the
animals for some months, if at all, were only occasionally glycosuric,
but later became more and more frequently so, until at last the condition
typical of complete pancreatectomy supervened. Similar results have
more recently been obtained by Thiroloix and Jacob, in France, and by
Allen in this country. These investigators point out that different re-
sults are to be expected according to whether the portion of pancreas
which is left does, or does not, remain in connection with the duodenal
duct. When this duct is ligated, atrophy of any remnant of pancreas
that is left is bound to occur, and this is associated with rapid emacia-
tion of the animal, diabetes and death. When the remnant surrounds a
still patent duct, a condition much more closely simulating diabetes in
man is likely to become developed — one, namely, in wMch there is, for
some months following the operation, a more or less mild diabetes,
which, however, usually passes later into the fatal type.
It is, of course, difficult to state accurately what proportion of the
pancreas must be left in order that the above described condition may super-
vene. Leaving a remnant amounting to from one-fifth to one-eighth
of the entire gland is commonly followed by a mild diabetes, whereas
if only one-ninth or less is left, a rapidly fatal type develops. As in
clinical experience, the distinguishing feature between the mild and the
severe types of experimental pancreatic diabetes is the tolerance toward
carbohydrates. In the mild form, no glycosuria develops unless carbo-
hydrate food is taken; in the severe form, it is present when the diet is
composed entirely of flesh. It is thus shown that "by removal of a
suitable proportion of the pancreas, it is possible to bring an animal
to the verge of diabetes, yet to know that the animal will never of itself
become diabetic. . . . Such animals, therefore, constitute valuable
test objects for judging the effects of various agencies with respect to
diabetes" — (Allen18). It therefore becomes theoretically possible to in-
vestigate, on the one hand, other conditions which will have an influence
680 METABOLISM
similar to removal of more of the gland, or, on the other, conditions
which might prevent the incidence of diabetes, even though this extra
portion of pancreas is removed.
From the work which he has already done, Allen believes that he has
sufficient evidence to show that the continued feeding with excess of
carbohydrate food will surely convert a mild into a severe case, and in
one experiment he succeeded in bringing about the same transition by
performing puncture of the medulla — that is, by creating an irritative
nervous lesion. By none of the other means usually employed to produce
experimental glycosuria could the bordering case be made diabetic,
although one such animal became acutely diabetic after ligature of the
portal vein. To the clinical worker the value of these results lies in the
fact that they furnish experimental proof that a so-called latent case
of diabetes — that is, one that has a low tolerance value for carbohy-
drates— may be prevented from developing into a severe case by proper
control of the diet. Attempts to show whether or not there are any
conditions which might bring about improvements in animals that were
just diabetic have not as yet been sufficiently made to warrant any con-
clusions that could help us in the treatment of human cases. The en-
couragement of the internal pancreatic secretion by diminution of that
discharge into the intestine may be of value.
The certainty with which diabetes results from pancreatectomy in dogs,
as well as the frequent occurrence of demonstrable lesions in the pan-
creas in diabetes'in man, leaves no doubt that this gland must be in some
way essential in the physiologic breakdown of carbohydrates in the
normal animal, but how, we can not at present tell. All we know is
that the first change to occur after the gland is removed, is a sweeping
out of all but a trace of the glycogen of the liver, although the muscles may
retain theirs ; indeed, in the cardiac muscle there may be more than
the usual amount.28 Nor can any glycogen be stored in the liver when
excess of carbohydrates is fed. After the glycogen has disappeared,
gluconeogenesis sets in, so that the tissues come to melt away into sugar,
and all the symptoms of acute starvation, associated with certain others
that are possibly due to a toxic action of the excess of sugar or other
abnormal products in the blood, make their appearance.
So far it might be permissible to consider an overproduction of glu-
cose as the cause of the hyperglycemia of pancreatic diabetes, just as
we have seen it to be of these forms of hyperglycemia that are due to
stimulation of the nervous system ; but this can not be the case, for
another very definite abnormality in metabolism becomes evident —
namely, an inability of the tissues to burn sugar. This fact is ascer-
tained by observing the respiratory quotient. When glucose is added
THE METABOLISM OF THE CARBOHYDRATES 681
to the blood in the case of a completely diabetic animal, no change oc-
curs in the quotient.
There are, therefore, two essential disturbances of carbohydrate
metabolism in pancreatic diabetes — overproduction of sugar and aboli-
tion of the ability of the tissues to use it. It becomes important for us
to see whether the tissues exhibit this inability to use sugar when they
are isolated from the animal ; for if they should, a much more searching
investigation of the essential cause of their inability would be possible
than is the case when they are functioning along with the other organs
and tissues. The earlier experiments of Lepine and his pupils, which seemed
to show that diabetic blood did not possess the glycolytic power of
normal blood ; and those of Cohnheim, from which it was concluded that
mixtures of the expressed juices of muscle (liver) and pancreas, although
ordinarily destroying glucose, failed to do so when they were taken from
a diabetic animal, are now known to be erroneous.
The failure to show a depression of glycolytic power by these methods
prompted Knowlton and Starling24 to investigate the question whether
any difference is evident in the rate with which glucose disappears from
a mixture of blood and saline solution used to perfuse a heart outside
the body, according to whether the heart was from a normal or a dia-
betic dog. In the first series of observations which these workers made,
it was thought that the normal heart used glucose at the rate of about
4 mg. per 100 gm. of heart substance per minute ; whereas that of a dia-
betic (depancreatized) animal used less than 1 mg. If such striking
differences in the rate of sugar consumption could make themselves
manifest for so relatively small a mass of muscular tissue as that of the
heart, it is permissible to assume that a much more striking difference
could be demonstrated when the perfusion fluid is made to traverse all
or practically all of the skeletal muscles, as well as the heart. For this
purpose an eviscerated animal may be employed — that is, one in which
the abdominal viscera are removed after ligaticn of the celiac axis and
mesenteric arteries, and the liver is eliminated by mass ligation of its
lobes. Using such preparations, R. Gr. Pearce and Macleod29 found that
the rate at which glucose disappears from the blood, although very
irregular, is in no way different in completely diabetic as compared
with normal dogs. They were thus unable to confirm any of Knowlton
and Starling's earlier conclusions. Patterson and Starling subsequently
pointed out that a serious error was involved in the earlier perfusion
experiments, partly on account of a remarkable but irregular disap-
pearance of glucose from the lungs, and partly because the diabetic
heart may contain a considerable excess of glycogen, from which its
682 METABOLISM
demands for sugar may be met without calling on that of the perfusion
fluid.
In spite of the failure to show that the isolated tissues of diabetic
animals have a lower glucose-consuming power than those of normal
animals, it is . important from a practical standpoint that we should
know something regarding the possible nature of the disturbance which
a removal of the pancreas entails. Even if we could not tell exactly
how this disturbance operates, it would be of value to know whether
it depends on the removal from the organism of some hormone that is
essential to carbohydrate utilization, for, if this were proved to be the
case, encouragement would be offered to seek for the chemical nature
of this hormone so that we might administer it with the object of re-
moving the diabetic state. The hope of a fruitful outcome of such an
investigation is encouraged by the success of researches on diseases of
other ductless glands, particularly the thyroid.
The removal of some hormone necessary for proper sugar metab-
olism is, however, by no means the only way by which the results can
be explained, for we can assume that the pancreas owes its influence
over sugar metabolism to some change occurring in the composition of
the blood as this circulates through the gland — a change which is de-
pendent on the integrity of the gland and not on any one enzyme or
hormone which it produces. It is obvious that the results of removal
of the gland could be explained in terms of either view, and indeed
there is but one experiment which would permit us to decide which of
them is correct. This consists in seeing whether the symptoms which
follow pancreatectomy are removed, and a normal condition reestab-
lished, when means are taken to supply the supposed missing internal
secretion to the organism; if they should be, conclusive evidence would
be furnished that it is by "internal secretion" and not by "local in-
fluence" that the gland functionates.
The experiments have been of two types: in the one, variously pre-
pared extracts of the glands have been employed, and in the other,
blood which is presumably rich in the internal secretion. The most
recent work with pancreatic extracts has shown that injection of pan-
creatic extracts into a depancreatized animal produces no change in the
respiratory quotient, although injections of extracts of pancreas and
duodenum may produce a temporary fall in the dextrose excretion in
the urine on account of the alkalinity of the extract. Neither have
experiments with blood transfusions yielded results that are any more
satisfactory. In undertaking these experiments it is of course assumed
that the internal secretion is present in the blood, and that if this blood
is supplied to an animal suffering from diabetes because of the loss of
THE METABOLISM OF THE CARBOHYDRATES 683
its pancreas, it will restore it to a nondiabetic state. The general con-
clusion that may be drawn from the numerous researches of this nature,
is that there is no evidence that the blood of a normal animal, even
when it is from the pancreatic vein, contains an internal secretion that
can restore to a diabetic animal any of its lost power to utilize carbo-
hydrates. When the extent of glycosuria alone is used as the criterion
of the state of carbohydrate metabolism, serious errors in judgment are
liable to be drawn. The condition of the blood sugar and the extent
and character of the respiratory exchange are the most reliable indexes.
DIABETIC ACIDOSIS OR KETOSIS
Nature and Cause. — Much confusion has existed in medical literature
over the correct definition of acidosis, mainly because the term was first
used for the particular variety of the condition observed in the later
stages of diabetes mellitus. The acids which accumulate in the tissue
fluids in this disease are acetoacetic and /?-oxybutyric, and they are
oxidation products of acetone, which is again derived from fatty acids
by a faulty metabolism (see page 709). The essential cause of the
acidosis is therefore entirely different from that in nephritis; in dia-
betes foreign acids are added to the blood, whereas in nephritis the
acids of a normal metabolism accumulate because of faulty excretion
through the kidneys. The usual signs of acidosis exist in both cases,
because the surplus of acid depletes the store of bicarbonate and
causes changes in the alveolar C02, in the C02-absorbing power of the
blood, in the reserve alkalinity, and in the acid excretion by the kidney.
It is important to recognize the special nature of diabetic acidosis by a
separate name — ketosis.
The chemical processes by which the ketone bodies are produced is
discussed elsewhere (page 709). It remains for us to consider the
general nature of the metabolic disturbance responsible for their ap-
pearance in diabetes.
For the thorough combustion of fat in the animal body a certain
amount of carbohydrate must be simultaneously burned. Fat evidently
is a less readily oxidized foodstuff than sugar; it needs the fire of the
burning sugar to consume it. If the carbohydrate fires do not burn
briskly enough, the fat is incompletely consumed; it smokes, as it were,
and the smoke is represented in metabolism by the ketones and derived
acids. Such a closing down of the carbohydrate furnaces may be
brought about either by curtailment of the intake of carbohydrates, as
in starvation (page 569), or by some fault in the mechanism of the
furnace itself, as in diabetes. Besides fat, protein may also contribute
684 METABOLISM
to the production of ketones when carbohydrate combustion is de-
pressed. Fundamentally, therefore ketosis in diabetes is due to the
same cause as in starvation — namely, an improper adjustment between
the metabolisms of fat and carbohydrate.
Bearing these principles in mind, it is easy to see how the intensity
of acidosis which develops during starvation will depend upon the re-
lative metabolism of carbohydrate, on the one hand, and of fat and
protein, on the other; it will therefore depend on the amounts of these
foodstuffs which have been stored in the organism, and this again will
depend on the nature of the diet previous to the starvation period. For
the first few days following entire abstinence from food in a healthy,
well-nourished individual, very few if any ketones will be excreted in
the urine, because the carbohydrate stored in the body as glycogen has
sufficed during this time to maintain the proper proportion between fat
and carbohydrate. Afterwards, however, their appearance is to be ex-
pected, because the glycogen stores become exhausted long before those
of fat. If starvation is still further prolonged^ a stage will come when
the fat, as well as the carbohydrate, is used up so that the organism has
now to subsist on protein alone. When this stage arrives, the ketones
will diminish, for, although they might be derived from certain of the
amino acids, yet this does not actually occur, because a large part of the
protein molecule (nearly half) also becomes changed into glucose, which
by burning, as above explained, prevents the formation of ketones from
the other part of the molecule. For the same reasons, marked acidosis
will not be expected to occur during any stage of starvation in lean
persons, who from the start must utilize mainly their stored protein to
supply the fuel upon which to live.
In diabetes exactly the same principles apply, but to an organism in
which the ability to metabolize carbohydrate has been depressed, so that
"the maximum rate at which dextrose can be oxidized is fixed at some
level which is absolutely lower than in health."30 Therefore, since a cer-
tain proportionality must exist between the rates of combustion of fat
and carbohydrate, the diabetic can thoroughly oxidize less fat; in other
words, an amount of fat which could readily be burned in a healthy body
is improperly burned by the diabetic, and ketones and their acids ac-
cumulate.
Starvation Treatment. — "In order to check a diabetic acidosis, it is
necessary to restore the proper ratio of fatty acid to glucose oxidation,"
which can best be done by starvation, rest in bed and warmth. But this
treatment may not at first suffice, because we have to deal not only with
the acidosis bodies derived from fat, but with those which can be derived
from protein on account of the diabetic organism having lost the power
THE METABOLISM OF THE CARBOHYDRATES 685
even of burning the glucose which is derived from this foodstuff. By
persistence in the starvation, however, the ability of the organism to
utilize carbohydrate usually becomes so far restored that enough burns to
prevent acidosis. Every case of diabetes can not, therefore, be expected
lo react in the same way to starvation, the determining condition being
the relation between the quantities of glycogen and fat stored in the body
at the outset of the fasting period. This relationship depends on the
nature of the previous diet.
To sum up, ' ' fasting will lower acidosis either in. health or in diabetes,
if it has the effect of stopping a one-sided metabolism and throwing the
tissues on a more nearly balanced ration of fatty acids and glucose" —
(Woodyatt). A practical point may be noted here — namely, that there
is likely to be more danger of serious acidosis developing during starva-
tion in fat than in lean diabetics. The importance of our appreciation of
these facts in the starvation treatment of diabetes will be self-evident.
CHAPTER LXXVII
FAT METABOLISM
Before considering, the physiology of fats, a few of the most essential
points regarding their chemistry may be of assistance.
THE CHEMISTRY OF FATTY SUBSTANCES
It is usual to classify all substances that are soluble in ether as lipoids.
They include fatty acids, neutral fats, cholesterols, cholesterol esters, and
phospholipins.
The fatty acids belong to two main homologous series, which differ from
each other with regard to whether they are saturated or unsaturated. A
saturated fatty acid is typified by palmitic, whose formula is CH3-CH2-CH2-
CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH, or CH3-
(CH2)14-COOH; that is to say it is a higher member of the series to which
acetic acid (CH3-COOH) belongs, differing from the latter in having four-
teen extra methyl radicles, each joined to its neighbor by one bond or satu-
rated linking on either side. Another member of this series is stearic, in
which there are sixteen extra CH2 groups (CH3(CH2)16-COOH). An un-
saturated fatty acid is oleic (CH3(CH2)7— CH29 = CH-(CH2)7-COOH).
Its unsaturation is represented in the formula by the double bond or
unsaturated linking, which it will be seen occupies a position in the mid-
dle of the molecule, the other methyl radicles being linked together by
single bonds.
The fatty acids readily combine with alkali to form soaps; thus,
CH3(CH2)14-COOH + KOH=CH3(CH2)]4-COOK + H20, the reaction being
(palmitic acid) (soap)
analogous to that by which acetic acid forms an acetate with alkalies.
In place of being combined with alkali, the COOH (carboxyl) group of
fatty acids may combine with alcohols to form substances called esters,
Thus, acetic acid and ethyl alcohol form ethyl acetate,
CH3COO JH + OHi C2H5=CH3COO-C2H5 + H20. When the alcohol thus
(acetic (ethyl (ethyl acetate)
acid) alcohol)
united with fatty acid is glycerol (glycerine), in which there are three
686
FAT METABOLISM 687
OH (hydroxyl) groups, the resulting ester — called triglyceride — is neu-
tral fat, Tripalmitin has the formula:
CHj — OOC " C15H31
CH -OOC-G15H31
CH2-OOC-C15H31.
By boiling neutral fats with alkali the fatty acid radicles are split off
as soaps, leaving the glycerol. This process is called saponification, and
it may be effected in many other ways, as for example by heating with
steam or by the action of special enzymes called Upases, which are widely
distributed in plants and animals.
The natural fats are usually a mixture of triglycerides, and their dif-
ferences in properties are dependent upon the relative amounts of fatty
acids present. The three most important in animal fats are tripalmitin,
tristearin and triolein. It is essential in the study of fat metabolism that
we should know the most important methods ~by which the proportion of
fatty acids present in a mixed fat is determined. These methods are as
follows :
1. The melting point. Olein is liquid at 0° C.; palmitic acid melts at
62.6° C.; and stearic at 69.3° 0. The solidity of animal fats depends on
the proportion of olein, palmitin and stearin present. Mutton fat, for ex-
ample, is much stiffer than pig fat because it contains less olein and more
stearin. The melting points of fats from different parts of the body may
also vary.
2. The acid number indicates the amount of free fatty acid mixed with
the fat, and is determined by titrating a solution of a weighed quantity of
the fat in alcohol with a N/10 ' alcoholic solution of KOH, phenolphtha-
lein being used as indicator.
3. The saponification value indicates the total amount of fatty acid
present, both that which is free and that combined with glycerol. It is
determined by heating a weighed amount of fat with an exactly known
amount of alcoholic KOH (determined by titration with standard acid).
After saponification is complete, titration of the mixture shows how much
alkali has been used to combine with the fatty acid. This is the saponi-
fication value.
4. The ester value indicates the amount of fatty acid combined with
glyeerol, and is obtained by subtracting the acid value from the saponi-
fication value.
Besides these there are two values, known as the iodine and the Reichert-
Meissl values, that are of importance because they depend on certain char-
acteristics of the fatty-acid radicles.
688 METABOLISM
5. The iodine value indicates the amount of unsaturated fatty acids pres-
ent, or the number of double bonds. It depends on the fact that iodine,
like many other substances, is capable of directly attaching itself to the
fatty-acid chain wherever double bonds exist.
6. The Reich ert-Meissl value indicates the amount of volatile soluble
acid present in the fat. It is determined by first of all saponifying the
fat, then decomposing the soap by mixing it with mineral acid and dis-
tilling the liberated fatty acid, the distillate being collected in a known
amottnt of standard alkali and titrated. It is a value that is not of very
great use in physiologic investigations, but it is so in connection with
food chemistry. Since volatile acids are present in butter, the Reichert-
.Meissl value helps us to distinguish between butter and margarine.
Fat is insoluble in water but soap is soluble, forming a colloidal solu-
tion which presents the phenomenon of surface aggregation of molecules.
This consists in the concentration of the soap both at the free surface of
the liquid, where a skin may form, and at the interfaces between the
soap solution and any undissolved particles present in it. This pellicle-
formation around the particles prevents them from running together so
that they remain suspended, thus forming an emulsion. An emulsion
may therefore be formed either of neutral fat of any other physically
similar substance. When fat itself is used, there is usually enough free
fatty acid admixed with it to make it unnecessary in forming the emul-
sion to do more than shake the fat with weak sodium-carbonate solution.
With other substances not containing any free fatty acid, some soaps
should be added. To preserve the emulsion it is often useful to add some
mucilage. In the emulsified state, neutral fats are much more readily
attacked by lipases than when they are present in an unemulsified state.
Thus, emulsified fats are "digested" by the relatively small amounts of
lipase present in the stomach, whereas neutral fats themselves are not so.
Fatty acids also exist in nature in combination not with the triatomic
alcohol, glycerol, but with monatomic alcohols such as cholesterol. These
cholesterol fats differ from the glycerol fats in being very resistant to-
wards enzymes and microorganisms. They are therefore used for pro-
tective purposes in the animal economy; for example, they occur in the
sebum, the secretion of the sebaceous glands, where they serve to moisten
the hairs and skin. They are also present in cells, in which it is prob-
able they take an important part in forming the skeleton of the cell.
Cholesterol is absorbed from the intestine ; it is always present in the blood
both in plasma and in corpuscles ; and it is an important constituent of bile,
from which it may separate out in the bile passages and form calculi
(gallstones).
In the cells themselves the lipoids are represented mainly by compounds
FAT METABOLISM 689
of a somewhat more complex structure — namely, the phospholipins. As
their name indicates, these consist chemically of phosphoric acid combined
with neutral fat and with a nitrogenous base, cholin. The best known of
the phospholipins is lecithin, which is widely distributed in the animal body
(present in blood and bile as well as in all cells). Other phospholipins
present in nervous tissue are cephalin, cuorin and sphingomyelin. There
are various lecithins distinguished from one another by the fatty-acid
radicles which they contain. Distearyl-lecithin, for example, has the
formula:
CH2 - O - OC (CH2) M - CHS
CH -0-OC(CH2)14-CH3
(stearic acid)
CH2-0 O
(glycerol) P
/ \
OH OCH2-CH2-]Sr(CH3)2
(phosphoric |
acid) OH
(choline)
This complex molecule can readily be split up by hydrolysis (warming
with baryta water) into:
glycero-phosphoric acid, CH, - OH
CH -OH
CH2 - O O f C2H.OH
\ /
P ; choline, N < (CH3)3 (oxy-ethyl-ammonium
/ \
OH OH [OH
hydroxide) ; and fatty acids.
With hydrochloric acid, choline forms a salt which readily forms a
double salt with platinic chloride. Since this double salt forms charac-
teristic crystals, it is used to identify and separate lecithins. For quan-
titative purposes, however, it is more suitable to determine lecithin in-
directly by the amount of phosphoric acid present in an ethereal ex-
tract of the organ or tissue.
Evidence is constantly accumulating to show that lecithin is an ex-
tremely important constituent of cells; indeed, it seems to be the inter-
mediate stage in the utilization of neutral fats by protoplasm. Its phos-
phorus also probably serves as the source of this element for the con-
struction of nucleic acid (see page 637). In nervous tissues it is often
associated with carbohydrate molecules (galactose), forming the sub-
stance known as cerebrin. It may therefore have some role to play in
carbohydrate metabolism. Some Avorkers also attribute to lecithin an
690 METABOLISM
important function in the transference of substances through cell mem-
branes. When mixed with water it swells up by imbibition, and if crys-
talloids or other substances are dissolved in the water, a means is offered
for bringing water-soluble and fat-soluble substances into intimate con-
tact.
DIGESTION OF FATS
A certain amount of fat, especially when it is in an emulsified condi-
tion, can be digested in the stomach by the lipase contained in the gas-
tric juice. Most of it, however, is digested in the small intestine, into
which as we have seen, it is gradually discharged suspended in the chyme.
For this intestinal digestion of fat both pancreatic juice and bile are nec-
essary. This is easily shown in the rabbit, in which the pancreatic duct
enters the intestine at a considerable distance below the bile duct. If the
mesentery is inspected during the absorption of fatty food, no fat in-
jection of the lymphatics will be noted between the bile and the pan-
creatic ducts but only below the latter. In the dog, in which both the bile
and the main pancreatic ducts enter the intestine at about the same level,
fat injection of the lymphatics starts at this point, but if the bile duct
(or rather the gall bladder) is transplanted at some distance down the
intestine, it will be found that the injection of the lymphatics with fat
occurs only below the new point of insertion of the bile duct.
Removal of the pancreas interferes very materially with the absorption
of fat. In man, for example, absence of the pancreatic juice alone di-
minishes the absorption of fat by 50 or 60 per cent. If the bile is also
absent, the diminution amounts to 80 or 90 per cent, and in such cases,
as is well known, the administration of bile or pancreas powder greatly
improves fat absorption. In the dog, although ligation of the pancreatic
duct apparently only slightly influences fat absorption, removal of the
pancreas itself greatly interferes with the process; from which fact some
observers have concluded that the pancreas, in addition to its external
secretion into the intestine, must produce an internal secretion into the
blood which has something to do with the efficient absorption of the
fat (Pratt, McClure and Vincent'*8). It is, however, improbable that such a'n
hypothesis is necessary, for it is very likely that the moribund condi-
tion into which an animal is brought by extirpation of the pancreas,
adequately accounts for the suppression of the fat-absorbing function.
As to the relative roles of pancreatic juice and bile in the digestion of
fat, we know of course that in the pancreatic juice there exists a lipolytic
enzyme, lipase, which, under suitable conditions has the power of split-
ting neutral fat into fatty acids and glycerine. If bile is examined, no
lipolytic enzyme will be found in it. It is entirely inactive on fat, but
PAT METABOLISM 691
if we mix bile with fresh pancreatic juice, which by itself only slowly
digests fat, we shall find that the bile very materially increases the lipo-
lytic activity of the pancreatic juice. It has been found that the salts
of cholalic acid, the so-called bile salts, are the constituents of bile
that are responsible for this activation of lipase, this fact having been
demonstrated with bile salts prepared in such a way that there was no
possible chance of any other biliary constituent being present as an
impurity. It is important to remember, however, that lipase itself be-
comes slowly activated on standing, which explains why it should be
that bile added to pancreatic juice that has stood for some time, has a
less evident activating influence than bile added to fresh juice. It is
probable that the activating influence of bile salts is due to some physico-
chemical change induced in the digestion mixture.
One may ask how it happens that, when bile and pancreatic juice are
both absent from the intestine, the fat which appears in the feces is not
neutral fat but fatty acid. The reason is that the neutral fat that has
escaped digestion in the small intestine becomes acted on by the intestinal
bacteria, particularly in the large intestine. Under these conditions,
however, the fatty acid that is split off is not absorbed, because the
epithelium of the lower parts on the intestinal tract can not perform this
function.
Besides assisting the action of lipase, bile facilitates fat digestion in
other ways. Thus, by its containing alkali and mucin-like substances
it assists in the emulsification of fat. Although eiriulsification is no es-
sential part of fat absorption, yet it greatly facilitates the process by
breaking up the fat into small globules on which the lipase can act
much more efficiently. The alkali also combines with the fatty acids,
as they are liberated by the digestive process, to form water-soluble
soaps, which are readily absorbed by the epithelial cells. The bile salts
further assist in the solution of the fatty acids, and they lower the sur-
face tension of fluids in which they are contained and so bring the fat
and lipase into closer contact.
ABSORPTION OF FATS
After its digestion fat lies in contact with the intestinal border of the
epithelial cells as fatty acid and glycerine. The fatty acid is combined
either with alkali to form a water-soluble soap, or with bile salts to
form a compound, which is also soluble. The glycerine and the dissolved
fatty acids are separately absorbed into the epithelial cells of the in-
testine, in the protoplasm of which — after the fatty acid has been set
free from the alkali or bile salt — they become united or resynthesized
to form neutral fat, which gradually finds its way by the central lac-
692 METABOLISM
teals into the villi, and then by way of the lymphatics to the thoracic
duct.
The chemical explanation of the absorption of fat is very different from
that formerly held by histologists who maintained that the fine particles of
emulsified fat in the intestine penetrate by a mechanical process through
the striated border of the epithelial cell into its protoplasm. The histologic
evidence for this view seemed very convincing, for fine fat globules can
readily be seen in the epithelial cells of the intestine after fatty food
has been taken, while they are absent during starvation. These par-
ticles seemed to have passed directly from the intestinal canal into the
epithelial cells because, when the fat was stained with characteristic fat
stains before feeding it to the animal, the globules in the epithelial cells
were found to be similarly stained. The supporters of this mechanistic
view of fat absorption maintained that the appearance of the stained fat
globules in the epithelial cells could not be explained in any other way
than by supposing that the fat globules had wandered unbroken into
the epithelial cells. Such a conclusion is, however, unwarranted, for the
stains that are soluble in fat are also soluble in soap, so that when the
fat splits up, the stain will remain attached to the soap and be carried
along with it into the intestinal epithelium.
Absolute proof that the chemical theory is the correct one has been
supplied by a large number of experiments. The following may be
cited: (1) When the lymph flowing from the thoracic duct is examined
after feeding with fatty acids instead of neutral fat, it is found to contain
only neutral fat, indicating that a synthesis must have occurred between
glycerine and fatty acid during the absorption. The glycerine for this
synthesis is furnished from sources which will be described later. (2)
When an emulsion made partly of neutral fats and partly of some hy-
drocarbon, such as albolene, is fed and the feces are examined for these
substances, it has been found that all the fat but none of the hydrocar-
bon is absorbed; the feces contain all of the albolene but none of the fat.
This experiment supplies very strong evidence against the mechanistic
theory, for microscopic examination of the above described emulsion
shows the particles of neutral fat and hydrocarbon to be of exactly the
same size. (3) By examining the properties of the fatty substances in
the thoracic lymph collected during the absorption of such an emulsion
as that described above, nothing but neutral fat has been found present.
(4) Similar results are obtained when wool fat, which is an ester of
cholesterol and fatty acid, is fed.
We may conclude that fatty substances which are insoluble in water or
can not ~be changed by digestion into substances (soap) that are soluble
in water, are not absorbed, however like fat they may be in other particulars.
FAT METABOLISM 693
The chemical theory of fat absorption further explains why there should
be such large quantities of soapy substances in the intestinal contents,
and also why the globules of fat present in the epithelial cells of the
intestine are so very much smaller than those which lie on the surface of
the epithelium.
It might be objected to the conclusions just stated that, although unde-
tectable, there is really some essential physical difference between emul-
sified fat and emulsified hydrocarbon. In order entirely to prove the case
for the chemical theory, it is necessary to feed a neutral fat possessing
some characteristic that depends on the manner of union existing between
fatty acid and glycerine, and then to see whether it appears in an un-
changed condition in the thoracic duct. If it does so, the fat must have
been absorbed through the intestinal epithelium in an unbroken, unsapon-
ified condition, for it is unlikely that, in the resynthesis which occurs in
the intestinal epithelium, the fatty-acid molecules would recombine with
the glycerine molecules in exactly the same manner as before.
There are, however, but very few qualities of neutral fats, apart from
those of the fatty acids which compose them, by which they can be char-
acterized. The most likely one is that of optical activity. None of the
ordinary fats is optically active, although from chemical considerations
it is quite conceivable that some should be so. In order to obtain such a
fat Bloor49 conducted numerous experiments with the esters of stearic
acid.* In a series of experiments Bloor fed isomannid-dilaurate, a syn-
thetic fat of dextrorotatory power and as readily absorbed as natural fats,
and by examination of the neutral fat present in the chyle flowing from
the thoracic duct, found no evidence of the dextrorotatory fat. This result
confirms previous work by Frank, who found that the ethyl esters of
fatty acids are not absorbed unchanged. The results of both workers
emphasize the probability that readily saponifiable fatty-acid esters do
not escape saponification under the favorable conditions of the normal
intestine. In other words, had the fats been absorbed unchanged, as
would be required by the mechanistic theory of fat absorption, they
would have appeared in the chyle in optically active conditions.
These most important conclusions lead us to inquire as to the reason
for the change in fat during its absorption. It can not be for the purpose
of preventing the absorption of undesirable fatty substances, such as the
petroleum hydrocarbons or the wool fats, because such substances are
so rarely present in our food. It is most probable that the breakdown
*Bloor prepared an optically active mannitan distearate, but found it to have a very high melt-
ing point and to be only half as digestible as the ordinary fats. Its absorption was too slow and
unsatisfactory to make it suitable for the above purposes. He, therefore, proceeded to prepare the
di-ester of isomannitan with lauric acid, and he found the resulting compounds to be as well-ab-
sorbed as ordinary fat, and yet to possess very marked dextrorotatory power, which, of course,
they lose on saponification. This fat seemed suitable, therefore, for testing the above question.
694 METABOLISM
and resynthesis of neutral fat occurs for the same reason that similar
processes occur during the absorption and assimilation of protein. It
will be remembered that protein is entirely disintegrated in the intestine
into its so-called building stones. These are absorbed separately into
the blood, which carries them to the tissues, in which they become re-
synthesized to form the body protein. And so it appears to be in the
case of fats. The process, in other words, permits of the rearrangement
of fatty-acid molecules, as a result of which the newly formed fat is more
adaptable for use in the organism. It comes to be more like the char-
acteristic fat of the animal. There may be another reason for the proc-
ess. It will be remembered that lecithins, which constitute the most
important of the fatty substances of the cell itself, are mixed glycerides —
that is to say, are compounds containing a variety of fatty acids. The
rearrangement of the molecules of neutral fat. which occurs during ab-
sorption may be the first step in the transformation of fat into lecithin.
In order to throw further light on the question, Bloor has performed
a number of interesting experiments in which the chemical properties
of fats before and after absorption were compared. The criteria which
he took were melting point, iodine value, and mean molecular weight;
the melting point representing the solidity of the fat, and the iodine
value, its degree of unsaturation — that is, the number of double links in
the fatty-acid chain. It was found that during absorption very con-
siderable changes occur in these two characteristics; for example, when
fat with high melting point and low iodine value was fed, the fat in the
thoracic lymph was of distinctly lower melting point and higher iodine
value. "When fat with a low melting point and a high iodine value was
fed, the reverse change occurred, for the melting point of the thoracic
lymph fat was higher and the iodine value lower. These results could
be explained as due in the first case to the addition of oleic acid to the
fat during its synthesis in the intestinal epithelium, and in the second
case to the addition of some saturated fatty acid.
When a fat consisting mainly of glyceride and saturated fatty acid,
but with a low melting point, was fed, the addition of oleic acid was still
found to occur, as judged from the iodine value. This indicates that the
change is, not merely in order that the melting point of the absorbed fat
may be lowered, but also for some chemical reason. In a fourth series
of experiments, a lowering of iodine value occurred after feeding with
cod-liver oil, which contains a high percentage of glycerides of highly
unsaturated fatty acid.
Evidently, then, the intestine possesses the power of modifying the com-
position of fat during its absorption, and this modification is apparently
of such a nature that it causes a change toward the production of a
FAT METABOLISM 695
uniform chyle fat, presumably characteristic of the animal body. The
changes are probably greater than could be produced by admixture of
the absorbed fat present in the normal fasting chyle, but the source of
the oleic acid or of the saturated acid required for this synthesis is at
present unknown.
CHAPTER LXXVIII
FAT METABOLISM (Cont'd)
THE FAT OF BLOOD
Methods of Determination. — Normally the blood contains only a small
percentage of fat, but after a fatty meal it may contain so large an
amount that the fat actually rises to the surface of the blood like a cream.
By means of the ultramicroscope, examination of the blood in the dark
field after a fat-rich meal reveals the presence of glancing particles,
the so-called "fat dust." These particles are most abundant about six
hours after the meal has been taken, and they gradually disappear by
the twelfth hour. They do not appear after a meal when the thoracic
duct is ligated. They disappear when oxygen is bubbled through the
blood.
Fat dust has also been found abundantly present in the blood of em-
bryo guinea pigs at full time, but not in the mother's blood. This would
indicate that the placenta must have the power of taking the constitu-
ents of fat from the mother's blood and building them into fat, which
then passes into the blood of the fetus. The placenta under these condi-
tions acts like the mammary gland. In this connection it is of interest
that there is also much fat present in the blood of pregnant women. The
fat content of the placenta is, however, greater in the early stages of
pregnancy than later.
Although these facts have been known for some time, it has been
impossible, either on account of the large quantities of blood required
for a chemical examination or because of the difficulty in estimating
the amount of fat from the density of the ' ' fat dust, ' ' to follow with any
great degree of accuracy the exact chemical changes that take place in
the fat of the blood. Recently, however, Bloor has succeeded in elab-
orating methods by which the fat content of the blood can be determined
with satisfactory accuracy in small quantities of blood, so that a con-
tinuous series of observations can be made over a considerable period.
The fat is extracted from the blood by an alcohol-ether mixture with moderate heat.
An aliquot portion of the nitrate is evaporated in the presence of sodium ethylate, which
saponifies the fat. The residue, consisting of soap, is well washed and then treated
with hydrochloric acid so as to precipitate the fatty acid. The density of the precipitate
696
FAT METABOLISM 697
thus produced is compared in an optical apparatus, called a nephelometer, with a
standard solution of two milligrams of oleic acid treated in the same way. The fatty
acids in human blood are mainly oleic and palmitic.
The lecithin and cholesterol may also be estimated in the same blood extract. For
lecithin the above extract of blood, after the removal of the alcohol and ether, is digested
by heating with concentrated HNO3 and H2SO4. This decomposes the lecithin, liberating
the phosphorus, a solution of the resulting ash being rendered faintly alkaline to phenol-
phthalein and then slowly added to a silver nitrate solution. The density of the pre-
cipitate thus produced is compared in the nephelometer with that of a precipitate pro-
duced in the same amount of silver nitrate by adding to it a standard phosphoric acid
solution.
For cholesterol an aliquot portion of the above extract is saponified with sodium
ethylate and then saturated with chloroform ; the chloroform extract is mixed with acetic
anhydrid and H2SO4 (con.) until the bluish color is fully developed (Liebermann reac-
tion), the intensity of which is then compared in a colorimeter with that obtained by
similar treatment from a standard cholesterol solution.
Variations in Blood Fat. — In the dog the percentage of fat in the
blood is remarkably constant under normal conditions. After a fatty
meal the increase in fat begins in about an hour, and reaches its maxi-
mum in about six. The increase is not found in animals in which the
thoracic duct has been li gated. Although this result would seem to
contradict the view held by some that part of the fat which can not be
accounted for in the thoracic-duct lymph 'is absorbed by way of the
portal vein, it does not by itself disprove the hypothesis, for it has been
found that the fat content of the portal blood is always higher than that
of the jugular.
Very interesting results have been obtained following the intravenous
injection of emulsions of oil, either the so-called casein emulsion or col-
loidal suspensions. Up to a dose of 0.4 gram per kilogram of body
weight — which by calculation would suffice to raise the fat content of
the blood by 100 per cent — there was no increase in fat content. In or-
der to explain this disappearance of fat, it might be imagined that the
injected fat particles formed emboli in the smaller Capillaries. Against
such a view, however, is the fact that the particles of fat in these emul-
sions are one-half to one-seventh the size of a red corpuscle. Although
this argument is no doubt of some weight, it should be remembered
that the physical condition of these fine fat globules is not the same as
that of the red blood corpuscle. Their surface condition may be such
that they readily agglutinate so as to form small masses, which may
stick at the branching of the smaller arterioles and capillaries. Bloor
himself suggests that the injected fat may be stored, possibly in the liver,
since the fat in this organ, as we shall see later, increases under similar
conditions. When twice the above quantity was fed in the form of egg-
698 METABOLISM
yolk fat, some of it persisted in the blood for several hours. This in-
crease may have been owing to the flooding of the temporary storehouse
with fat, or, more probably, to a retarding influence that lecithin may
have on fat assimilation, for lecithin itself persists in the blood for a
long time after intravenous injection.
During fasting, no increase in blood fat was found unless the animal,
by special feeding, had been stuffed with excess of fat prior to the fast-
ing period. The lipemia in this case indicates that fat is being trans-
ported from one place to another to serve as fuel for the starving tissues.
Narcotics were found to produce an increase in blood fat. Ether pro-
duced this increase during the narcosis, whereas morphine and chloro-
form did not do so until after recovery. The explanation given for the
ether effect is that a mixture of blood and ether has higher solvent power
for fat than blood alone. The explanation for the chloroform and mor-
phine effects is that a certain amount of breakdown of the tissue cells,
in which lipins are set free, supervenes upon the action of these narcotics.
The blood fat also becomes enormously increased in about forty hours
after the administration of phlprhizin, and on the second or third day
after the administration of phosphorus. The special significance of
these facts we shall consider .later in connection with the relationship of
the liver to fat metabolism.
By comparison of the fatty acid, lecithin, and cholesterol contents of
blood during fat absorption, it has been found that there is a steady but
very variable increase in fatty acid, accompanied by no variation in
cholesterol, but with an increase in lecithin, which varies from 10 to 35
per cent, but does not run strictly parallel with the fatty-acid increase.
It is probable that this increase in lecithin represents that part of the
absorbed fat which is intended for immediate use in the tissues (page
705). The more or less independent increase in lecithin is of significance
in connection with the fact that in many pathologic conditions of so-
called lipemia the increase does not affect the fats of the blood but rather
the lipoids (i.e., lecithin and cholesterol). Separate analyses of blood
plasma and whole blood show the increase of lecithin to be much more
marked in the corpuscles than in the plasma, whereas the fatty-acid
increase is -confined to the plasma.
To illustrate some of these points the following table will be of value.
In it is shown the average distribution of fatty acid, lecithin and choles-
terol in' normal individuals and in cases of diabetes, in which disease,
as has been known for long, there is marked disturbance of fat metab-
olism.
FAT METABOLISM 699
BLOOD LIPOIDS IN NORMAL AND IN DIABETIC PERSONS
NORMAL
PER CENT
MILD
DIABETES
PER CENT
MODERATE
DIABETES
PER CENT
SEVERE
DIABETES
PER CENT
Fat by Bloor's |
Method |
Whole Blood
Plasma
0.59
0.62
0.83
0.90
0.91
1.06
1.41
1.80
Total Fatty AcidsJ
Whole Blood
Plasma
Corpuscles
0.37
0.39
0.34
0.59
0.64
0.45
0.65
0.75
0.48
1.01
1.28
0.62
Lecithin
Whole Blood
Plasma
Corpuscles
0.30
0.21
0.42
0.32
0.24
0.42
0.33
0.28
0.40
0.40
0.40
0.40
Cholesterol J
Whole Blood
Plasma
Corpuscles
0.22
0.23
0.20
0.24
0.26
0.21
0.26
0.30
0.20
0.41
0.51
0.24
Glycerides |
Plasma
Corpuscles
0.10
0
0.38
0.18
0.46
0.23
0.84
0.38
Total Lipoids
Plasma
0.68
0.98
1.16
1.98
It will be observed that there is about 0.7 per cent of total fatty sub-
stances in normal blood. The fatty acids (palmitic and oleic) amount to
about 0.4 per cent, and are equally distributed between plasma and
corpuscles; the lecithin, about 0.3 per cent, being twice as abundant in
corpuscles as in plasma, and the cholesterol, 0.2 per cent, about equally
distributed. In diabetes all of these substances are seen to be increased
in proportion to the severity of the disease, the increase being mostly
in the plasma. The increase in cholesterol (confined mainly to the
plasma) is particularly interesting, since the substance is unaffected in
amount by excessive feeding with fat.
The Destination of the Fat of the Blood. — In general, it may be said
that the blood fat is transported to three places: (1) the depots for fat; (2)
the liver; and (3) the tissues. The fat present in each of these places
differs from that in the others, as is revealed by chemical examination
by the methods described on page 687. The depot fat usually yields about
95 per cent of its total weight as fatty acid. The tissue fat, on the other
hand, yields only about 60 per cent of its total weight as fatty acid.
This difference indicates that the fatty acid must be combined in the
tissues with a much larger molecule than is the case in the fat of the
depots. This large molecule is probably that of lecithin or other phos-
pholipin, and the smaller molecule in the depots, that of neutral fat.
The liver fat takes an intermediate position between depot fat and tissue
fat in its yield of fatty acid. When no active metabolism of fat is go-
ing on, the liver fat is like that of the tissues ; but when fat metabolism
is active, the liver fat occupies an intermediate position between liver
fat and depot fat.
700 METABOLISM
Another difference among the fats in these three places is Avith regard
to the degree of saturation of the fatty-acid radicles. This, it will be
remembered, is indicated by the iodine value; the higher the iodine
value, the greater the desaturation of fatty acid. In depot fat this value
is relatively low — for example, about 30 in the goat and about 65 in man ;
depending somewhat on the fat taken in the food, compared with which
it is usually a little higher. The fat in the tissues, on the other hand,
has a high iodine value, possibly 110 to 130. The iodine value of the
fat of the liver is remarkably inconstant, being about the same as that
of the tissues when fatty-acid metabolism is not particularly active, but
approximating that of the depots when fat mobilization is proceeding.
The significance of this fact we shall consider later.
The Depot Fat. — The places in the animal body where depot fat is
deposited in greatest amount are the subcutaneous and retropsritoneal
tissues. These fat depots may sometimes become of enormous size, as
in the case of the famous dog of Pfliiger, of whose total body weight
40 per cent was due to fat. Bloor suggests that there may really be two
different types of fat storage: one of a purely temporary character,
which readily takes up and liberates the fat, but which is of limited
capacity and possibly under the control of some quickly acting regulat-
ing mechanism, like that of the glycogenic function of the liver; and
another of a more permanent nature, into which the fat is slowly taken
up, but the capacity of which is very much greater.
TAVO questions present themselves concerning this depot fat: (1) Avhere
does it come from, and (2) what becomes of it? Regarding the source
of tlie depot fat, there is no doubt that it comes partly from the fat and
partly from the carbohydrate of the food; in other words, it is either
taken ready-made with the food or manufactured in the organism. That
some of it comes from the fat of food is now a Avell-established fact, the
evidence for AArhich need not detain us long. The best-knOAvn experiment
consists in first of all starving an animal until his stores of fat are
nearly exhausted and then feeding him Avith some "ear-marked" fat —
that is, Avith some fat having a characteristic property AA^hich it Avill
not lose during absorption. It Avill be found that the depot fat thereby
deposited presents many of the qualities of the fed fat. The "ear-
marking" of the fat may be secured by using fats of different melting
points, such as mutton fat, Avhich has a high M.P., or olive oil, which has
a low M.P. On feeding a previously starved dog with mutton fat, the
M.P. of the depot fat approaches that of mutton fat — he becomes a
dog in sheep's clothing; Avhereas when olive oil is fed, the subcutaneous
fat becomes oily. Or again Ave may "ear-mark" the fat by combining it
Avith bromine, when the deposited fat will likeAvise be brominized fat.
FAT METABOLISM 701
It must not be imagined, however, that no change takes place in the
fat during its absorption and before it becomes deposited in the tissues.
-On the contrary, the stamp of individuality is put upon the fat, for, as
we have already seen, its iodine value may become altered and its melt-
ing point changed during the process of absorption. In other words,
although the absorbed fat does not become entirely adapted to conform
with the ordinary qualities of the depot fat, yet it tends to change in
this direction.
That some of the depot fat comes from carbohydrate is wrell known to
stock raisers. When, for example, an animal is fed on large quantities
of carbohydrate and kept without doing muscular exercise, its tissues
become loaded with fat. If we desire strict scientific proof for this, we
do not need to go further than the old experiments of Lawes and Gil-
bert, who, it will be remembered, showed that the fat deposited in the
tissues of a growing pig is greatly in excess of the fat that could have
been derived from the fat or protein which was meanwhile metabolized.
The experiment was performed on two young pigs from the same litter
and of approximately equal weight ; one was killed and the exact amounts
of fat and nitrogen in the body determined; the other was fed for several
months on a diet the fat and protein contents of which were accurately
ascertained. When after four months this pig was killed and the fat
determined, it was found that much more had become deposited than
could be accounted for by the fat and protein of the food, even suppos-
ing that all the available carbon of the protein had become converted
into fat. The only conclusion is that the carbohydrate must have been
an important source of the extra fat.
The Destination of the Depot Fat. — The depot fat becomes mobilized
and transported by the blood to the active tissues whenever the energy
requirements of the body demand it. During starvation, as we have
seen, the depot fat is used to supply 90 per cent of the energy on which
the animal maintains its existence. Before the fat is transported, it is
probably broken down into fatty acid and glycerine, as which it passes
through the cell walls to be again reconstructed into neutral fat in the
blood. What agency effects this constant breakdown and resynthesis
of fat it is difficult to say. Two ester-splitting enzymes are present in
blood, one acting mainly on simple esters, the other on glycerides; but
it has been impossible to demonstrate any evident relationship between
either of them and the extent of fat mobilization.
The Fat in the Liver. — The physiology of the liver fat has been very
diligently studied, particularly by Leathes and his pupils.50 The out-
come of this work has been to show that the liver occupies an extremely
important position in the metabolism of fat, being, as it were, the half-
702 METABOLISM
way house in the preparation of the fatty-acid molecule for consumption
in the tissues. Fat is a material containing large quantities of poten-
tial energy. While in the depots this potential energy is so locked away
as to be unavailable for tissue use. To make it available the depot fat
is carried to the liver, where the energy becomes unlocked but not actu-
ally liberated. The fat is then transported to the tissues, and the libera-
tion of the energy occurs. Neutral fat is like wet gunpowder: it con-
tains much potential energy, but not in a suitable condition for explo-
sion. The liver, as it were, dries this gunpowder, whence it is sent to
the tissues to be exploded.
The great importance of the liver in fat metabolism is indicated by
comparison of the percentages of fat — or better of fatty acid — contained
in it under different conditions of nutrition. In the ordinary run of
slaughter-house animals the liver contains from 2 to 4 per cent of higher
fatty acid, but in about one in every eight animals a much higher per-
centage will be found to occur. The same is true in laboratory animals.
In the case of the human liver as obtained' from autopsies in certain
classes of patients, from 60 to 70 per cent of the dry weight- of the
organ, or 23 per cent of the moist weight, may be fatty acid. There is
no other organ in the animal body that is ever loaded with fat to this
extent. As in the depots, this liver fat might be derived either from fat
carried to the organ from elsewhere in the body, or it might represent
a surplus of manufactured fat.
That transportation of fat to the liver occurs is very readily demon-
strable both in the laboratory and in the clinic. About forty hours
after giving phlorhizin to a dog, it has been found that enormous quan-
tities of fat appear in the liver; a few hours later, hoAvever, this excess
of fat may have entirely disappeared. Fatty infiltration of the liver
is also observed in phosphorus poisoning, although in this case the fat
usually persists till the death of the animal. In man, in delayed chlo-
roform poisoning and in cyclical vomiting, enormous quantities of fat
may be present in the liver within a very short period of time after the
onset of the condition. There can therefore be no doubt that fat is
transported to the liver under abnormal conditions, but this can not
be taken as evidence that the liver has anything to do with fat metab-
olism in the normal animal. Such evidence has been supplied by Coope
and Mottram,51 who have been able to show that, at least in rabbits, a
similar invasion of the liver with fat occurs in late pregnancy and early
lactation. They also found that the fatty acid deposited in the liver
in late pregnancy gives an iodine value which lies nearer to that of the
mesenteric fatty acid than is the case in normal animals. Mottram con-
cludes that "wherever . . . there is abundant fat metabolism, the
FAT METABOLISM
703
liver is found to be infiltrated with fats, presumably to be handed on
elsewhere when worked up." It is interesting that the fetus is greedy
of unsaturated fatty acids.
The most likely source of the fat transported to the liver is the fat pres-
ent in the depots, unless when digestion is in progress, when it may be
the fat from the intestine. That much of it comes from the depots is
easily demonstrated. Thus, the more extensive the infiltration of the
liver with fat, the more closely will this fat be found to agree with the
depot fat in its chemical characteristics. This has been very clearly
shown by, first of all, starving an animal so as to clear the depots of fat
as much as possible; then feeding it on some "ear-marked" fat (unusual
melting-point or a brominized fat) ; and after another day or so of
starvation, so as to clear the liver- itself of fat, poisoning the animal
with phosphorus or phlorhizin. The liver will be found shortly after-
wards to be invaded with fat which has all the ear-marks of that on
which the animal had been fed.
Evidence of the same character has been furnished in a series of clin-
ical cases by observations on the amount of fat and the iodine value of
the fatty acid of the liver. This is shown in the accompanying table.
FATTY ACIDS OF LIVEK
CAUSE OF DEATH
HIGHER FATTY
ACIDS PER CENT
OF DRY WEIGHT
IODINE VALUE
OF FATTY ACIDS
1. Pernicious anemia
12.1
116.8
Normal
2. Lobar pneumonia
3. Pernicious anemia
13.7
14.25
116.8
116.0
figures
4. Diabetes
14.4
119.6
5. Toxemic jaundice
15.6
109.5
Commencing
6. Accident
17.2
103.5
fatty
7. Empyema
21.5
96.0
change
8. Phthisis
25.4
96.4
9. Broncho-pneumonia
38.4
84.9
10. Appendicitis
44.0
91.1
Marked
11. Carcinoma of bladder
47.2
77.8
fatty
12. Broncho-pneumonia
54.6
71.8
change
1?>. Ulcerative colitis
60.9
80.3
14. Accident
66.3
63.0
15. Dysentery
73.5
69.1
This table clearly shows that the more fat there is in the liver, the
nearer this fat approaches in character that stored in the depots.
That some of the fat in the liver may come directly from the fat re-
cently absorbed from the intestine is also very readily demonstrable.
Thus, when cocoanut oil was placed in the intestine of anesthetized an-
imals, along with bile salts and glycerine, it was found by Raper52 that
30 per cent of the absorbed oil appeared in the liver.
704 METABOLISM
The characteristic feature of cocoanut o.il is that its fatty acids are volatile in steam
and are saturated. Some of the fatty acids of the liver are volatile in steam, but they
are unsaturated. By distillation in steam of the fatty acids obtained by saponification
of the liver, it is possible to determine how much of the cocoanut oil has passed to the
liver.
Similar results have been obtained when unsaturated fatty acids, such
as those contained in cod-liver oil, are fed. In all these cases the rela-
tionship of the liver fat to that of the food is even more evident than
that between food fat and depot fat, because in the liver the newly absorbed
fat is not diluted by that deposited it may be months previously, as is
the case in the connective tissues.
The question now arises: What happens to the fat during its stay in
the liver? An indication of the nature of the change is furnished by
observing the iodine value of the fat. This, it will be remembered, in-
dicates the degree to which the fatty acid is unsaturated. It does not
necessarily indicate the number of unsaturated bonds present in the fatty-
acid molecule, for the difference in iodine-absorbing power may depend
not on the number of such bonds but on the position in the chain at
which a given double bond is inserted. Even with this reservation, how-
ever, it is evident that the increase observed in the iodine values shows
that the liver has the power of desaturating fat. The advantage of
this change depends on the fact that the desaturated fatty acid will
be more liable to break up than the saturated fatty acid. In other words,
the double linkage" will weaken the chain with the consequence that it is
liable to fall apart at this place; such at least is the natural interpreta-
tion which the chemist would put on the result. It may not, however,
be the correct interpretation, for it has been shown that, although un-
saturated fatty acids are more susceptible to chemical change in the
laboratory than saturated, yet when fed to animals they appear to be
more stable than many saturated acids. It may then be wrong to con-
clude that the introduction of a double linkage in fat necessarily means
the liability of the fatty-acid chain to break at that point. However
this may be, it seems likely that one function of the liver consists in
introducing double linkages at places in the fatty-acid chain, as a result
of which the chain breaks at these places, and the fragments then undergo
further oxidation.
Double linkages may be introduced not only in one place in a fatty-
acid chain, but in several. For example, it has been found in the liver
of the pig, after oxidizing the fatty acids with permanganate, that oxida-
tion products are obtained indicating the existence of unsaturated acid
with four double links. Permanganate (in alkaline solution) is used for
detecting the position of these double bonds, because, when it is allowed
FAT METABOLISM 705
to act on unsaturated fatty acids in the cold, it causes hydroxyl groups to
be introduced in the position of the double bonds. When .the oxidation is
performed at a moderate temperature, the fatty acid falls apart at the
hydroxyl groups. A fatty acid with eight hydroxyl groups has been
obtained in this way from the liver of the pig. The presence of the hy-
droxyl groups has been confirmed by finding that an octobromide is ob-
tained by treatment with bromine. An acid of the same formula is said to
be present in cod-liver oil. To sum up, we may conclude that there are
certain positions, in the chains of carbon atoms which constitute the fatty-
acid radicle, where the liver introduces double bonds, and that the weak-
ened radicles then circulate to the tissues, where they break up at those
positions.
But this is probably not the only way in which the liver assists in
the metabolism of fat. It may also take part in the building of fatty-
acid radicles into the complex molecule of lecithin. The process of de-
saturation that we have just considered is probably a preliminary step
to this incorporation of the fatty-acid molecule into lecithin, for it is
well known that lecithin contains highly unsaturated fatty-acid radi-
cles. In support of such a view it is interesting to note that in alcohol-
ether extracts from normal and pathologic livers, the lecithins, which are
precipitated by acetone, have higher iodine values (i. e., are more unsat-
urated) than the neutral fats extracted from the same liver, which also
have higher iodine values than the depot fat of the same animal. The
desaturation process must, therefore, involve the fatty acids before these
become built into the lecithin molecule.
The liver is probably not the only place in the animal body where the
desaturation of fatty acids is brought about. The relative activity of
the different tissues in this regard has been studied by feeding cats
with fatty fish and then determining the iodine value of fat from various
places in the body. The absorbed fat was more obvious in the liver than
in the subcutaneous tissues, because it had not become diluted with fat
deposited it may have been months previously, which would be the
case in the fat of the fat depots; and it was found that, although the
iodine value of the subcutaneous fat was slightly raised, that of the
liver was much more so, indicating that the desaturation process had
been more active in this organ, but had also occurred to a certain extent
in the depots.
Before leaving this subject of fat in the liver, it is important to re-
call the old observation of Rosenthal, that a more or less reciprocal
relationship exists between glycogen and fat in the liver. When much
glycogen is present there is little or no fat, and vice versa. It is impor-
706 METABOLISM
taut to note that the exact locations of fat and carbohydrate in the he-
patic lobule are somewhat different in the two cases.
A practical clinical application of the above work is found; in the fact
that fats will be more readily utilized by the body when they contain a
high percentage of unsaturated fatty acids. It is probably for this
reason that Norwegian cod-liver oil is of such undoubted nutritive value.
It is much more so than Newfoundland cod-liver oil, because in the prep-
aration of this variety oxidation occurs, which makes it no longer unsat-
urated. Fish oils in general are more unsaturated than other animal
oils, and are for this reason more nutritious.
The fat in the tissues differs very materially from that of the liver or
the depots. Only 60 per cent of this fat consists of fatty acid, which is
present very largely as part of the lecithin molecule, thus accounting for
the high iodine value. Some is probably also present as simple glyceride,
in a highly unsaturated and therefore very fragile condition.
CHAPTER LXXIX
FAT METABOLISM (Cont'd)
Two very important questions of fatty-acid metabolism may now be
considered: namely, (1) how is fatty acid formed from carbohydrate?
and (2) what becomes of the fragments into which the fatty-acid molecule
is split as the result of the desaturation process? Although these prob-
lems involve chemical details of a somewhat complex nature, we must
not on this account fail to consider them; for, as we shall see, much of
what is known has an important practical application depending on the
fact that certain of the intermediary substances may accumulate in the
organism and develop a toxic action.
The Production of Fatty Acid out of Carbohydrate.— If we place the
formulas for glucose and palmitic acid side by side, thus:
CH2OH-(CHOH)<-CHO (glucose), and
CH3-(CIL,)14-COOH (palmitic acid);
we shall see that this transformation must involve: (1) a considerable
alteration in the structure of the molecule, (2) the removal of oxygen,
and (3) the fusion of several glucose molecules into one molecule of fatty
acid.
The conversion of carbohydrate to fat therefore involves a process of
reduction, and the resulting molecule must be capable of yielding more
energy when it is oxidized than the original one of carbohydrate, for
obviously the system 02 - CH2 (which corresponds to fat) will develop
more energy than that of 02 - CHO (which corresponds to carbohydrate) ;
just as a piece of wood when it is burned Avill develop more heat than a
piece of charcoal. This explains why one gram of fat yields 9.3 calories
of heat, and one gram of carbohydrate, only 4.1 (page 535). Fatty
acid therefore contains more potential energy than sugar, and in explain-
ing its synthesis from sugar in the animal body we must indicate the
source of the extra energy. This is dependent on oxidation of some sugar
molecules — which do not themselves become changed to fatty acid —
proceeding side by side with the reduction which affects the others and
represented in the outcome of the reaction by the combustion products
C02 and H,0, thus:
6C6H1206 + 13 02 = 20 C02 + C16H3202 + 20 H20.
(glucose) (fatty acid)
707
708 METABOLISM
What evidence have we that such a process actually occurs in the body?
If we compare the intake of oxygen with the output of carbon dioxide
in the respired air, we shall find that usually there is less of the latter;
that is to say, the respiratory quotient, as this ratio is called, is usually
less than unity. During the extensive conversion of carbohydrate into
fat, however, which occurs during the fall months in hibernating animals,
the K.Q. has been found to rise as high as 1.4. The great excess of
C02 - output over 02 - intake which such a quotient indicates conforms
with the above equation.
The entire dissimilarity in chemical structure between the molecules
of fat and carbohydrate suggests that the primary step in the conversion
must be a thorough breakdown of the carbohydrate chain into compara-
tively simple molecules, from which the fat molecules are then recon-
structed and the unnecessary oxygen set free. The problem is to ascer-
tain the chemical structure of these simpler molecules and the manner
of their union into fatty acid.
Of the several suggestions which have been made, that of Smedleyss seems the most
likely. As will be seen from the following equations, the first step is the conversion of
glucose to pyruvic acid (page 600, No. 1 in equations). By enzymie action pyruvic
acid is converted into acetaldehyde (No. 2), which then condenses with another pyruvic-
acid molecule to form a higher ketonic acid (No. 3), from which by the loss of (XX,
as in the case of the production of • acetaldehyde from pyruvic acid, an aldehyde is pro-
duced (No. 4). This higher aldehyde behaves like acetaldehyde in again combining with
pyruvic acid, forming a still higher ketonic acid; and so on until at last a long fatty-
acid chain is built up, thus :
( 1 ) C^H^O,, + O, = 2CH3COCOOH + 2H2O
( glucose ) ( pyruvic acid )
( 2 ) CH3COCOOH — CH3CHO + CO2
(acetaldehyde)
( 3 ) CII3CHO + CH3COCOOH — CH3CH : CHCOCOOH -f H2O
(unsaturated ketonic acid)
(4) CH3CH : CHCOCOOH i= CH^CH : CHCHO + CO2 ; and so on.
(higher aldehyde)
(5) From the ketonic aldehyde formed at any stage, an unsaturated fatty acid (with
one less C-atom) is readily formed, and this by taking up H may become saturated:
CH3CH:CH CO COOH + O = CH3 CH:CH COOH + CO,.
During the butyric-acid fermentation of sugar a slightly different process may occur —
namely, the lactic acid, which we know is readily produced from glucose, may break down
into acetaldehyde (and formic acid), and two such molecules condense to form /3-oxy-
butyric aldehyde; and this again condense to form higher fatty acids, thus:
( 1 ) C0H12O6 r= 2CH3CHOHCOOH;
(glucose) (lactic acid)
( 2 ) 2CH3CHOHCOOH = 2CH3CHO + H.COOH
(acetaldehyde)
(3) 2CH3CHO = CH3CHOHCH2CHO ; and so on.
(/3-oxybutyric aldehyde)
FAT METABOLISM 709
That higher fatty acids, such as caproic (C6H12O2) and caprylic (C8H16O2), have
actually been isolated from the products of this fermentation is a very significant fact,
and it is of interest to note that Leathes has sometimes found an increase in higher fatty
acids to occur during the aseptic incubation of liver pulp. Unfortunately, however, the
increase of fatty acid could not be shown to be affected by adding substances to the
liver which, according to the above equations, should yield fatty acid.
The Method by Which the Fatty Acid is Broken Down. — In the chemi-
cal laboratory, ordinary oxidizing agents attack the fatty-acid chain at the
C-atom next the carboxyl (COOH) group (the alpha C-atom). But
this can not occur in the animal body, because it would leave behind a
smaller chain containing an uneven number of C-atoms, and such chains
are never found present in the animal fats. On the contrary, the com-
moner fats all contain an even number of C-atoms; thus : Butyric, C4H802 ;
palmitic, C1CH3202; stearic, C18H3(.02; oleic, C18H3402.
The intermediary substances which are produced during the gradual
breakdown of the fatty-acid molecule in the normal animal are of a very
transitory character so much so indeed that it is impossible for any one
of them to accumulate in sufficient amount to permit of isolation, or even
detection, by chemical means. How then are we to identify the inter-
mediary products? This has been rendered possible by the discovery that,
when anything occurs to disturb the normal course of fat metabolism, as,
for example, when the tissues are deprived of carbohydrates (as in star-
vation or in severe diabetes), the oxidation of the fatty-acid chain stops
short when a chain of four C-atoms still remains unbroken. These last
four C-atoms seem to form a residue that is more resistant to oxidation
than the remainder of the fatty-acid molecule. It is a residue, therefore,
which is quite readily further oxidized to C02 and H20 under normal con-
ditions, but which, although incapable of becoming completely oxidized
Avhen the metabolism is upset, does undergo a partial oxidation, result-
ing in the production of various intermediary products. These accumu-
late in the body in sufficient amount to overflow into the urine, from
which they can be isolated and identified.
The fatty acid with 4 C-atoms is 'butyric, CH3CH2CH2COOH, and the
first oxidation product formed from it in the body seems to be fi-oxybuiy-
ric acid, CH3CHOHCH2COOH. This then becomes oxidized to form a
body having the formula CH3COCH2COOH, acetoacetic acid, which, on
further oxidation, readily yields CH3COCH3, or acetone. These sub-
stances (/?-oxybutyric acid, acetoacetic acid and acetone) appear in the
urine during carbohydrate starvation, as in diabetes.
It might be objected, however, that a chemical process occurring under
abnormal conditions need not also occur in the normal animal. That it
probably does, hoAvever, is indicated by the results of the experiments
710 METABOLISM
of Knoop and of Embden and his coworkers. Knoop conceived the idea
of introducing into the fatty-acid molecule some group which is resistant
to oxidation in the body. The phenyl group (C6H5) was found to have
this effect. By feeding an animal with the phenyl derivatives of acetic,
propionic, butyric, and valeric acids, it was found that the urine con-
tained either hippuric (see page 630) or phenaceturic acid. Both of
these are compounds of aromatic acids with glycocoll or aminoacetic
acid (CH2NH2C'OOH), one of the protein building-stones and always
available in the organism to form such compounds, thus:
(1) C8H5COOH + CH2NILCOOHz=C6H/X)NHCH2 COOH.
(benzoic (glycocoll) (hippuric acid)
acid)
(2) C6H6CH2COOH + CHJSTH^COOH = C6H5CH2CONHCH2COOH.
(phenylacetic (glycocoll) (phenaceturic acid)
acid)
When either benzoic acid (C6H5COOH) or phenylacetic acid (CGH5CH2-
COOH) is formed in the body as a result of the oxidation of phenyl
derivatives of the higher fatty acids, the acid combines with glycocoll
according to the above equations. From this it folloAvs that if oxidation
occurs so that two C-atoms are thrown off at a time (/^-oxidation), fatty
acids with an even C-atom chain should yield hippuric acid, and those
with an uneven chain, phenaceturic. This was found to be the case, as
the accompanying table shows.
ACID FED
OXIDATION
PRODUCT
EXCRETED AS
Benzoie acid, CCH5.COOH
Phonylacotic acid, C6II5 . CH; . COOH «
Not oxidized
Not oxidized
Hippuric acid
Phenaceturic
acid
Phenylwopioiiic acid, C8H, . CH, . CH, . COOH C6H5.COOH Hippuric acid
Phenylbutyric acid, CeH5 . CH, . CH2 . CH2 . COOH C6H3 . CH2 . COOH Phenaceturic
acid
Phenyl valeric acid, C6H5 . CH2 . CH2 . CH . CH2 . COOH C6HC . COOH Hippuric acid
(From Dakin.)
Embden 's experiments are equally convincing. He studied the forma-
tion of acetone in defibrinated blood perfused through the freshly excised
liver. Normally only a trace of this substance is formed, but when fatty
acids with an even number of carbon atoms were added to the blood,
they gave rise to a marked increase in acetone, whereas those with an
uneven chain failed to cause any change. The acetone was found to be
derived immediately from acetoacetic acid. The following table shows
the results.
FAT METABOLISM 711
NORMAL FATTY ACID
FORMATION OF
ACETOACETIC ACID
Acetic acid
Propionic acid
Butyric acid
Valeric acid
Caproic acid
Heptylic acid
Octoic acid
Nonoic acid
Decoic acid
CH3.COOH
CH3.CH2.COOH
CH3.CH2.CH2.COOH
CH3 . CH2 . CH2 . CH2 . COOH
CH3 . OH2 . CH2 . CH2 . CH, . COOH
CH3 . OH, . CH2 . CH2 . CH2 . OH., . COOH
CH3 . CH"2 . CH2 . CH2 . CH2 . CH", . CH2 . COOH
CH3 . CH2 . CH2 . CH2 . CH2 . CH, . CH2 . CH2 . COOH
CH,.CH2.CH,.CH,.CH2.CHj.CH,.CH2.CH,.CH2.
+
+
-f
COOH +
(From Dakin.)
For a long time it was difficult for chemists to understand how such
a process of oxidation at the /?-C-atom could occur, since they were
unable to bring it about in the laboratory by the use of the ordinary
oxidizing agents, but recently Dakin has removed the difficulty by show-
ing that hydrogen peroxide (H202) oxidizes fatty acids just exactly in
this way.
We may sum up the results of these experiments and observations by
stating that normal saturated fatty acids and their phenyl derivatives can
undergo oxidation, not only in the animal body, but also in vitro, in such
a manner that the two (or some multiple thereof} termial G -atoms are
removed at each successive step in their decomposition.
But we must not be too hasty in concluding from these experiments that
the steps in the process are necessarily in the order of first, the produc-
tion of a /3-hydroxy acid, and second, the oxidation of this to a ketone
group. The mere presence, side by side, of /?-hydroxybutyric acid and of
acetone in the' above experiments does not indicate which is the ante-
cedent of the other,' and indeed there are several experimental facts that
seem to show that the hydroxy acid may be derived from the ketone.
For example, when acetoacetic acid is added to minced liver and the
mixture incubated, /?-hydroxybutyric acid is formed (a reduction process),
although lees usually the reverse action (oxidation) may occur when
/?-hydroxy acid is added. A reversible reaction must therefore be capable
of occurring between these two substances, thus:
reduction
CH3.CHOH.CH2.COOH < CH, . CO . CH, . COOH.
oxidation
(/3-oxy butyric acid) — • • '•> (acetoacetic acid)
We know practically nothing as to the conditions determining whether
oxidation or reduction shall predominate, but there are two significant
facts that one should bear in mind: (1) that a plentiful supply of oxy-
gen is necessary for the oxidative process, and (2) that the presence of
readily oxidizable material in the liver (e.g., carbohydrates) may deter-
mine the direction which the reaction shall take. It is commonly said
that fats burn in the fire of carbohydrates, and it may be that the un-
712 METABOLISM
doubted diminution in acidosis which occurs in diabetes when carbo-
hydrate food is given is dependent upon the directive influence which its
combustion in the liver has on the above processes. But we must be
cautious not to transfer results obtained by experiments with minced
liver in judging of the reactions Avhich occur during life. Provisionally,
then, we must assume either that /Miydroxybutyric acid is a necessary
stage in the oxidation of butyric acid or that it is formed by reduction
of acetoacetic acid, which is really the first step in that process.
Of course there is no evidence in the above experiments that the higher
fatty acids are also broken down by the removal of two C-atoms at a
time, nor has it been possible to detect any ketonic or /3-hydroxy deriv-
atives of them in the animal body. We can only reason from analogy
that a similar process may occur, although some support is furnished
for such a view by the fact that ketonic fatty acids have been found in
vegetable organisms.
What, then, it may be asked, is the relation of the desaturation of fatty
acids which we have seen occurs in the liver (and probably elsewhere) to
the ft oxidation? There can be no doubt that both processes can occur
in the animal body, indeed in the same organ, e.g., the liver; and it is
important to ascertain their relationship to each other. The conclusion
Avhich would seem to conform best with the known facts is that the
desaturation process occurs (in the liver) so as to break up the long
fatty-acid chain into smaller chains, which are then capable of /? oxida-
tion (in the tissues) ; desaturation may be the process by which the mole-
cule is rough hewn, and /? oxidation that by which the resulting pieces
are finally split to their smallest pieces — that is, to molecules of the size
of acetic acid, which are finally completely burnt to carbonic acid and
water.
The increase of iodine value observed by Leathes and his coworkers need not, as has
already been pointed out, necessarily indicate that new double linkages have been intro-
duced in the fatty-acid chain ; it may merely indicate that structurally isomeric deriva-
tives which absorb iodine more readily have been formed. Direct evidence of desatura-
tion has, however, been offered by Hartley, who isolated the unsaturated fatty acids (by
dissolving the lead soaps in ether) from pig's liver and then proceeded to oxidize them
with alkaline permanganate. When the olein of the depot fat is thus treated at a low
temperature, two hydroxyl groups become attached where the double linkage existed
(forming dioxystearic acid), and when the mixture is now warmed, the molecule splits
into two at this place, forming two lower acids (pelargonic and azelaic) :
(1) CH,-(CH,),CH:CH(CH2).COOH;
(oleic acid)
(2) CH,-(CH2)I-CH
OH OH
CH (CH2)7COOH;
(dioxystearic acid)
(3) CH, (CHS)7COOH + COOH-(CH2)7COOH.
(pelargonic acid) (azelaic acid)
FAT METABOLISM 713
We may conclude from this that the double linkage in the oleic acid of the depot fat
exists between the ninth and tenth C-atoms. But it is otherwise in the case of the un-
saturated acid from the liver (pig's), for under the above process of oxidation this
yielded caproic acid, which, since this acid has six C-atoms, would indicate that the
double linkage existed between the sixth and seventh C-atoms. Another interesting fact
brought to light by the experiments was that a tetraoxystearic acid was formed, which
fell apart in such a way as to indicate that the hydroxyl groups occurred between the sixth
and seventh and between the ninth and tenth C-atoms. The occurrence of this substance
would be satisfactorily explained by the introduction into the molecule of oleic acid of a
second double bond— i. e., between the sixth and seventh C-atoms. ' ' The acids found in
the pig's liver may be accounted for, in other words, by supposing that desaturation
of stearic acid and of the ordinary (depot) oleic acid occurs by the introduction of a
double link between the sixth and seventh carbon atoms in each ease" — (Leathes). Still
other double links may, however, be introduced into the fatty-acid chain, for acids of the
linolie acid series are present in cod-liver oil. Finally, it is of interest to note that caproic
acid is a product of the above oxidation process, for it has an even number of C-atoms
and therefore will form /3-oxybutyric acid.
To go into these chemical problems any further here would be out of
place. One other fact, should, however, be borne in mind — namely, that
the unsaturated acids may be formed from saturated acids through the
intermediate formation of /?-hydroxy and /8-ketonic acids. Their mere
presence, in other words, should not be taken as evidence that the oxida-
tion of fatty acids is initiated by the introduction of an hydroxyl group
at the ft position in the chain.
CHAPTER LXXX
The classification of animals into two groups — warm-blooded and cold-
blooded— according to their ability to maintain the body temperature at
a constant level, is more or less arbitrary. Between the two groups an-
other exists, represented mainly by hibernating animals, in which at
certain times of the year the animal is warm-blooded and at other times
cold-blooded. The ability of the higher mammals to maintain a constant
body temperature may or may not be present at the time of birth. The
heat-regulating mechanism of the human infant for example remains ill
developed for some time, so that exposure to cold is liable to lower the
body temperature to a dangerous degree.
VARIATIONS IN BODY TEMPERATURE
In animals in which the heat-regulating mechanism is fully developed,
there is not, even during perfect health, entire constancy in temperature
in the different parts of the body or in the same part at different periods
of the day. The average rectal temperature of man is usually stated as
being 37° C. (98.6° F.), but the diurnal variation may amount to 1° C.,
being highest in the late afternoon and lowest during the night. There
are probably several causes for this variation, and they are in part at
least dependent upon the greater metabolic activities of the waking
hours and upon the taking of food. Apart from these influences, how-
ever, others which are less evident appear to operate ; for it has been
found that, when the daily program is reversed by night work, the usual
diurnal variation, although much less pronounced, still remains evident
even although this reversal in habit may have been kept up. for years.
It is of interest to note in this connection that nocturnal birds have their
maximum temperature at night and their minimum during the day.
Regarding the temperature in different parts of the body, that of the
rectum is usually about 1° C. higher than that of the mouth, and this
again higher than that of the axilla. Of these three the mouth tempera-
ture is the most variable, for many conditions, such as mouth breathing,
talking, drinking cool liquids and even exposure to cold air, are sufficient
to lower markedly the temperature of this region. When the mouth
714
CONTROL OF BODY TEMPERATURE AND FEVER 715
temperature is carefully taken by leaving the bulb of the thermometer
under the tongue for a minute or more, it is practically the same as the
temperature of the arterial blood of the hand when this is exposed to the
ordinary conditions of outside temperature. Greater differences than
1° C. in the temperature of different regions of the body are often ob-
served in feeble individuals and in those with some circulatory disturb-
ance.
FACTORS IN MAINTAINING THE BODY TEMPERATURE
The body temperature represents the balance between heat production
and heat loss. The production is effected mainly in the muscles by the
oxidative processes which are constantly ensuing there. When the
activity of the muscles is abolished by paralyzing the terminations of
the motor nerves with curare, the temperature of warm-blooded animals
immediately falls or rises according to the temperature of the environ-
ment. A curarized warm-blooded animal is thus made to behave like a
cold-blooded one. Increased muscular activity, on the other hand,
promptly raises the body temperature by 1° or 2° C., above which, how-
ever, a further rise does not occur, provided nothing has been done to
interfere writh the mechanism by which the excess of heat is got rid of
from the body. The temperature in such cases adjusts itself at a higher
level, at which it remains fairly constant however strenuous the exer-
cise. It is possible that a certain amount of heat may also be generated
by the chemical processes occurring in the liver and other viscera, but
when compared with the muscles this source of heat is undoubtedly in-
significant. Many of these chemical processes, as in the liver, instead
of producing actually absorb heat, so that the balance between heat-
producing and heat-evolving mechanisms may or may not come out in
favor of the liberation of heat.
The production of heat goes on all the time in muscles on account of
the condition of tonic contraction in which they are held (see page 814),
and which is also necessary for keeping the joints in the proper degree
of flexion or extension. When more heat is required by 4he animal body,
the tone of the muscles increases independently of the function which
they may be performing in controlling the position of the joints. This
increased tone may become so pronounced that it causes visible contrac-
tions, which AVC recognize as shivering. Whenever the "insensible hyper-
tonicity and the shivering are inadequate to produce a sufficient amount
of heat, the animal instinctively moves about in order that the greater
contractions may liberate more heat.
The heat is produced in the muscles by oxidation of the .foodstuffs that
have been assimilated from the blood. Even during the process of as-
716 METABOLISM
similation itself a certain amount of heat is generated; this is known
as the specific dynamic action of the foodstuff, and is most pronounced
with protein and least so with carbohydrate (page 538). Advantage
may be taken of this heating power of protein to produce more heat
when the cooling conditions are excessive ; in winter, for example, there
is an inclination to take more protein food than during summer, and the
per capita consumption of such food is "much greater in peoples living in
temperate zones than in those living in the tropics. The ultimate amount
of heat produced by oxidation is greatest with fat and least with carbo-
hydrate.
Heat loss in man is effected partly through the lungs, but mainly
through the skin. Through the latter pathway heat is lost by the physical
processes of heat conduction and radiation and by the evaporation of the
sweat. Through the lungs it is lost mainly in the vaporization of the
water contained in the expired air (latent heat of vapor). The amount
of heat lost from the skin by conduction and radiation depends on the
temperature of the skin, which again depends on the rate at which the
blood is circulating through the cutaneous vessels. Under ordinary con-
ditions of external temperature two or three times as much heat is lost
by these methods as by evaporation. The losses by evaporation, under
conditions of rest and average external temperature, are about equally
divided between the lungs and the skin.
From all these facts, it is evident that heat loss occurs mainly by the
skin and only to a small degree by the lungs. This means that under
average conditions in man the main regulation of heat loss is effected by
variations in the skin temperature brought about by peripheral vaso-con-
striction and dilatation. The marked sensitivity of the cutaneous
blood supply to changes in the temperature of the environment has been
very clearly shown by observations made with the hand calorimeter of
Stewart described elsewhere (page 281). When the bloodflow through
the hand is examined in a person who has been exposed to the outside
air, it may be little more than half that which it attains after he has
been in a warm room for some time. In the outside air the vessels con-
strict to prevent heat loss by conduction and radiation; in the warm room
they dilate to facilitate this loss. The afferent impulses which reflexly
control the change in the cutaneous blood circulation may be set up by
local applications of heat or cold, as can be shown in the hand-calorim-
eter experiments by applying a cold pad to the skin of the correspond-
ing forearm, when an immediate curtailment of bloodfloAV takes place.
Or the reflex may be excited from distant skin areas, as illustrated in
the curtailment in bloodflow observed when the opposite hand to that
on which the observation is being made is placed in cold water. The
CONTROL OF BODY TEMPERATURE AND FEVER 717
magnitude of the change in cutaneous circulation is nevertheless depend-
ent upon the extent of the area of the body that is opposed to the change
in temperature, as seen in the dilatation of the skin vessels prior to a
rise in body temperature when a person is immersed in a warm bath.
Although afferent impulses from the skin are therefore of great im-
portance in adjusting the cutaneous blood supply according to the
amount of surface cooling that has to occur, a further effect is also pro-
duced on them by the action on the nerve centers of temperature dif-
ferences in the blood itself. Thus, when the temperature of blood going
to the brain is raised by placing the carotid arteries on some heating de-
vice or when the region of the corpora striata is directly warmed, the
skin vessels become dilated as if the animal had been exposed to general
warmth.
When the loss of heat by radiation and conduction is no longer ade-
quate to prevent a rise in body temperature, or when the processes can
not operate on account of a high temperature in the environment, the
loss of heat from the skin is mainly dependent upon the evaporation of
sweat. Under ordinary conditions this evaporation takes place at such
a rate that there is no visible perspiration on the surface of the body —
the so-called insensible perspiration. "When the heat loss by this channel
must become greater, the perspiration is produced in larger amount, so
that it collects on the surface of the body ; and, provided the conditions of
the environment are such that evaporation can readily take place (low
relative humidity), the amount of cooling of the body that can be effected
becomes very great. A man may exist without any marked rise in body
temperature in a very hot environment even when he is exposed to an out-
side temperature that is the same as that of his body, or even greater. To
encourage evaporation, however, he should be naked or very lightly clad,
and the air should be kept in constant motion so that the layers of air
next to the skin, which ordinarily very quickly become saturated with
vapor, are transferred and replaced by dryer air. Movement of the air
also increases the heat loss by conduction, provided the temperature of
the air is not too near that of the body.
The importance of the movement of air in the regulation of heat loss
has been clearly demonstrated by Leonard Hill,54 F. S. Lee, and others, who
have found that a great part of the discomfort experienced by living in
stagnant air can be obviated by putting the air in motion by electric fans
without doing anything to improve its chemical purity. In one famous
experiment a number of young men were placed in an air-tight cabinet
at the ordinary temperature of the room. After a time they began to
exhibit the symptoms usually attributed to polluted air; they became
drowsy and some of them developed headaches, etc. A small electric
718 METABOLISM
fan was then started so as to set the air in motion. Immediately all of
the men recovered and remained in a perfectly comfortable condition
so long as the fan was kept going. The practical application of these
facts to the hygienic control of the working conditions in mine shafts,
in submarines, in workshops, etc., will be self-evident.
The stimulus to increased SAveating seems to be dependent mainly on
changes in the temperature of the Wood; for sweating does not im-
mediately set in when the body is subjected to heat, as by a warm bath or a
hot pack. It usually takes from ten to twenty minutes after the person
has been placed in the bath or surrounded by the warm blankets of the
pack before SAveating becomes pronounced. It can usually be sho\vn that
before it sets in the body temperature has been raised from 0.1 to 0.8
degrees C. (0.2 to 1.4 degrees F.). In this regard, therefore, the response
of the SAA'eat glands does not occur so promptly as does the dilatation of
the cutaneous vessels.
Loss of heat by evaporation of SAA'eat occurs only in certain animals.
It is practically absent, for example, in the dog. The degree to which
it may occur also varies in different individuals of the same species. The
power of Avithstanding high temperatures is proportional in man to the
facility AAdth Avhich he perspires. "Where SAveating is interfered Avith by
skin diseases, — by ichthyosis, for example, — exposure to heat or in-
creased heat production, as by muscular actiAdty, may raise the body
temperature to a dangerous degree.
Another factor upon Avhich the efficiency of eAraporation of sweat in
cooling the body depends is the relative humidity of the air. When this
is high, evaporation of Avater into it can not occur, and it is on this
account that an increase in body temperature is much more likely to
occur in AArarm, humid atmospheres than in those that are dry. At the
same temperature people can live in perfect comfort in the dry air of the
open plains, but suffer immediately from rise of temperature Avhen they
go into the humid air of the river valleys. Similarly, work in hot fac-
tories or in mines is quite possible at very high temperatures if the air
is kept dry and in motion, but becomes impossible when the air is moist.
In judging of the adequacy of air from this point of vieAV, it is there-
fore important to take not the ordinary dry-bulb thermometer reading
but that of the Avet-bulb.*
In animals, like the dog, that do not perspire over the surface of the
body, vaporization of the water in the expired air is the most important
method of regulation of heat loss. When such an animal is exposed to
*The wet-bulb thermometer registers a temperature that is lower than that of the dry-bulb in
proportion to the relative humidity of the air. When the air is completely saturated with moisture,
the temperature recorded by the two instruments will be the same; when it is perfectly dry, the
difference will be maximal.
CONTROL OF. BODY TEMPERATURE AND FEVER 719
warmth or when the region of the corpora striata is artificially warmed,
the breathing immediately becomes much quicker and deeper, so that
pulmonic ventilation is greatly increased and much more water is carried
out as vapor with the expired air. To vaporize the water large quanti-
ties of heat are required (seen in the latent heat of steam). In man this
method is, ordinarily, not of great importance, but it may become so
when sweating is interfered with, as in ichthyosis. The more rapid
breathing also facilitates cooling by increasing the conduction of heat
from the mucous membranes of the tongue, mouth, throat, etc. The im-
portance of this method of cooling has been shown by finding that after
the introduction of a trachea! cannula a dog can not withstand an in-
crease of external temperature nearly so well as a normal animal.
There are many other questions concerning the control of heat loss
from the human body that might be considered, but it is scarcely nec-
essary to do so here. It should be added, however, that the relative
humidity of the air in the control of heat loss has a different significance
when the temperature is high from that when it is low. High relative
humidity at high temperatures, as we have seen, interferes with evapora-
tion of sweat, whereas high relative humidity at low temperatures in-
creases the heat-conducting power of the air and therefore tends to cool
off the surface of the body by greater conduction. It is on this account
that it is much more comfortable to live at a low temperature when the
air is dry than when it is moist. On the dry plains of the West a tem-
perature of many degrees below zero causes less sense of cold to be ex-
perienced than in the moist atmosphere at a considerably higher tem-
perature along the Great Lakes and in the river valleys.
THE CONTROL OF TEMPERATURE
In the case of man the body temperature is very largely under volun-
tary control, as by the choice of clothing and the artificial heating of the
room. Desirable as this voluntary control of heat loss may be, there can
be little doubt that it is often managed to the detriment of good health.
Living in overheated rooms during the cooler months of the year so
diminishes the loss of heat from the body that the tone and heat-produc-
ing powers of the muscular system are lowered. Not only does this
diminish the resistance to cold, but it causes the food to be incompletely
metabolized so that it is stored away as fat. The superficial capillaries
also become constricted and the skin bloodless and "pasty." It is not
looks alone that suffer, however, but health as well, for by having so
little to do the heat-regulating mechanism gets, as- it were, out of gear,
720 METABOLISM
so that when it is required to act, as when the person goes outside to
the cold air, it may not do so as promptly as, it should, with the result
that the body temperature falls somewhat and catarrh, etc., are the
result. There can be little doubt that much of the benefit of open-air
sleeping is owing to the constant stimulation of the metabolic processes
which it causes.
As will be inferred from what has been said above, the control between
heat production and heat loss is effected through a nerve center located
in or near the corpora striata. In most animals, when the spinal cord
is cut in the cervical region, the body temperature quickly falls unless
artifically maintained. In the case of man, on the other hand, it has
usually been observed, after accidental section of the spinal cord in the
cervical region, that a rise in temperature occurs. In twenty-four un-
complicated cases of spinal-cord injury in man, collected from the rec-
ords of Guy's Hospital by Gardiner and Pembrey, it was found that
nineteen showed hyperthermia (sometimes amounting to 43.9° C.), and
only five, hypothermia (sometimes 27.6° C.). If the patient lived, the
ultimate effect of the section, as in the lower animals, would no doubt
be the loss of the power of maintaining a constant temperature.
The extent to which the animal comes to behave as if cold-blooded after
section of the spinal cord varies considerably according to the level of
the lesion; if the cord is cut in the upper thoracic region, for example,
the regulation against cold, although distinctly less efficient than normal,
is far better than when the section is made through the cervical cord.
This difference is dependent on the fact that after the lower lesion much
larger muscular groups and skin areas are left intact, so as to make
regulation possible. Section of the dorsal cord in mice has been found
by Pembrey to abolish entirely the increased metabolism which occurs
in normal mice when they are exposed to cold.
In the light of these experiments it is probable that the difference in
the effects produced on body temperature by section of the cervical
spinal cord in man and the lower animals depends on the relative im-
portance of the heat-producing and heat-dissipating mechanisms. When
the control of heat loss is paralyzed in the smaller animals, the cooling
of the body becomes excessive in relation to the amount of heat produced
in the paralyzed muscles, because the body surface is extensive in com-
parison with the body weight (see page 551). In the larger animals such
as man, on the other hand, the cooling effect is much less marked, espe-
cially when, as is common after such an accident, the patient is kept
unusually warm.
CONTROL OF BODY TEMPERATURE AND FEVER 721
FEVER
The clinical application of a Knowledge of the mechanism of heat regu-
lation in the animal body concerns the causes of fever. In the most
familiar form fever is produced by infectious processes, but it may also
be owing to various other causes, among which may be mentioned the
parenteral injection of foreign protein, excessive destruction of protein
substances in the body itself, the action of certain drugs, and lastly,
injury to the base of the brain or lesions of the upper levels of the spinal
cord. Various types of fever are recognized: when the temperature re-
mains constantly above the normal, it is known as continuous fever;
when oscillations occur but the temperature never falls to the normal
level, it is known as remittent; when it attains the normal level at cer-
tain periods during the day, it is known as intermittent.
Causes of Fever
During a sudden rise in temperature -there is, on the one hand, in-
creased heat production in the muscles, and on the other, dimin-
ished heat loss from the surface of the body. The fever is therefore
due to an exaggeration of the processes by which the body normally re-
acts to conditions which tend to lower the body temperature. The increased
muscular activity thus induced often causes visible contractions, familiar
as shivering; and the constriction of the cutaneous blood vessels pro-
duces the subjective sensation of chills, and causes the skin to become
pale and cold to the touch. The skin muscles also contract, producing
"goose skin." During this stage, objective demonstration of the cur-
tailment of the skin circulation can be secured by observation of the
bloodflow. through the hands and feet (page 283). When the temperature
suddenly falls again, the crisis, as it is called, muscles become ' flaccid
and produce less heat, and the cutaneous blood vessels dilate, as has
been shown by measurements of the bloodflow of the hands and feet.
At the same time also, the sweat glands are stimulated and marked per-
spiration occurs.
Concerning the cause of continuous fever, it must be assumed that the
balance between heat production and heat loss has been adjusted at a
higher plane than normal. We can not explain the fever on the basis
either that heat production is permanently increased or that heat loss
is permanently diminished, for in neither of these cases would the tem-
perature stand at a permanent level but would steadily rise or fall, ac-
cording to which mechanism was disturbed. .While set at this higher
plane of fever, the thermogenic nerve centers are still capable of re-
sponding in the usual way to the influences which cause the body tern-
722 METABOLISM
perature to change in a normal person. For example, when a fever pa-
tient is subjected to a hot bath so that his body temperature rises about
0.2 to 0.5 degrees C., sweating occurs just as in a normal individual; or
if exercise is taken the. increased amount of heat thereby produced in
the muscles is dissipated in the usual way. When, on the other hand,
the patient is exposed to cold, the vessels of the skin contract and he
shivers.
Although fever is not caused by an actual disturbance of balance be-
tween heat production and heat loss, neither of these processes is pro-
ceeding at its normal level. That there is a distinct increase in the total
heat production of the body in acute fevers in well-developed persons
has been shown by means of the respiration calorimeter. This increased
heat production is not observed in patients who have been brought into
a weakened condition and in whom the muscular tissues have become
atrophied by long-continued fever. The increased heat production in
continuous fever is mainly dependent upon the increase in body tem-
perature and is not one of its causes, as is evident from the fact that far
larger quantities of heat are frequently produced in normal individuals
as a result of muscular exercise or the taking of large quantities of
protein-rich food. The heat thus produced is, however, very quickly
dissipated, so that only a temporary rise in temperature occurs, (cf.
Hewlett.57)
Similarly, it can be shown that in continuous fever there is a relative
inefficiency in the mechanism of heat dissipation. When the temperature
of a normal person is artificially raised through about 1° C., a marked
increase in cutaneous bloodflow and profuse perspiration are invariably
noted. In a patient with fever of the same degree, on the other hand,
there is practically no change in the skin circulation; indeed, it is usually
diminished, and there is no unusual perspiration. The heat-regulating
mechanism is now fixed on a plane that is higher than the normal, so
that although further increase in body temperature, as we have seen,
calls forth responses like those in a normal individual, yet at the fever
temperature itself there are none of the reactions which a normal individ-
ual would exhibit if his temperature were artificially raised to that level.57
The adjustment of the temperature at the higher level is by no means
so perfect as it is at the normal level of health, so that a normal subject
is more resistant to the effects of cold than is a patient with fever. The
degree of response of the fever patient, however, varies considerably
from time to time ; a cold bath in typhoid fever, for example, lowers the
body temperature much less effectively at an early stage in the disease,
when the fever is more or less continuous, than later when it is becoming
of the intermittent type. In the third week of the disease the cold bath
CONTROL OF BODY TEMPERATURE AND FEVER 723
more readily brings down the temperature and keeps it down for a longer
time than during the first or second week. The mechanism for heat loss
is also deranged in fever, which explains the rise in temperature that is
likely to follow the performance of even moderate muscular exercise or
the taking of too hearty a meal in tuberculous and convalescent typhoid
patients.
Changes in the Body During Fever
In seeking for the cause of fever which is evidently of an obscure
nature, it is necessary to collect all the information we can regarding
the metabolic changes that are then occurring in the animal body. A
few of the most significant facts that have so far been collected may
be mentioned here. Some of the most important concern the dis-
turbance in nitrogenous equilibrium caused by the considerable loss of
nitrogen which takes place in fever patients when they are fed on
the usual hospital diet prescribed for such cases. This loss of nitro-
gen is.no doubt the result of the partial starvation in which the pa-
tient is kept; for it has been shown by Shaffer and Coleman-"'5 that
patients with typhoid fever may be maintained in nitrogenous equi-
librium by feeding them with relatively large amounts of carbohy-
drate, which acts by protecting the protein of the body from disintegra-
tion (see page 571). Even with a diet excessively rich in carbohydrates
that no more than covers the calorie requirements of the patient, nitrog-
enous equilibrium has also been attained. The protein minimum to
which fever patients can be reduced is nevertheless considerably higher
than the minimum in normal individuals.
From the above results as a whole, it is probably safe to conclude that
there is a specific destruction of protein going on in the body during fever.
Further evidence of such a destruction is furnished by the presence in
the urine of excessive amounts of creatinin, of purine bases, and, it is
said, of incompletely hydrolyzed proteins, such as the albumoses (pro-
teoses.) Moreover, when the fever suddenly terminates in crisis, there
is a marked increase in the excretion of urea (the epicritical urea in-
crease), which indicates that an extensive deamination of protein build-
ing stones (ammo acids) is occurring. The so-called "diazo reaction"
obtained in the urine during the fever is also believed to depend on the
presence of abnormal protein-disintegration products.
As to the specific cause of the increased protein disintegration, little
is known. Several factors may operate: (1) the partial starvation of the
patient, entailing an increased breakdown of protein to meet the calorie
requirements; (2) the high temperature, which in itself may stimulate
increased protein metabolism, for it has been shown that, when normal
724 METABOLISM
animals are artificially warmed, protein metabolism becomes increased ;
and (3) toxic protein-decomposition products specifically causing an ex-
cessive breakdown of protein.
Although there is increased protein breakdown during fever, it must,
not be forgotten that only about 20 per cent of the total expenditure
of the body is derived from this foodstuff, 80 per. cent coming from non-
nitrogenous material, which must be fat, because the available carbo-
hydrates are used up at an early stage.
Since the general metabolism is increased, the excessive breakdown of
the fatty substances, occurring as it does in the presence of a diminished
combustion of carbohydrates, interferes with the proper oxidation of the
fatty-acid molecules and leads to the appearance of so-called acidosis
products in the urine, and consequently to a relative increase in the
urinary ammonia (page 616). A tendency to acidosis therefore exists.
The acidosis may reach a considerable degree of severity and cause the
tension of carbon dioxide in the alveolar air to become diminished. Since
a similar degree of acidosis may be produced in partially starved ani-
mals by overheating them with moist air, but not so if the animals are
liberally fed with carbohydrates, it is probably safe to conclude that
abundance of carbohydrate is advisable in the food that is furnished to
fever patients.
Another interesting metabolic change in fever concerns the salt bal-
ance. This is studied by observing the amount of sodium chloride excreted
by the urine. As is well known, this becomes markedly diminished until
the crisis of the fever, when it suddenly increases. Salt retention is more
marked in certain types of fever than in others, and it is essentially dif-
ferent in nature from the salt retention that has been observed to occur
in nephritis. This difference has been brought to light by examination
of the chloride content of the blood. In nephritis, the concentration of
chlorides in the blood is considerably increased, whereas in fever it is
markedly diminished. The deficiency in salt elimination can not be at-
tributed to a deficiency of salt in the food, for it sets in before the diet
has been curtailed and, when salt is given to a febrile patient, it is re-
tained in the body to a greater degree than is the case in the normal
individual. For some reason the tissues in fever have acquired the
property of retaining large quantities of salt. •
Attempts to study the water balance during fever have frequently been
made, but the technical difficulties of such investigations make the re-
sults uncertain and of little value. That some retention of water occurs
during fever is, however, evidenced by the dilution of the blood. At the
crisis this hydremia quickly disappears at the same time as the increased
CONTROL OF BODY TEMPERATURE AND FEVER 725
elimination of chlorides is going on. Chlorides and water would there-
fore seem to behave in a similar fashion during fever.
The Heat-regulating Center
In all discussions on the regulation of hody temperature and the
causes of fever, it is assumed that a heat-regulating or thermogenic
center exists somewhere in the brain. It is believed to be located
about the optic thalami or corpora striata, for it has been found in
rabbits that destruction of the brain anterior to this region does not
cause any change in body temperature, whereas destruction behind it
is followed by an entire upset in the heat-regulating mechanism. Fur-
thermore, artificial puncture of this part of the brain causes marked
elevation in body temperature in rabbits (heat puncture). Most in-
teresting experiments have been recorded by Barbour,56 who succeeded
in applying heat or cold locally in the region of the centers. By the
application of cold, increased muscular metabolism/ on the one hand,
and diminished heat loss, on the other, were excited; and conversely,
when warmth was applied, an increased heat loss and a diminished heat
production were observed. Irritation of this region of the brain in man,
as after cerebral hemorrhage, is also accompanied by remarkable dis-
turbances in heat regulation. It is believed by many that the essential
cause of infectious fever is an action on these centers by toxic substances
which develop in the blood.
The centers may also be acted on by various drugs, some of which
excite them to increase the body temperature, others, to lower the tem-
perature when this has already been elevated. When solutions of sodium
chloride are injected intravenously or subcutaneously or even sometimes,
particularly in children, when administered by mouth, more or less fever
may result. This must be a specific action of the Na ion, for, if instead
of pure solutions of NaCl. solutions containing calcium and potassium
salts as well as those of sodium are injected, no fever is induced. This
fact, taken along with the close similarity between puncture diabetes
and heat puncture, lends support to the view that in its initial stages
experimental fever of this type is the result of an excessive breakdown
of glycogen in the liver. It must not be imagined, however, that persist-
ent fever can be attributed to such a cause, since the fever remains after
the glycogen has all been removed. Other chemical substances produc-
ing fever are caffeine, certain other purines, and particularly tetra-hydro-
naphthylamin.
Belonging to this group of fevers must also be considered the im-
portant ones produced by the intravenous injection of certain forms of
protein, as those of egg white or those derived from the bodies of bac-
726 METABOLISM
teria or from the laked corpuscles of a foreign blood. The fever in
these cases is 110 doubt caused by a mechanism closely related to that
responsible for anaphylaxis (see page 89). Such injections do not pro-
duce fever in animals after division of the cervical spinal cord or ex-
cision of the midbrain. It is believed that many cases of so-called asep-
tic fever, occurring after severe contusions or other wounds, may be
the result of destruction of proteins within the body. Similarly the rise
in temperature during infections may be owing to the breakdown protein
of the microorganism in the cells.
Significance of Fever in the Organism
It is impossible at present to state definitely whether fever is a re-
action of the organism against some infection and therefore of benefit
in assisting the organism to combat it, or whether it is in itself an un-
favorable condition. The question can certainly not be answered by
observing the behavior of bacteria growing at different temperatures
in various media outside the body. That certain bacteria should be
found not to thrive at incubator temperatures equal to those found in
the body during fever, does not at all prove that this fever is of sig-
nificance as a means of combating the growth of the bacteria in the
body. It is undoubted that, where the body temperature becomes ex-
cessively high, the correct treatment is to keep it down as much as
possible. On the other hand, the reduced mortality that has followed
the introduction of the cold-bath treatment in typhoid fever may not
be due so much to the reduction in body temperature itself as to
the favorable effect produced on the nervous system and circulation.
We certainly know that in normal animals moderate degrees of hyper-
pyrexia produced by exposure to moist heat are well borne for consider-
able periods of time, thus indicating that it is the infection and not the
hyperthermia that causes the serious damage to the body in infectious
fevers.
METABOLISM REFERENCES
(Monographs and Original Papers)
iLusk, Graham: The Elements of the Science of Nutrition, W. B. Saunders Co., ed.
3, 1917.
^Cathcart, E. P.: The Physiology of Protein Metabolism, Monographs on Bio-
chemistry, Longmans, Green & Co., 1912.
^Taylor, A. E.: Digestion and Metabolism, Lea & Febiger, New York, 3912.
iUnderhill, P. P. : The Physiology of the Amino Acids, Yale Press, New Haven, 1915.
sMacleod, J. J. E.: Diabetes, Its Pathological Physiology, E. Arnold, 1913.
saFiirth, von: The Problems of Physiological and Pathological Chemistry, etc., J. B.
Lippincott Co., 1916.
5b Jones, W.: Nucleic Acids, Monographs in Biochemistry, Longmans, Green & Co.,
1914.
scMendel, Lafayette B.: Ergebnisse der Physiologic, 1911.
CONTROL OF BODY TEMPERATURE AND FEVER 727
sdLeathes, J. B.: The Fats, Monographs in Biochemistry, Longmans, Green & Co.
seMathews, A. P.: Physiological Chemistry, Wm. Wood & Co., 1917.
s'Dakin, H. K. : Oxidations and Reductions in the Animal Body, Monographs in Bio-
chemistry, Longmans, Green & Co., 1912.
5gLeathes, J. B.: Problems in Animal Metabolism, 1906.
6Du Bois, E. F., and collaborators : Clinical Chemistry, Papers 1 to 25, Arch. Int. Med.,
1915-17, xvi-xix.
7 Benedict, F. G.: Am. Jour. Physiol., 1916, xli, 275 and 292.
sMendel. Lafayette B.: Harvey Lecture, J. B. Lippincott Co., 1914-1915, p. 101.
sMcCollum, E. V., and collaborators: Numerous papers in Jour. Biol. Chem., be-
ginning 1913.
"Hopkins, F. Gowland, and Willcock, E. G.: Jour. Physiol., 1906, xxxv, 88.
nBayliss, W. M.: The Physiology of Food and Economy in Diet, Longmans, Green
& Co., 1917.
i2McCollum, E. V.: Harvey Lecture, Jour. Am. Med. Assn., 1917.
"Sweet, J. E., Carson-White, E. P., and Saxon, G. J.: Jour. Biol. Chem., 1913, xv,
181; ibid., 1915, xxi, 309.
"Stepp, W.: Biochem. Ztschr., 1909, xxii, 452.
isFunk, Casimir: Ergebnisse der Physiologic, 1915.
ifiMcKillop, M.: Food Values: What They Are and How to Calculate Them,
Rutledge.
K'aMcCoy, D. Major: The Protein Element in Nutrition, E. Arnold, London, 1912.
"Pembrey, M. S.: Chemistry of Eespiration, in Schafer's Text Book of Physiology,
1898, i.
isAllen, F. P.: Glycosuria and Diabetes, Boston, 1913.
isJoslin: Diabetes.
aoWoodyatt, R. T., Sansum, W. D., and Wilder, R. M.: Jour. Am. Med. Assn., 1915,
Ixv, 2067. Also Taylor, A. E., and Hulton, F.: Jour. Biol. Chem., 1916, xxv,
173.
ziMacleod, J. J. R., and Fulk, M. E.: Am. Jour. Physiol., 1917, xlii, 193.
22Hamman, L., and Hirschbaum: Arch. Int. Med., 1917, xx, 761-788.
23Cannon, W. B.: Bodily Changes in Pain, Hunger, Fear and Rage, D. Appleton &
Co., 1915.
2<Knowlton, F. P., and Starling, E. H.: Jour. Physiol., 1912, xlv, 146.
2spatterson, S. W., and Starling, E. H. : Jour. Physiol., 1913, xlvii, 135; also Cruick-
shank and Patterson: Ibid., p. 113.
ssMacleod, J. J. R.: Glycolysis, Jour. Biol. Chem., 1913, xv, 497.
27Murlin, J, R.: Jour. Biol. Chem., 1913, xvi, 79.
2sCruickshank: Jour. Physiol., 1913, xlvii, 1.
29Macleod, J. J. R., and Poarce, R. G.: Zentralbl. f. Physiol., 1913, xxvi, 1311.
soWoodyatt, R. T.: Jour. Am. Med. Assn., 1916, Ixvi, 1910.
aiVan Slyke, D. D.: The Present Significance of the Amino Acids in Physiology and
Pathology, Harvey Lectures, J. B. Lippincott & Co., 1915-1916, p. 146. Also
papers in Jour. Biol. Chem., 1911, ix, 1S5; xii, 275; ibid., 1912, xii, 301 and 399;
ibid., 1913, xiii, 121, 125 and 187.
32Folin, O., and Denis, W. : Jour. Biol. Chem., xi, 87 and 493; ibid., 1912, xii, 14 and
253.
saAbel, J. J.: The Mellon Lecture, Science, 1915, xlii, 135.
3-iHewlett, A. W., Gilbert, L. O., Wickett, A. D.: Arch. Int. Med., 1916, xviii, 636.
ssLosee, J. R., and Van Slyke, D. D.: Jour. Am. Med. Assn., 1917, cliii, 94.
36Shaffer, P. A.: Am. Jour. Physiol., 1908, xxviii, 1.
srCathcart, E. P.: Jour. Physiol., 1907, xxxv, 500.
ssMyers and Fine: Jour. Biol. Chem., 1913, xiv, 9.
39Levene, P. A.: Cf. W. Jones.*o
•to Jones, W.: Nucleic Acids, Monographs on Biochemistry, Longmans, Green & Co.,
1914.
•^Benedict, S. R.: Harvey Lecture, 1915-16.
42Hunter, A., and Givens, M. H.: Jour. Biol. Chem., 1914, xviii, 403.
43Burian, R., and Schur, H.: Cf. Macleod in Recent Advances in Physiology and Bio-
chemistry, ed. by Leonard Hill, E. Arnold, London, 1905.
44Mendel, Lafayette B., and Lyman, J. F.: Jour. Biol. Chem., 1910, viii, 115.
*5Taylor, A. E., and Rose, W. C. : Jour. Biol. Chem., 1913, xiv, 419.
46Hopkins, F. G., and Hope, W. B.: Jour. Physiol., 1899, xxiii, 277.
728 METABOLISM
47Ascoli, M., and Izar, G.: Ztschr. f. Physiol. Chem., 1909, Mil, 529; ibid., 1911, Ixiii,
319.
•isMcClure, 0. W., Vincent, B., and Pratt, J. H. : Am. Jour. Physiol., 1916, xlii, 596.
*9Bloor, W. E.: Jour. Biol. Chem., 1912, xi, 429; ibid., 1913, xv, 105; ibid., 1914, xvi,
517; ibid., 1912, xi, 141; ibid., 1915, xxi, 421; ibid., 1914, xix, 1; ibid., 1915,
xxiii, 317; ibid., 1914, xvii, 317; ibid, 1915, xxii, 133. Also Bloor and Knudson:
Jour. Biol. Chem., 1916, xxvii, 107; ibid., 1916, xxiv, 447; Bloor, Joslin and
Homer: Ibid., 1916, xxvi, 417; ibid., 1916, xxv, 577.
suLeathes, J. B.: The Fats, Monographs on Biochemistry, Longmans, Green & Co.
"Coope, E., and Mottram, V. H.: Jour. Physiol., 1914, xlix, 23; ibid., 1915, xlix, 157.
saEaper, H. 8.: Jour. Biol. Chem., 1913, xiv, 117.
53Smedley, I. D. : Proc. Phys. Soc., Jour. Physiol., 1912, xlv, 25.
s-tHill, Leonard: Address to the Phys. Sec. Brit. Assn. for the Adv. of Sci., Section,
J, 1912.
ssShaffer, P. A., and Coleman, W.: Arch. Int. Med., 1909, iv, 538.
ssBarbour, H. G.: Arch f. Exper. Path. u. Pharmac., 1912, Ixx, 1. Also Barbour and
Wing, S. S. : Jour. Pharmac. and Exper. Therap., 1913, v, 105.
"Hewlett, A. "W.: Monographic Medicine, D. Appleton & Co., 1917, i.
PART VIII
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
CHAPTER LXXXI
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
In order that the various activities of the animal organism may act
efficiently as a whole, it is necessary that those of one part be correlated
with those of another. This correlation of function is mediated either
through the nervous system or through the action on one part of the
body of substances produced in another part and carried between them by
the blood. Control through the nervous system is especially developed for
those functions which have to be brought promptly into play, such as
muscular movement and the other physiologic processes concerned in the
adjustment of the organism to quickly changing conditions of its environ-
ment. Control through the blood is the mechanism by which the metabolic
activities of different organs are mainly correlated. The chemical sub-
stances involved are often called internal secretions.
Some of these internal secretions are merely by-products of metabolism,
and are only incidentally used for the purpose of bringing about control
between different parts of the body. To this group belong carbon dioxide,
which may act on the respiratory and other nerve centers, and urea, which
may stimulate increased activity of the kidneys. Indeed, the list of sub-
stances included under such a definition of internal secretions is almost
illimitable, and to designate by the special name of hormone every con-
stituent that can affect physiologic functions, as some have done, can lead
only to confusion. The internal secretions with which we are more
directly concerned are those that are specially produced for the purpose
of controlling the metabolic functions. They are given the general name
of autacoids (E. A. Schafer).60 Autacoids may be either the sole product
of some special gland or a secondary product of glands which have other
functions. To the former class belong the autacoids produced by the para-
thyroid, thyroid, pituitary and adrenal glands, and to the latter, those
produced by the pancreas and generative glands.
Autacoids have further been subdivided by Schafer into two classes
729
730
according to whether they excite metabolic processes or depress them.
Examples of excitatory autacoids, also designated as hormones, are the
epinephrine produced by the adrenal glands, which excites the termina-
tions of the sympathetic nervous system, and pituitrin produced by the
posterior lobe of the pituitary gland, which excites plain muscular fiber.
Inhibiting autacoids, also called chalones, are not so commonly known, but
are illustrated by the substance contained in extract of the placenta,
which tends to prevent the secretion of milk.
Autacoids may have either an immediate or a delayed action ; the effect
which they produce may be like that with which we are familiar as the
result of stimulation of the nerve supply of a gland, being illustrated
again by the effect of epinephrine, or they may act so slowly that it is
only after a considerable period of time during which they have been
iii the organism in excess, that any apparent effect is produced. The
slowly acting autacoids have been called morphogenetic, and they are
well illustrated in the internal secretions of the anterior lobe of the
pituitary and of the generative glands — secretions which affect growth.
Regarding the chemical nature of autacoids, certain facts stand out
prominently. Being very largely the products of glands, it might be
imagined that they would be enzymic in nature, for enzymes are now
known to be the most important active agents in bioplasm as Avell as the
active agents in many of the external secretions, like those of the sali-
vary, gastric and intestinal glands. Autacoids, however, are not enzymes.
They are far simpler in chemical structure, and are not destroyed by
heat in the presence of water. They are represented by a, comparatively
small molecule, and- are therefore dialyzable. This latter fact justifies
the hope that it may be possible to prepare them or their simpler salts
in crystalline form — a hope which has already been realized in the case
of at least one of them — epinephrine. Great progress has likewise been
made in isolating the active principles of the thyroid and of; the anterior
and posterior lobes of the pituitary glands. To sum up, then, we. may
say that an autacoid is a specific organic substance, formed by the cells
of one organ and secreted into the circulating fluid, which carries it to
other organs, upon which it produces effects similar to those drugs.
Methods of Investigation
To investigate the function of an autacoid, careful studies are made of
the effects produced (1) by excision of the gland which furnishes the
autacoid and (2) by administering intravenously or subcutaneously or
orally extracts prepared from the gland. Frequently, also light is thrown
on the function of the autacoid by observing the effect which fol-
IOAVS prolonged feeding with the endocrine organ that manufactures it
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 731
and by observing the pathologic changes in the various endocrine organs
in diseased conditions. Embryologic and histologic studies are also of
the greatest importance. A difficulty in investigating the function of
an endocrine organ lies in the fact that the secretion of no one gland
acts independently of those from other glands. On the contrary, there is
undoubtedly a close association of function, so that we can not tell
whether a change of function observed after removal of some gland or
administration of some extract is a direct consequence of the experi-
mental procedure, or is induced by some secondary effect developed on
another endocrine organ. It will no doubt take many years before suf-
ficient data have been collected to enable us definitely to state what the
particular function of each endocrine organ may be. Since most progress
has been made in connection with the adrenal gland, it will be advan-
tageous to consider the functions of this gland first.
ADRENAL GLAND
In mammals the adrenal gland is composed of two parts, the cortex and
the medulla. In other groups of animals however, these two are more or
less separate, being completely so in fishes. This not infrequent separa-
tion of cortex and medulla suggests a different function for the two
structures. Experimental investigation supports this view.
The Cortex
The cortex on microscopic examination is seen to be composed of rows
of epithelial cells arranged more or less in columns except at the
periphery, where they form glomerular masses, and next the medulla,
where they assume a reticular formation. The cells of the greater part
of the cortex, unlike those of the medulla, contain no granules with
special staining qualities, but they do contain particles which are be-
lieved to be composed of cholesterol esters and lecithin. In the cells of
the reticular portion of the cortex, however, pigment particles are not
infrequently observed. The blood supply of the cortex is not nearly .so
rich as that of the medulla, being represented by fine arterioles which
run inwards from the capsule towards the medulla in the connective tis-
sue that lies between the columns of cortical cells. Nerves similarly
penetrate into the cortex, some supplying its blood vessels and cell
columns, but most of them proceeding to the medulla. They are derived
from a network of nerve fibers in the capsule of the organ, and the nerve
supply of this network comes partly from the suprarenal plexus, and
partly from the splanchnic nerve. Embryologically the cortex is de-
veloped from the cells of the genital ridge, that is, from mesodermic
cells.
732 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
Very little is known concerning the function of the adrenal cortex,
although there is little 'doubt that it is closely related with the develop-
ment of the sexual organs. The evidence for this is as follows: (1) in
cases of sexual precocity it is found that the adrenal cortex is much
hypertrophied ; (2) it becomes hypertrophied during pregnancy; (3) it
is ill developed in sexual deficiency; (4) changes occur in it during the
estrual cycle in many animals; (5) after castration it is said to be hyper-
trophied; (6) the innermost portion of the cortex, sometimes called the
boundary zone, is much hypertrophied in the human fetus, but this hyper-
trophy entirely disappears after the first year of extrauterine life.
Whether the cortex possesses other functions is difficult to say. Some
facts would indicate that it does. For example, the passage of blood
through the cortex before reaching the medulla, would seem to indicate
that some change which is preparatory to the main change occurring in
the medulla takes place in the blood while it is in the cortex. This VICAV
is partly substantiated by the observation that when an excised portion
of cortex is incubated at body temperature, a substance develops in it
which has an action like that of the hormone of the medulla — epi-
nephrine. It is possible, however, that this action is due to the fact that cer-
tain of the decomposition products of protein develop an epinephrine-
like action (see page 502).
The Medulla
Histologically the medulla is composed of masses of polygonal cells
with blood sinuses between them. The blood supply is derived from ves-
sels that have proceeded to the medulla through the capsule, and it is
extremely rich, being indeed the richest blood supply to any organ in the
body, greater even than that to the thyroid gland. The nerves form a
dense plexus, extending into and between the secretory cells. The most
characteristic feature of the cells composing the medulla is the presence
in them of granules which stain readily with chromic acid, and are hence
often called chroma ffin cells. There are also some cells containing coarser
granules that are soluble in water and do not stain with chrome salts.
Embryologically the medulla is developed from the same neuroblastic
cells tliat give rise to the sympathetic nervous system. This evidence of
the close association between the medulla and the sympathetic nervous
system, we shall see to be substantiated by the results of experimental
investigation.
On account of the anatomic relationships, it is impossible to study the
effect of excision of the cortex and medulla separately, or, indeed, of the
action of pure extracts prepared from either of these portions of the
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 733
gland. Our investigations must concern the effect of removal of the
whole gland or of the injection of extracts of it; but as we proceed to
examine the data, it will become evident that most of the effects ob-
served to occur, with the exception of those already mentioned as
definitely associated with the cortical function, can be attributed to
the medulla.
»
Adrenalectomy
Excision of the adrenal gland in most animals is very quickly fatal,
the only well-known exception being in the case of the white rat, in which
excision of both adrenals may not be incompatible with life. For some
time after recovery from the anesthetic the animal upon which double
adrenalectomy has been performed usually behaves in a perfectly normal
fashion, although it may be less lively and less inclined to feed than
usual. • Very soon, however, generally within twenty-four or forty-
eight hours, definite symptoms of muscular weakness are apparent. This
Aveakness soon becomes extreme, and is accompanied by a feeble pulse,
a depression of body temperature, and, later, by dyspnea. After an
interval which is never longer than a few days, death supervenes, being
sometimes preceded by convulsions.
When only one adrenal is removed, very few animals succumb; and
if some time is allowed to elapse so that the immediate shock of the
operation has disappeared, it will usually be found that removal of the
remaining adrenal, although ultimately fatal, is not so quickly so as
when both glands are removed at one operation. The reason for this
result is that opportunity is given for a compensatory hypertrophy of
accessory adrenal bodies to occur. Such accessory adrenal bodies may
be composed of cortical or medullary tissue, but it is the> latter that is of
importance in the present connection. Chromaffin tissue is found in most
animals along the front of the aorta, between the renal arteries, where it
can usually be recognized by staining the tissue with chromic acid.
Sometimes accessory chromaffin tissue is located in distant parts,
as in the epididymis of the rat,- for example. It is said that life can
be maintained if one-eighth of the total amount of the adrenal substance
be present in the body. Attempts to prolong life after adrenalectomy
by adrenal transplantation have almost invariably met with negative
results, because the graft undergoes a rapid process of necrosis and dis-
appears; although it is said that transplantation may sometimes be suc-
cessfully accomplished if the grafting is done into the kidney. Adminis-
tration of suprarenal extract is also without definite benefit after
adrenalectomy.
734 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
Suprarenal Extracts — Preparation
Injection, particularly intravenous, of extract of the adrenal gland
has furnished us with most of the evidence upon which our knowledge
regarding the function of this organ depends. Such an extract is best
made by grinding the entire gland with fine sand in a mortar and then
extracting with a weak (decinormal) solution of hydrochloric acid. The
extract may then be boiled, filtered through muslin and nearly neutral-
ized, preferably by means of sodium acetate. If kept in this acid reac-
tion, the active principle of the extract does not materially deteriorate
with time, but if it be neutralized or considerably diluted, destruction
due to oxidation occurs, as evidenced by -a distinct browning of the
solution. The active principle of such extracts lias been isolated in a
crystalline form (Takamine and Abel). It has been given various names
(adrenalin, suprarenin, adrenin, etc.), but the tendency is definitely
towards the use of epinephrine. Chemically, epinephrine has been found
to be orthodioxyphenylethylolmethylamine.
H0_
\ -*CH(OH) - CH,NHCH3.
It will be noted that it is closely related to tyrosine (see page 604). It
is also closely related to a group of substances (amines) occurring in
putrid meat and to which the active principles of ergot belong. It
contains an asymmetric carbon atom (asterisked in formula), which
indicates that there must be three varieties of epinephrine, differing
from one another in the effect which they produce on the plane of
polarized light (i.e., a dextro- and a levo-rotatory and a racemic form).
Epinephrine can be prepared by synthetic means, the first product of
this synthesis being the racemic salt, which can then be split lay appro-
priate methods into dextro- and levo- varieties. The levo- variety ap-
pears to be identical in its pharmacologic action with the natural product.
The dextro- variety on the other hand has only poorly developed physio-
logic activities (about seven per cent that of the levo- variety), while
the racemic variety comes in between the two in its action. A valuable
assay of the amount of epinephrine in tissue extracts can be made by
the method of Cannon, Folin and Denis,62 in which an acid extract of
the gland is treated with phosphotungstie acid, and the blue color thereby
developed compared colorimetrically with a standard blue.
Physiologic Action
The physiologic effects of the intravenous injection of epinephrine are
markedly excitatory and slightly inhibitory in nature. We will consider
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 735
the excitatory action first. Immediately after the intravenous injection
of as small an amount as 0 '00008 milligrams per kilogram of body weight,
a distinct rise in arterial blood pressure may be observed. It is quite def-
inite with 0 '00008 milligrams per kilogram, and is accompanied by a
slowing of the pulse. This slowing is caused by stimulation of the .vagus
center, as is evidenced by the fact that if the vagus nerves are cut, or
sufficient atropine administered to paralyze them, the same dose of
epinephrine produces not a slowing but a quickening of the pulse, and
consequently a much greater rise in blood pressure. The vagus action
is developed not because of an effect of epinephrine on the vagus center,
but secondarily because of the rise in blood pressure.
These preliminary experiments indicate that the locus of action of
epinephrine, so far as the circulatory system is concerned, is mainly on
the small blood vessels, constricting them and thus raising the peripheral
resistance. This conclusion can readily be confirmed by applying the
epinephrine directly to the blood vessels of the exposed mesentery, or
by enclosing a vascular organ such as the kidney in a plethysmo graph
during the injection of epinephrine, when a great diminution in volume,
accompanying the rise of arterial blood pressure, will be observed. The
vasoconstricting effect of epinephrine does not become developed on the
large blood vessels near the heart on account of the deficiency in muscu-
lar tissue in their walls. Indeed, these vessels may become passively
.dilated because of the increased blood pressure. The arterioles of dif-
ferent parts of the circulation are not equally sensitive to epinephrine;
those of the splanchnic area are most sensitive, whereas those of the
heart — the coronary vessels — do not respond at all in most animals (see
page 257). The pulmonary and cerebral vessels have a variable reactivity
to epinephrine.
The effect on the vessels persists after complete destruction, not only
of the central nervous system, but also of the vasomotor nerves; epi-
nephrine still acts, for example, on vessels the nerve fibers of which
have been allowed to degenerate by cutting them several days before the
epinephrine is applied. This would seem to indicate that the epinephrine
acts directly on the muscular tissue in the Avails of the blood vessels,
but this does not appear to be the case, for it has been found that epi-
nephrine is incapable of acting on tissues which are devoid of sympathetic
nerve fibers, and is also inactive on those tissues in the embryo which have
not yet received any nerve supply. In brief, then, although epinephrine
acts only on blood vessels that are supplied by the sympathetic nervous
system, it is not on the nerve fibers that the epinephrine unfolds its
action. "We shall see immediately that this conclusion is in conformity
736 THE ENDOCRINE ORGANS, OR 'DUCTLESS GLANDS
with the results of observations made on structures other than the blood
vessels.
Other muscular structures excited by epinephrine are as follows:
(1) the dilator muscle of the pupils, especially after the nerve supply has
been destroyed by extirpation of the superior cervical ganglion; (2) the
sphincters of the pylorus and of the ileocecal valve; (3) the muscle fibers
of the spleen, the vagina, the uterus, the vas deferens, and the retractor
penis. Eegarding the action on the uterus, however, it should be noted
that a different response may be obtained according to whether the
uterus is pregnant or not. The plain muscles of the orbit and globe of
the eye are sometimes excited by suprarenal extract, causing the eyes to
protrude, the palpebral fissure to become large and the third eyelid to
be retracted, changes which are very like those which develop as a
result of 'fright.
Inhibitory effects of epinephrine on muscle are exhibited by the follow-
ing: (1) the muscle of the intestine; (2) the stomach; (3) the esophagus;
(4) the gall and urinary bladders.
The effect of epinephrine in inhibiting the rhythmic contractions of
an isolated portion of the intestine in oxygenated Ringer's solution is a
very striking phenomenon, and one which, as we shall see, may be very
successfully employed for detecting small quantities of epinephrine.
Extremely dilute solutions of epinephrine increase the contractions.
The effects of epinephrine on glandular structures are the same as those
which would be produced by stimulation of the sympathetic nerve supply
of the gland. Thus, the secretions of the lachrymal gland, the salivary
gland (in the cat), the mucous glands of the mouth and pharynx, the
gastric but not the pancreatic glands, can readily be shown to be
excited.
From these results as a whole, it is evident that the effect of epineph-
rine on muscles and glands is exactly the same as that which would be
produced by stimulation of their sympathetic nerve supply. This paral-
lelism of action between epinephrine and the sympathetic nervous sys-
tem becomes still more evident when we consider certain of the changes
in metabolism that follow administration of epinephrine. Injection of
epinephrine excites glycogenolyste in the liver so that hyperglycemia
and glycosuria become established, results which are also obtained by
stimulating the great splanchnic nerve. Epinephrine causes the clotting
time of the blood discharged from the liver to be very materially short-
ened, an effect also produced by stimulating the splanchnic nerve.63
As in the case of the blood vessels, the above results are obtained even
after the sympathetic nerves to the part have been allowed to undergo
degeneration, from which it is concluded that the tissues elaborate some
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 737
substance which reacts with epinephrine. This substance may be pro-
duced either at the junction between the nerve and muscle — the myo-
neural junction, — or perhaps throughout the protoplasm itself. It is
called the receptor substance of Langley, and is believed to react not
only with epinephrine, but also with various drugs. The receptor sub-
stance seems to increase, if not in amount, at least in sensitivity after
the removal of the nerve control.
Ergotoxin, which is an amine obtained from ergot and also from cer-
tain of the products of histidine, has an action on the receptor substance
which is inhibitory and therefore antagonistic to that of epinephrine.
The antagonistic action of ergotoxin affects the excitatory but not
the inhibitory actions of epinephrine. By using this drug we are en-
abled to show that, although the main effect of epinephrine on tissue is
excitatory, a less marked inhibitory influence may be simultaneously
developed. The inhibitory effect may also sometimes be evoked by
doses of epinephrine very much smaller than those used to produce
excitatory effects. These facts are well illustrated in the case of the
muscle fiber of the blood vessels. With an ordinary dose of epinephrine
constriction occurs; after ergotoxin the same dose of epinephrine causes
dilatation. Or this latter result may also be obtained by administer-
ing to a normal animal quantities of epinephrine that are very much
smaller than the usual quantity. The coexistence of inhibitory and ex-
citatory influence is also well noted in the case of the uterus. In some
animals the effect of epinephrine on this organ is to augment its rhythmic
contractions, in others to inhibit them. In the former case, however, if
ergotoxin is first of all administered, epinephrine in its usual dosage will
invariably produce an inhibitory effect. The ergotoxin no doubt acts on
the receptor substance, and similar effects have also been produced with
apocodeine.
Although it is especially on plain muscular fiber having a sympathetic
nerve supply that epinephrine unfolds its action, yet, according to Can-
non, it increases the contracting power of voluntary muscle and dimin-
ishes the tendency to fatigue.*
"For further details of these effects the papers of Iloskins"-' and IIartniaiir'1 should be consulted.
CHAPTER LXXXII
THE ADEENAL GLANDS (Cont'd)
Variations in Physiologic Activity
Since it is clearly established that the adrenal glands are indispensable
to life and that extracts of them have a v6ry pronounced physiologic action,
it remains to consider whether the glands produce this internal secre-
tion within the body, and if so, whether it is essential for the well-being
of the animal or required only under certain conditions. We must also
endeavor to find out upon which of the bodily functions of the intact
animal the internal secretion acts. These problems have been attacked
by three methods of investigation: (1) by comparing the epinephrine
content of similarly prepared extracts of the resting gland and of one
removed after a period of supposed increased activity; (2) by collecting
the blood as it flows into the vena cava from the adrenal vein and ex-
amining it for epinephrine by physiologic tests. These consist in observ-
ing the behavior of some tissue that is sensitive to -the action of epineph-
rine, such as the intestine or uterus, after applying the .blood or serum
to it, or by injecting the blood or serum intravenously into another ani-
mal and looking for epinephrine effects; and (3) by allowing the blood
of the adrenal vein to be discharged under certain conditions through
the vena cava into the blood vessels of the same animal, and observing
the effect produced on certain physiologic processes which in one way
or another have been sensitized toward the influence of epinephrine.
This autoinjection method has recently been used successfully by Stew-
art and Rogoff,66 their favorite structure upon which to observe the
epinephrine effect being the denervated pupil.
Assaying the Epinephrine Content of the Gland
With regard to the first mentioned of the methods, either chemical or
physiologic means may be employed to assay the strength of the ex-
tracts. The best chemical method is that of Cannon, Folin and Denis,ea
the principle of which has already been described. The physiologic
method yielding most satisfactory results is that of Elliott,67 which con-
sists in injecting a portion of the extract intravenously into animals
from which the influence of the nerve centers on the heart and blood
vessels has been removed by decapitation. The rise in arterial blood
738
THE ADRENAL GLANDS 730
pressure produced by the injection is then a very fair measure of the
amount of epinephrine contained in it. It has been shown that the re-
sults obtained by the chemical method agree very closely with those obtained
by the physiologic, but it should be remarked that it is difficult to see how the
physiologic method could be accurate in all cases, vsince it has been
shown that with great dilution of epinephrine a reversed effect — a vaso-
dilatation — may be obtained. Attempts to assay the strength of an
epinephrine solution by investigating the effects which it produces on
other preparations, such as isolated loops of intestine or uterus; or the
enucleated eyeball of the frog, must prove unsuccessful, since the effects
are by no means dependent on the concentration of epinephrine in the
extract. When such preparations are used for quantitative purposes,
the strength of the extract must be judged by finding the extent to
which it can be diluted and still remain active.
Quite apart from the foregoing possible sources of error, it must be
remembered that the results merely give us an idea of how much epineph-
rine may have been contained in the gland at the time of its excision.
They can not tell us how much epinephrine the gland was secreting. Prior
to excision as much of this hormone might have been undergoing a process
of manufacture in the gland as was being discharged from it, so that the
assayed amount would represent merely the balance of production and loss
of hormone by the gland. We might quite well find that the amount of
epinephrine in the excised gland was normal under conditions where
there had been an excessive discharge of it into the blood; that is to say,
loss and production might have been equal. Where, however, a marked
deficiency is found to exist, it probably indicates that exhaustion of the
power of producing epinephrine was taking place.
The Epinephrine Content of the Blood. — The second method, in which
blood from one animal is tested for its epinephrine effect by intravenous
injection into another animal or by applying it to some isolated prepara-
tion on which epinephrine acts, has yielded important results. Since
serum contains all the epinephrine of blood, it can be conveniently used
for the tests (Stewart and Rogoff). The isolated physiologic prepara-
tions that have been used in testing for epinephrine in the animal fluids
are as follows:
1. A segment of the small intestine of a rabbit, suspended in oxygen-
ated Locke's solution at body temperature.
2. A segment of the uterus of a nonpregnant rabbit similarly prepared.
The apparatus used for observing the contractions of either prepara-
tion consists of a small glass chamber furnished below with a hook to
which one end of the segment is attached, the other end being connected
740
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
to a muscle lever, so that the regular rhythmic contractions can be regis-
tered on a drum (Fig. 190).
Epinephrine inhibits the contractions of the intestine but stimulates
those of the uterus of most animals, the intestine preparation being the
more sensitive (Fig. 191). Indeed, it is said that the inhibition in this
case may be obtained with a solution containing 1 part of epinephrine in
20,000,000 of solution. In using this method, however, great care and
judgment must be exercised in drawing conclusions, because other sub-
stances present in the blood are liable to affect the contractions; thus,
Air. vent
Metal waterbath
38'c.
Harvard muscle
warmer with
radiated scale
n. metal
heating rod
soldered in
wall of
water bath
Fig. 190. — Arrangement of apparatus for recording contractions of a uterine strip, intestinal
strip, or ring, etc. The metal water-bath is made of a cheap metal water-pail with a heating rod
soldered through the side at the bottom. A short metal tube is soldered into a 1-inch opening in
the bottom to receive a perforated cork for connecting with the Harvard muscle-warmer inside.
(From Jackson.)
certain substances in blood serum which have been produced by the act
of blood clotting may cause augmentation of the beat in both the intes-
tinal and the uterine preparations. A certain amount of epinephrine in
Locke's solution is consequently more likely to cause inhibition of the
intestine than a similar amount added to blood serum, because in the lat-
ter case the pressor substance will neutralize the depressor effect of the
epinephrine. On the uterine preparation, both the blood serum and the
epinephrine have pressor effects. As has been pointed out by G. N.
Stewart,68 if both preparations are employed for testing a solution sup-
THE ADRENAL GLANDS
741
posed to contain epinephrine, little chance of error is likely to be in-
curred; that is, if the solution produces inhibition of the intestine along
with augmentation of the uterus, it must contain epinephrine.
3. The fresh carotid artery of the sheep. A ring cut from the artery
is suspended in oxygenated Locke's solution and attached below to a
Fig. 191. — Tracing showing the effect of epinephrine on the intestinal contractions and on the
arterial blood pressure. (The preliminary addition of barium to the nutritive fluid may be disre-
garded.) (From Jackson.)
small hook and above to a light muscle lever, by which the contraction
of the muscle fibers can be observed. Epinephrine causes the muscle to
contract, but the test is not so sensitive as the foregoing, especially in
the presence of blood serum, because the pressor substances therein con-
tained also cause contraction. Blood plasma does not contain the pres-
sor substances, so that oxalated plasma should be used in place of serum
742
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
in applying the test. 'To increase the sensitiveness of the muscle, the
artery ring should be slightly stretched. by loading the lever.
4. The Wood vessels of a frog. This method depends on the same prin-
Funnel, or
small pressure
bottle
Hook through
lower jaw
Cannula In
one aorta
I'tg. 192. — Arrangement of apparatus for pcrfusion of the vessels of a brainless frog. (From
Jackson.)
ciple as in that just described. The fluid supposed to contain epinephrine
is added to Locke's solution, which is meanwhile being perfused under
constant pressure through the blood vessels and the rate of outflow
THE ADRENAL GLANDS 743
noted (Fig. 192). If the fluid added to the inflowing fluid contains epi-
nephrine, the outflow will become diminished. This is a very satisfactory
method, although it is somewhat limited in scope unless large frogs are
procurable, because of the difficulty of getting the necessary cannulas
into the vessels (aorta and abdominal vein).
5. The pupil of the enucleated eye of the frog. Extremely small traces
of epinephrine are observed to cause a dilatation.
6. The denervated iris. The fluid to be tested is placed in the conjunc-
tival sac of an animal from which the superior cervical ganglion of the
corresponding side has been removed some days previously. Under such
conditions, if epinephrine is present in the fluid, dilatation of the pupil
occurs. Both of the preceding reactions we owe to Meltzer.70
It should be emphasized that, although each of these methods is in
itself very sensitive for the detection of epinephrine without being al-
ways specific, yet the result should not be considered conclusive unless
definite effects have been secured by at least two methods that are as
far as possible independent of each other.
As an outcome of investigations by these methods it has been found
that, when blood from the adrenal vein is collected in a pocket of vena
cava made by applying clamps above and below the entrance of the
adrenal veins, the presence of epinephrine can be revealed, the rate of
secretion being from 0.0003 to 0.001 mg. per kilogram of body weight
per minute (Stewart and Rogoff). The absolute amount of epinephrine
liberated from the gland can be measured only by finding the concen-
tration in the adrenal vein blood and the rate of bloodflow. This amount
is approximately constant, so that the concentration in the blood which
collects in the cava pocket varies inversely with the rate of bloodflow.
In asphyxia the bloodflow is decreased so that the concentration of epi-
nephrine increases, but there is no change in the absolute amount. Nei-
ther anesthesia nor trauma affects the amount. The concentration is
likely to rise late in an experiment because of the slowing of bloodflow.
Adrenal activity may, however, be excited by massage of the gland, or
by stimulation of its nerve supply through the great splanchnic nerve.
The presence of epinephrine in blood collected directly from the adrenal
veins does not justify us in concluding that, when mixed with the re-
mainder of the blood in the body, there would be a sufficient concentra-
tion of this substance to develop any of its activities. It has therefore
been necessary to devise methods by which this possibility could be
tested.
The Autoinjection Method. — Such a method was first of all success-
fully used by Asher, who employed an animal from which all the abdom-
inal viscera had been removed. On stimulation of the great splanchnic
744 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
nerve a rise in arterial blood pressure occurred provided the adrenal
veins were open, but not so if the adrenal veins were clamped. By re-
moving the viscera, the effect of splanchnic stimulation on the abdom-
inal blood vessels themselves is eliminated, and any constriction which
occurs in the blood vessels of the rest of the body must obviously be due
to the action of epinephrine.
The most satisfactory of these methods is that more recently employed
by Stewart, Rogoff and Gibson,69 which consists in observing the be-
havior of the pupil on the side from which the superior cervical ganglion
has been removed about one week previously. Of course the blood pres-
sure effect is also observed.
Among the most important results secured by this method it may be
mentioned that dilatation of the pupil occurs on stimulation of the great
splanchnic nerve, provided the vena cava and adrenal vein are unobstructed
so that the blood from the adrenal glands can get to the head. If the vena
cava is clamped and the splanchnic nerve stimulated, there is no pupil-
lary dilatation, but it immediately occurs after the clamp is removed.
Epinephrine continues to be discharged for a considerable period of time
after stimulating the splanchnic nerve, but the immediate increase which
follows the application of the stimulus does not last long, so that more
secretion can be obtained by intermittent than by continuous stimula-
tion. It does not seem to be' possible to exhaust the adrenal gland of its
supply of active material by stimulating the splanchnic — a fact which
would seem to throw considerable doubt on the reliability of the con-
clusions arrived at by the use of those methods in Avhich extracts of the
gland are assayed (see page 739).*
Many interesting facts concerning the nature of the innervation of the
gland have been secured by one or other of the above methods. After
section of the sympathetic chain and the great splanchnic nerves on both
sides (in the thorax), no epinephrine is secreted into the blood of the
adrenal vein, and when one gland is extirpated and the nerve connec-
tions of the other entirely cut, the epinephrine content of the adrenal
vein blood sinks to not more than 1/1000 of the normal amount. The
animals survive this latter operation and behave in a perfectly normal
fashion, indicating that the internal secretion of the adrenals can not have
the physiologic significance so often ascribed to it.
The splanchnic fibers concerned in the secretion of epinephrine seem
to come from a nerve center situated relatively low down in the spinal
cord. Section of the cord at the level of the last cervical segment does
not affect the spontaneous secretion, but this disappears when the section
is made beloAv the third thoracic segment. (Stewart and Rogoff.)
"Another great advanlage of tl'e aufoinjection method is that no confusion can be caused by
the development of prcssor substances through clotting.
THE ADRENAL GLANDS 745
In connection with these observations it is of interest to note that dur-
ing stimulation of the splanchnic nerve in a normal animal, the conse-
quent rise in blood pressure shows two peaks (see Fig: 29, page 137). The
first is no doubt due to direct stimulation of the splanchnic vasoconstric-
tors, and the second to the outpouring of epinephrine into the blood, the
justification for this conclusion being that the latter rise fails to appear
after removal of the adrenal glands.
Taking the results as a whole, it is indeed doubtful whether under nor-
mal conditions a sufficient amount of epinephrine is discharged into the
blood of the vena cava to affect appreciably the tone of the blood vessels,
and this conclusion seems all the more justified because of the fact that
small quantities of epinephrine have a dilating rather than a constricting
influence, at least on certain vessels (Hartman64). It maybe, however, that
the maintenance of vascular tone under certain conditions is greatly as-
sisted by the presence of epinephrine in the blood. Similarly the sympa-
thetic control of other functions may be facilitated by the presence of
small amounts. It has been found, for example, that, although stimula-
tion of the celiac plexus causes the glycogen stored in the liver to be con-
verted into sugar, this result is not as a rule obtained on stimulating
plexus shortly after removal of the adrenal glands. The presence
of epinephrine in the blood would, therefore, seem to be necessary to bring
about functional activity of the sympathetic nerve endings concerned in
the glycogenolytic process (see page 637);
Adrenalemia. — In the light of these researches it is important to point
out that a great part of the work done by clinical observers purporting to
show that in such conditions as nephritis and arteriosclerosis there is an
increase of epinephrine in the blood, has been found by Stewart and
others, using controlled methods, to be entirely unproven.70 Some inves-
tigators, however, still hold that temporary conditions, such as transient
rises of arterial blood pressure or temporary glycosuria, may sometimes be
due to increased adrenal discharge into the blood.
Ephinephrine has been thought to be a substance which is secreted into
the blood in supernormal amount when certain emergencies arise, the most
important of these being fright, or some other extreme emotion. This
belief has arisen partly from the similarity between the general behavior
of an animal following the intravenous injection of epinephrine and dur-
ing states of extreme excitement. Dilatation of the pupils, bristling of
the hair, salivation, rise in arterial blood pressure, inhibition of the intes-
tinal movements, protrusion of the eyeballs are all symptoms of fear just
as they are of epinephrine injection. Impressed by these resemblances
Cannon72 undertook an extended research to test the hypothesis that the
reaction of an animal to fear and other emotional states is dependent on
746 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
hypersecretion of epinephrine into the blood. The results seemed to con-
firm the hypothesis. In the first place, it was found that, whereas the blood
drawn from the vena cava opposite the entry of the adrenal veins (by
passing a catheter up the femoral vein till its free end lay at this level) in
a normal male cat did not give evidence of the presence of epinephrine
when tested by means of the intestinal segment method, it did so in a
cat that had previously been frightened by allowing a dog to bark at it.
Such results were not obtained after removal of the adrenal gland, or in
a female cat, which is usually indifferent to such a method of frightening.
Cannon also thought that many of the other adaptations which take place
in an animal in this condition could be attributed to the presence of an
excess of epinephrine in the blood. The three most important of these
are: (1) increased discharge of sugar from the liver into the blood; (2) in-
creased efficiency of muscular contraction; (3) diminished clotting time of
the blood — all of which are adaptations enabling the animal either to con-
quer the source of the fear or to be in a better position to recover from
any bodily injury involving a loss of blood should he suffer bodily dam-
age. Stewart and Rogoff have more recently thrown considerable doubt
on these conclusions by finding that cats in which both adrenal glands are
entirely removed from the influence of the nervous system, behave like
normal animals when frightened, and develop hyperglycemia when as-
phyxiated or etherized. It is scarcely necessary to point out that, until
it is definitely established by experimental investigation that epinephrine
may be discharged in excessive amounts under certain conditions, it is
irrational to assume that such may occur in disease. The surgical removal
of tlie adrenal gland is certainly not warranted under any circumstances,
The Association of the' Adrenal with Other Endocrine Organs
We have at present very little accurate and reliable information on the
association of the adrenal with other endocrine organs. That epinephrine
has an influence on many diverse organs and glands is an undoubted
fact, but this is more probably to be attributed to an activating influence
on sympathetic nerve endings than to any specific relationship between
the adrenal glands and the gland in question. The most important of the
results that have been obtained are the folloAving:
1. With the Thyroid and Parathyroid. — Cannon and Cattell, after con-
firming Bradford's discovery that an electric current of action is set up in
the salivary gland when it is excited to activtiy, proceeded to investigate
the occurrence of such a current in the thyroid gland.73 By placing one
nonpolarizable electrode on the gland itself and the other on the neigh-
boring subcutaneous tissues or on the trachea, a current was found to be
set up by stimulation of the sympathetic nerve supply of the thyroid, by
intravenous injection of epinephrine, .or by stimulation of the great
THE ADRENAL GLANDS 747
splanchnic nerve before it reaches the adrenal gland. This last result,
which is the most important in the present connection, Avas, however, not
observed when the blood of the inferior vena cava was prevented by the
application of a clamp from getting to the heart, but immediately ap-
peared, after stimulation, when the clamp was removed. This experiment
taken alone does not, however, justify the conclusion that there is any
direct relationship between the adrenal glands and the thyroid, because
there are in the thyroid gland structures such as the muscle fibers in the
blood vessels, which a hypersecretion of epinephrine might affect. Before
any direct relationship between the two glands could be claimed to exist,
it would be necessary to show that the thyroid action current is obtained
with a concentration of epinephrine in the blood lower than that affecting
the blood vessels.
2. With the Sexual Glands. — As mentioned above, a very direct rela-
tionship exists between the development of the sexual glands and that of
the suprarenals,. particularly the cortex of the glands. In addition to the
evidence above furnished, it may be mentioned that in hyperplasia of the
adrenals changes occur in the testicles, particularly in their interstitial
cells.
3. With the Liver. — Of the many functions of this gland that which is
most directly associated Avith epinephrine is the production of glucose
from glycogen — the glycogenolytic process (see page 669). The injection
of epinephrine causes an immediate discharge of such an excess of glucose
into the blood that hyperglycemia and glycosuria immediately folloAV.
This result is most striking Avhen the injection is made in glycogen-rich
animals. In animals from Avhich all the glycogen of the liver has been
removed by starvation, the injection of large amounts of epinephrine
causes glycogen to accumulate in the liver cells — a result Avhich it is
difficult to interpret.
In the light of the fact that stimulation of the great splanchnic nerve
causes a demonstrable increase of epinephrine in the blood, a natural con-
clusion is that the glycosuria and hyperglycemia Avhich are known to re-
sult from stimulation of the splanchnic nerve or of its center in the
medulla, must be dependent upon a hypersecretion of epinephrine.
Evidence supporting this hypothesis seemed to be furnished by the obser-
vation that, after the removal of the adrenal glands, stimulation of the
splanchnic or of the so-called " diabetic" center in the fourth ventricle
no longer produced glycosuria even in a glycogen-rich animal. But it is
difficult to see how such an important physiologic process as that of the
nerve control of the production of sugar by the liver should be dependent
on the hypersecretion of the adrenal gland, especially since the epineph-
rine Avould have to be carried by the blood around a considerable part of
748 'THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
the circulation before it arrived at the place on which it is to act. More-
over, it has been shown that "stimulation of the previously cut hepatic
nerve plexus (around the hepatic pedicle) in a normal animal produces
hyperglycogenolysis, in which case there can be no question of a hyper-
secretion of epinephrine.
No doubt the adrenal glands have some important relationship to the
nerve control of the glycogenolytic' process, for, in animals from which the
adrenal glands have been removed, stimulation of the hepatic plexus does
not produce hyperglycemia. From this result it would appear that the
presence of a certain amount of epinephrine in the blood is necessary for
the proper transmission of the nerve impulse from the sympathetic nerve
fibers to the liver cell. When the nervous system is stimulated in such
a way as to excite the glycogenolytic process, two effects both operat-
ing in the same direction with regard to the glycogenic function are
developed: the one, a hypersecretion of epinephrine, which activates
the sympathetic nerve endings, the other, the transmission of the nerve
impulse to the liver cell (Macleod arid-R. G. Pearce).74
4. With the Pancreas. — The function of the pancreas here concerned
is that of its supposed internal secretion from the Isles of Langerhans.
Since epinephrine readily produces glycosuria, and since excision of
the pancreas has the same effect, it has been natural to inquire whether
any relationship exists between the two glands, and some observers
have obtained results which they interpret as indicating that it does.
Certain observers even state that glycosuria does not occur after the
injection if at the same time extract of pancreas is injected. It is al-
most certain, however, that these results are not trustworthy. Thus,
removal of the adrenal glands in an animal suffering from pancreatic
diabetes does not restore any of the lost power of utilizing glucose
during the feAV hours that the animal remains alive.74 That some rela-
tionship may, however, exist is indicated by the fact that epinephrine
causes dilatation of the pupil when it is dropped into the eye of a per-
son suffering from diabetes, whereas it has no such effect in the normal
individual.
CHAPTER LXXXIII
THE THYROID AND PARATHYROID GLANDS
Structural Relationships
The thyroid and parathyroid glands are intimately associated, anatom-
ically, in most animals. The thyroid is present in all the vertebrates,
but the parathyroids do not occur below the amphibia. The thyroid
exists as two lateral lobes joined over the trachea by the so-called isthmus.
The parathyroids are very much smaller, being four in number and
located in pairs on the posterior aspect of the thyroid lobes.- The two upper
parathyroids are usually more or less embedded in the thyroid tissue,
whereas the lower ones are much more loosely attached to the thyroid;
indeed, in some animals they are quite separate from it and may be
located at a distance, as in the mediastinum. Accessory thyroid and
parathyroid glands are sometimes present in the tissues of the neck, or
in the anterior mediastinum, accessory parathyroids being common in
the rabbit and rat, and parathyroid tissue being present in the thymus
in 5 per cent of dogs (Marine75). Before these anatomic relationships
were thoroughly worked out, there was much confusion in the interpre-
tation of the results following removal of one or the other gland.
In their histologic structure and embryologic derivation, the two
glands are very different. The parathyroids are developed as an out-
growth from the third and fourth branchial pouches, and they are com-
posed of masses of epithelial-like cells, sometimes more or less divided
up into lobules or trabeculae by bands of connective tissue. The cells
contain granules, some of which are of a fatty nature. Sometimes col-
loid-like material is found between the cells, or it may be enclosed in
small vesicles not unlike those of the thyroid, although usually consider-
ably smaller. -The blood vessels are extremely numerous, and form
sinus-like capillaries, which come into close relationship Avith the epi-
thelial cells of the glands. Nerves also are abundant and pass both to
the vessels and to the secreting cells. The blood vessels are derived from
the inferior thyroid artery.
The thyroid is developed by immediate outgrowth from the entoderm
lining the floor of the pharynx, at a level between the first and second
branchial pouches. Represented at first by a solid column of cells,
there very soon occurs a division at the lower end into two lateral por-
749
7f)0 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
tions, and the original solid column becomes hollowed out. The two
lateral branches of the original column divide again and again so as to
form a system of hollow tubes lined with epithelium. These afterward
become cut up so as to form the closed vesicles characteristic of the
gland. Each vesicle is more or less spheroidal in shape, and has no
basement membrane, but its walls are formed by a layer of epithelial
cells, which may be columnar, cubical, or flattened in shape. Each vesicle
is filled with the so-called colloid material, which is peculiar in con-
taining iodine, and between the vesicles is a layer of connective tissue
often containing small cells, some of which are not unlike those of the
parathyroid. The connective tissue also contains the blood vessels,
which are very numerous — indeed, the thyroid, in proportion to its size,
receives more than five times as much blood as the kidneys, the only
tissue that surpasses it in this regard being the medulla of the adrenal
gland (see page 211). The nerves arise from both the vagus and the
sympathetic systems and have been traced to the secreting epithelial
cells. The above description applies to a strictly normal gland.
THE THYROID GLAND
Condition of the Gland
In the crowded communities of the Great Lakes Basin of this conti-
nent, it has been found that in most animals the thyroid gland is more or
less abnormal. In Cleveland, for example, Marine has found this to be
the case in well over 90 per cent of the dogs brought to the laboratory.77
The condition usually goes under the name of simple goiter, which in-
cludes all thyroid enlargements except those of exophthalmic goiter.
In man the goiter originates usually about the age of adolescence and
more frequently in girls than in boys. It may sometimes pass over into
the exophthalmic type. The exact pathologic changes in the goitrous
gland vary with the species of animal and with the duration of the dis-
ease. In man, besides the cystic or colloid goiter an adenomatous type
is very common although rare in other animals.
From the numerous observations that have been made on the glands of
domestic animals, it has been clearly established that the very earliest
sign of goiter is a diminution in the iodine content of the gland; fol-
lowed by an increase in the epithelial cells and in the blood supply and a
decrease in the colloid. Such kyperplasia may be induced in what re-
mains after removal of a large part of a normal gland (compensatory
hyperplasia), or if a similar operation be performed early in pregnancy,
the young when born will be found to have hyperplastic thyroids. A
certain degree of hyperplasia exists as an accompaniment of pregnancy,
THE THYROID AND PARATHYROID GLANDS
751
and it can be produced in normal animals (particularly rats) by placing
them on an excessive meat diet. Important observations bearing on this
point have been made by Marine on brook trout, in which it has been
found that the so-called carcinoma that develops when the fish kept in
hatcheries are fed with unsuitable food and overcrowded, is really a
typical hyperplasia. In its second stage this develops into what is known
/^*v
c.
Fig. 193. — Microphotographs of thyroid gland of dog. A, normal hyperplasia; B, active hyper-
plasia; C, colloid goiter. (From Marine and L,enhart.)
as colloid goiter which is produced by a deposition of colloid material
between the rows of cells so as to cause an opening out again of the
vesicles (Fig. 193), with a consequent tendency to a reversion to the
normal histologic structure, so far as this is possible. The vesicles in
such a gland are of enormous size, and the lining epithelium, low cubical,
or almost flat in shape.
The outstanding characteristic feature of the colloid material is that
752 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
it contains iodine, which exists in combination with a nonprotein nitrog-
enous base, and is usually called iodothyrin. In the gland itself the
iodothyrin may be in combination with protein, forming iodothyro-
globulin. E. C. Kendall79 has recently succeeded in isolating a pure
crystalline substance of perfectly constant composition and containing
over 60 per cent of iodine. It is called alpha-iodine, and has been identi-
fied'as an indole compound. In extremely minute dosage it greatly affects
the energy metabolism, and is said to induce symptoms like exophthalmic
goiter. Its therapeutic value in cases of thyroid deficiency is remarkable.
Kendall believes this substance to be the active constituent of the thyroid
and to be associated with the metabolism of amino acids. For one thing,
when it is given alone no change occurs in pulse rate, whereas if amino
acids are given along with it, there is acceleration.
The importance of the relationship between the function of the thyroid
and the iodine-containing material is indicated by the changes which
occur in the percentage of iodine in the glands under varying condi-
tions of activity. Marine observed that the amount of iodine is inversely
proportional to the degree of hyperplasia of the gland, and when the
hyperplastic condition becomes fully developed, scarcely a trace of
iodine is contained in the gland. Later, when the hyperplasia gives
place to colloid goiter, the iodine increases again, both absolutely and
relatively. Moreover, it has been found that if iodide be administered
to an animal suffering from hyperplasia, the hyperplastic condition very
quickly disappears (Fig. 192) and the animal becomes normal. Thus, in
brook trout, the poor nutritive condition of the fish when hyperplasia has
developed can be immediately remedied by placing them in larger quan-
tities of running water or by adding small traces of iodide to the water.
The administration of small amounts of iodine as in ordinary salt from
salt deposits also prevents goiter in farm stock, this having been first
noted in the State of Michigan, where prior to the discovery of salt
deposits sheep breeding was an entire failure. The importance of admin-
istering small doses of iodides to school children living in goitrous dis-
tricts has recently been emphasized by Marine and Kimball.78 As small
a dose as 0.001 gm. at weekly intervals prevents goiter in puppies sus-
ceptible to it.
Experimental Thyroidectomy
A correct interpretation of the functional changes and symptoms which
follow upon partial or complete removal of the thyroid gland, or from
its disease, has proved a very difficult problem, partly because sufficient
care has not been taken to note how much parathyroid tissue was re-
moved along with the thyroid, and partly because the fact has been over-
THE THYROID AND PARATHYROID GLANDS 753
looked that the effects produced by thyroidectomy and parathyroid-
ectomy are often very different in animals of the same kind at dif-
ferent ages. Speaking generally, it may be said that the influence of the
parathyroid is focused mainly on the nerve centers and only to a second-
ary degree on the metabolic functions, whereas the reverse is the case
with the thyroid, its main effect being on metabolism, although it prob-
ably also exercises a secondary effect on the nerve centers. More so
than in the case of any other endocrine organ, our knowledge concerning
the function of the thyroid has been gained by clinical experience, and
it is difficult to say whether the cJinical or the experimental method has
contributed the greater amount of information.
The results of experimental extirpation of the thyroid vary accord-
ing to the age of the animal, and frequently they are by no means
marked, provided sufficient parathyroid tissue has been undamaged.
The symptoms are in general thickening and drying of the skin, with a
tendency to adiposity and a loss of tone of the muscle. The body tem-
perature is low and the sexual functions become subnormal. Nervous
symptoms in the direction of mental dullness and lethargy are also
usually present. Surgical removal of the thyroid in man produces the
condition known as CQchexia strumipriva. The symptoms may first of
all become apparent a few days after the operation, or they may remain
latent for years, and then develop so as' to produce the condition known
as myxedema. When nervous symptoms are prominent in cachexia
strumipriva, it is usually taken as evidence that an excessive amount
of parathyroid tissue has been destroyed. Kocher states that after com-
plete loss of the thyroid, life is impossible for more than seven years,
and that to prevent ultimate ill effects, at least one-fourth of the organ
should be left intact.
Disease of the Thyroid
The symptoms of diseased conditions of the thyroid may be inter-
preted as the consequence of increased or diminished functioning of the
gland. Sometimes, however, the less active gland is really increased in
bulk, this increase being caused by the accumulation in it of very large
quantities of colloid material accompanied by an attenuated condition
of the vesicular cells (see page 751). When the gland is atrophied at
birth, the condition of cretinism soon becomes developed (Fig. 194). The
characteristic features of cretinism are: (1) An arrest of growth, espe-
cially of the skeleton, accompanied by incomplete ossification of the long
bones and failure of the fontanelles of the skull to close properly. (2)
Poor development of the muscular system. (3) An unhealthy, dry, swollen
condition of the skin, so that it is yellowish in color, the face being pale
754
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
and puffy. (4) An abnormal development of the connective tissues
causing a shapeless condition of the surface; the abdomen is always
swollen, the hands and feet are shapeless, and the nose depressed. (5)
The nervous system also fails to develop properly, so that at the age of
puberty or over, the child remains like an infant in his mental behavior,
idiotism being common. Indeed, the whole clinical picture is so char-
acteristic that once having seen a case no one can fail afterward to
Fig. 194. — Cretin, nineteen years old. The treatment with thyroid extract started too late to be ot
benefit. (Patient of Dr. S. J. Webster.)
recognize the disease. Besides being due to congenital absence of the
thyroid (sporadic type), cretinism may also occur as a result of goitrous
degeneration of the gland. This forms the so-called endemic variety of
the disease, and is more commonly seen in goitrous districts, being not
infrequently associated with disease of the parathyroid, in which case
the nervous symptoms are very prominent.
Atrophy of the thyroid in adults causes the clinical condition known
THE THYROID AND PARATHYROID GLANDS
755
as myxedema, and here again the symptoms are very characteristic (Fig.
195). The skin is dry and thick, with a deposition of connective tissue
often containing fat in its deeper layers; the hands and feet become
unshapely; the lips thick and the tongue somewhat enlarged, so that
when the person attempts to speak, it appears as if the tongue were too
large for the mouth ; the hair falls out ; there is a low body temperature,
and it can be shown that the energy metabolism is greatly depressed, and
that a deficiency of oxygen is being consumed. It is said the person can
take a larger quantity of sugar than an ordinary individual without the
development of glycosuria, but the depression of the metabolic function
causes the patient to take sparingly of food, in spite of which, however,
the body weight may steadily increase. The sexual function becomes
A. B.
Fig. 195. — A, Case of myxedema; B, Same after seven months' treatment. (From Tigerstedt.)
depressed, and there is involvement of the nervous system as shown by
mental dullness and lethargy.
Although the thyroid gland is much atrophied in myxedema, symptoms
that are very similar may also occur when the gland is enormously en-
larged. As already explained, however, this enlargement is due merely
to an accumulation of colloidal material and is really an atrophic con-
dition. A patient suffering from endemic goiter may at first exhibit
symptoms which are usually attributed to a hypersecretion of thyroid
material into the blood (the symptoms will be described immediately),
but later these give place to symptoms not unlike those of myxedema.
It is concluded that the above conditions are due to deficiency of
thyroid function, or hypothyroidism, because: (1) the gland is atrophied,
756 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
and (2) similar symptoms to those exhibited by the clinical conditions
can be produced experimentally by the removal of the gland in animals.
By observations on the effect of administration of thyroid extract to
cretinous or myxedematous patients, prompt amelioration of the symp-
toms occurs, which certainly suggests that the real cause is the absence
of an internal secretion. There is probably nothing more striking in
the whole domain of therapeutics than this effect from the administration
of thyroid extract or, more so still, of alpha-iodine. If the treatment is
started early enough, the cretinous child from being an ill-developed
idiot quickly 'catches up with children of his own age and becomes in
every respect normal. Even if this treatment is not undertaken until
the child is several years of age, it is remarkable how quickly the benefit
may show itself. In myxedema and cachexia strumipriva also, the
symptoms very quickly disappear and the person becomes perfectly nor-
mal by the treatment. In all these conditions, however, the thyroid
extract must be administered continuously in order to prevent the reap-
pearance of symptoms.
Quite distinct from the above described conditions of hypothyroidism
are those produced by an excess of thyroid autacoid in the blood, namely,
hyperthyroidism. Such a condition can be produced experimentally in
normal animals by the administration of thyroid extract or alpha-iodine
(Kendall). In man large doses are soon followed by great quickening
of the pulse with some irregularity, flushing of the skin, increased per-
spiration, tremor in the limbs, emaciation, and marked nervous excita-
bility. Along with these symptoms, metabolic investigations have shown
that the energy output per square meter of surface is greatly increased,
being sometimes nearly doubled; that the nitrogen excretion is exces-
sive; and that alimentary glycosuria is very commonly present. The
body temperature is not, however, as a rule increased, because although
metabolism is excited, yet heat loss is correspondingly increased. Ex-
ophthalmos is said to develop very occasionally after such administra-
tion, but this is doubtful. Lastly, there are usually digestive disturb-
ances, although the appetite is likely to be increased. The pulse is quick-
ened after administration of alpha-iodine only when protein food is also
taken. This is believed by Kendall to be due to the association between
the thyroid hormone and the metabolism of the ammo acids.
The symptoms following the injection of the extract are very similar
to those of the disease known as exophthalmic goiter. Indeed, the symp-
toms are so much alike in the tAvo conditions that it is scarcely neces-
sary to describe them specially for the disease except to mention that
the exophthalmos is much more likely to be present.
Like simple goiter this variety is from three to four times more fre-
THE THYROID AND PARATHYROID GLANDS 757
quent in women than in men, a fact of significance when we recall the
evidence of association between the thyroid gland and the generative
organs. It is said that the disease is usually coupled with persistence of
the thymus gland. The thyroid gland in exophthalmic goiter is enlarged,
sometimes in one lobe; it is hard and pulpy, and on auscultation a mur-
mur is heard. Histologically the gland presents a picture very like
that which has been described above as hyperplasia ; that is to say, the
vesicles have a deficiency of colloid material ; their epithelium is colum-
nar and folded up into the vesicles; and the interstitial tissue between
the vesicles is very markedly increased.
Exophthalmic goiter is almost universally claimed to be due to hyper-
secretion of the thyroid, because: (1) the symptoms of the disease are not
unlike those produced by excessive administration of thyroid to a normal
individual; and (2) they are in general opposite in character to the symp-
toms found in cases where the thyroid gland is atrophied. The blood of
a person with exophthalmic goiter when injected into mice increases their
resistance to the toxic action of acetonitrile, which is also the case after
thyroid extract has been injected. In many cases of exophthalmic goiter
partial removal of the gland is said to ameliorate the symptoms. Other
clinicians, however, state that if the patient is given proper medical
treatment, rest, and diet, equally beneficial results can be obtained.
Certain investigators, however, deny that it has yet been conclusively
demonstrated that exophthalmic goiter is due to hypersecretion of the thy-
roid (Marine). It is pointed out that, if hypersecretion were the cause of
the disease, one would expect that the injection into animals of the blood
of patients suffering from it would produce symptoms similar to those
following the injection of thyroid extract. The results of such experi-
ments, however, have been extremely confusing and very indecisive, since
it is difficult to recognize in laboratory animals many of the characteristic
symptoms, especially those affecting the skin and eyes and the general
bodily nutrition. Another difficulty in accepting the hypersecretion hypoth-
esis is the fact that an extract of a gland removed from an exophthalmic
patient has no different physiologic action on a normal animal from an
extract of a normal gland containing the same percentage of iodine.
The evidence is by no means conclusive one way or the other, and it may
well be that the observed changes in the thyroid gland are not the cause
of the symptoms of exophthalmic goiter, but merely, like the other symp-
toms of this disease, a result of some condition located elsewhere.
The Relationship of the Thyroid with OtheT Endocrine Organs
1. With the Generative Organs. — Evidence of an association between
the female generative organs and the thyroid is very strong; thus, the
758 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
thyroid becomes enlarged at puberty, during the menses, and during
pregnancy, and in thyroidectomized young animals the sexual glands
fail to develop properly.
2. With the Adrenal Glands. — (See page 746.)
3. With the Pituitary Body. — After removal of the thyroid, the pitu-
itary becomes greatly altered and enlarged, particularly the pars an-
terior, in which it is not uncommon to find that a certain amount of
vesicles containing colloid, not unlike those of the thyroid, become devel-
oped. This colloid material, however, does not contain iodine. It is said
that this increase of the pituitary after thyroidectomy does not occur if
thyroid extract be administered. Increased activity of the pars inter-
media of the pituitary is also quite plain. These facts Avould at first
sight seem to indicate that the pituitary and the thyroid can act vica-
riously, but this is very doubtful, for it has not been found that pitu-
itary extract has any beneficial effect in the treatment of goiter and myx-
edema. Nevertheless the association in function of the two glands must
be more or less close, not alone for the above reasons, but also because they
are both associated to much the same degree with the sexual organs,
and both act on the higher functions of the nervous system in much the
same manner.
4. With the Thymus Gland. — The persistence of the thymus in ex-
ophthalmic goiter, as well as the anatomic and embryologic relationship
between thymus and thyroid, is taken to indicate some close relationship.
THE PARATHYROIDS
Experimental Parathyroidectomy
Experimental parathyroidectomy yields results which vary in dif-
ferent groups of animals, undoubtedly because of the fact that in some,
such as the rat and rabbit, accessory parathyroids may exist. In . gen-
eral, however, it has been found that if more than two of the four
parathyroids be removed, very definite and pronounced nervous symp-
toms soon supervene and if all four glands be removed, a quickly fatal
result is inevitable. The most acute symptoms are exhibited by the
carnivora. They may not be apparent for a day or two after the opera-
tion, although during the period the animal is in a depressed state, re-
fusing food and losing weight rapidly. The muscles are also more or less
stiff during this stage. When more definite symptoms appear, they con-
sist of a marked abnormality of muscular contraction, leading to the
occurrence of fibrillar contractions, or tremors and, later, to cramp-like
and clonic contractions. When spontaneous movements are made, a
THE THYROID AND PARATHYROID GLANDS 759
peculiar shaking of the foot, like that made by a normal animal to shake
water off its pads, is a characteristic symptom. The slightest stimulation
of the peripheral nerves is sufficient to induce one of these attacks, which
recur with ever increasing frequency, becoming at the same time more
pronounced and accompanied by other disturbances, such as diarrhea,
profuse salivation, rapid pulse, and dyspnea (in the dog but not in the
cat). In cases that are not quickly fatal, the hair tends to be shed, and
the teeth to be improperly calcified (in young animals). Where a certain
amount of parathyroid tissue has been left — for example, one of the four
lobes — the symptoms may not appear except under conditions of special
strain to the animal economy, such as pregnancy or improper diet.
Thus, in a bitch from which three of the four glands had been removed,
no symptoms of tetany occurred until she became pregnant. Under the
same conditions it has been found that a diet of flesh is much more apt
to bring about the condition than one of vegetables or milk.
Tetany, as the above condition is called, may also become developed
in man either as the result of surgical removal of the parathyroids or
because of their improper development. The symptoms in man are very
similar to those observed in laboratory animals, the only difference being
that the muscular contractions are more likely to be tonic in. character.
Certain symptoms that may develop during pregnancy or in the course
of infectious diseases or in newborn infants have also been found to be
associated with degeneration of or hemorrhage into the parathyroid
(idiopathic tetany), and certain obscure nervous diseases in adults,
such as paralysis agitans, may possibly also be associated with changes
in this gland. Chorea, epilepsy, and eclampsia have likewise been
thought to be associated with it.
The parathyroid gland, besides influencing the nerve centers, has also
an influence on metabolism. The symptoms produced are: (1) rapid
emaciation and failure to grow; (2) a tendency to the production of
glycosuria, often detected by finding that the assimilation limit for
carbohydrate is lowered (page 652) ; and (3) most definitely of all, an
interference with calcium metabolism, as illustrated by the failure of
the teeth and bones to calcify properly. This interference with normal
metabolism led Kellogg and Voegtlin81 to study the effect produced on
parathyroidectomized animals by the administration of calcium. It was
found that the symptoms were considerably ameliorated. These authors
concluded from their results that the essential cause of tetany is a
deficiency of calcium in the blood. It is possible however that the bene-
ficial action of calcium salts in this condition is that it decreases the
excitability of the nervous system, an action which it is known to
possess.
760 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
When the tetany is the result of a complete extirpation of all parathy-
roid tissue, the symptoms can be combated by a successful transplan-
tation or graft of parathyroid tissue made from an animal of the same
species. Indeed, it has been found that the success of a graft of parathy-
roid is assured only when the graft is derived from the same kind of
animal as that from which the parathyroid has been removed. Implan-
tation into the subcutaneous tissue of a tetany patient of parathyroid
tissue obtained fresh from the deadhouse has been performed with bene-
ficial outcome.
Noel Paton, Findlay and Watson80 have recently contributed greatly
to our knowledge of the physiologic pathology of tetania thyreopriva,
as the above condition is called. The symptoms are not due to any con-
dition affecting the muscles themselves, since they disappear after sec-
tion of the nerves. Nor are they primarily dependent upon the cere-
brum or cerebellum, since ablation of neither abolishes them. This does
not imply that secondary involvement of the higher centers never oc-
curs; on the contrary, the epileptiform convulsions and disturbances of
equilibrium sometimes observed indicate cerebral or cerebellar involve-
ment, respectively. This leaves some part of the lower neuron reflex
arcs as the site of involvement. It is not the afferent neuron, since the
tremors and jerkings persist after section of the posterior roots, leaving
the afferent neuron as the affected structure.
The foregoing conclusion led Paton and his co-workers to compare the
response of muscle and nerve to electric stimulation in normal and
parathyroidectomized animals. Although there are considerable varia-
tions in the responses of a normal animal, they are very definitely ex-
aggerated in tetany when either the motor neuron or the muscle itself
is stimulated, the exaggeration in the latter case being dependent upon
alterations in the neural structures (nerve endings) in the muscle. The
increased electric excitability can not, however, be taken as a measure
of the severity of the condition, for it may be no more marked in cases
in which there is involvement of the cerebral hemisphere (causing epilep-
tiform fits) than in milder cases.
As to the cause of the symptoms, many possibilities have to be con-
sidered. In the first place, no direct relationship exists between the
thyroid and parathyroid in this connection. One cause might be the
absence of some substance which checks the activity of the nervous sys-
tem, some chalone in Schafer's sense. That such is not the case is shown
among other things by the fact that bleeding and transfusing normal
saline immediately removes the symptoms for some time. Moreover,
the metabolic disturbances go on when the nervous symptoms are slight.
It had previously been thought by W. G. Macallum81 that, since symp-
THE THYROID AND PARATHYROID GLANDS 761
toms like those of tetany can be induced by deficiency of calcium in the
body and the symptoms of parathyroidectomy relieved by administration
of this cation, calcium deficiency is the cause of the symptoms. While
not denying that these ions may have some relationship to the symptoms,
Noel Paton ascribes tliem to intoxication by yiianidine (page 605). The
evidence is as follows: (1) Guanidine and methyl guanidine admin-
istered to normal animals produce symptoms that are identical with those
following parathyroidectomy. (2) There is a marked increase in the
amount of these substances in the blood and urine of parathyroidec-
tomized dogs and in the urine of children suffering from idiopathic
tetany. (3) In certain cases the serum of parathyroidectomized dogs
acts upon the muscles of the frog similarly to weak solutions of guani-
dine and methyl guanidine. (4) There is a striking similarity in the
relative amounts of the nitrogenous metabolites in the urine of parathy-
roidectomized dogs and of normal animals injected with guanidine.
It is concluded that the parathyroids control the metabolism of guani-
dine "by preventing its development in undue amounts. In this way
they probably exercise a regulative action upon the tone of the skeletal
muscles." Since it is similar with regard to its characters and metabo-
lism to the condition following thyroidectomy, it is believed that disease
of the parathyroids is the cause of idiopathic tetany.
The Relationship of the Parathyroid with Other Endocrine
Organs
We know very little of the relationship of the parathyroid with other
endocrine organs. Vincent and others have stated that after removal
of the thyroid itself enlargement of the parathyroid may occur with the
formation of colloid material between the rows of cells, but the con-
clusion that this represents a vicarious function between the thyroid and
parathyroid glands is not generally accepted. The supposed relation-
ships among the parathyroid and the pituitary and adrenal glands are
also based upon uncertain evidence.
CHAPTER LXXXIV
THE PITUITARY BODY
Structural Relationships
Situated at the base of the brain and lying in the sella turcica, the
pituitary body in man does not weigh much more than half a gram. It
is connected with the brain by a funnel-shaped stalk, the infundibulum.
On account of a natural cleft, which runs across the gland in an oblique
plane, it is an easy matter to split it into two portions, an anterior, or
pars glandularis, and a posterior, or pars nervosa. This cleft in the
case of man is usually found to be more or less broken up into isolated
cysts containing a colloid-like material, and it represents the remains of
the original tubular structure from which the pars glandularis is de-
veloped; namely, a pouch growing out from the buccal ectoderm.
On histologic examination it will be found that the pars glandularis
consists of masses of epithelial cells with large sinus-like blood capil-
laries lying between them. These blood vessels are very numerous, so
that in an injected gland this portion of the pituitary stands out very
prominently. The vessels are derived from about twenty small arterioles
that converge toward the pituitary from the circle of Willis, and enter
the gland by the infundibulum or stalk by which the gland is connected
with the base of the brain. Three types of cell can be differentiated:
nonstaining (chromaphobe) and granular (chromaphil), of which latter
there are cells with acid-staining and others with base-staining granules,
the former being by far the more numerous (Schafer).60 In some
animals such as the cat, the cells of the pars anterior are arranged around
the blood sinuses in rows as in a columnar epithelium. The cells with
acid-staining granules are said to become much increased in number in
pregnancy and also in the enlarged gland of acromegaly (see page 772).
After thyroidectomy it has been observed that colloid-like masses ac-
cumulate in the pars glandularis, the cells sometimes arranging them-
selves around these masses as in the thyroid gland. The colloid, how-
ever, contains no iodine.
The posterior part of the gland, or pars nervosa, is composed almost
entirely of neuroglia, cells, and fibers, usually with some hyaline or
granular material lying between them, particularly in the neighborhood
762
THE PITUITARY BODY
763
of the infundibulum, into which it may be traced. It is believed that
the active principle of the gland is represented by this material. The
blood supply of the pars nervosa is relatively scanty.
Between the pars nervosa and the intraglandular cleft above referred
to is a layer of cells differing from those of either the anterior or the
posterior lobe. This layer of cells constitutes the so-called pars inter-
media. The cells are somewhat like those of the pars glandularis, except
that they are distinctly granular, the granules being of the neutrophile
variety, that is to say, they stain with neither basic nor acid dyes. Well-
defined vesicles containing an oxyphile colloid material are often found
Fig. 196. — Drawing from a photograph of a mesial sagittal section through the pituitary gland
of a human fetus (5th month): a, optic chiasma; c, third ventricle; d, pars glandularis; e, infun-
dibulum surrounded by epithelial cells; /, pars intermedia; g, intraglandular cleft; h, pars nervosa.
(Herring, from Howell's Physiology.')
between them. The blood supply is much less abundant than that of the
pars glandularis. Although well separated by the cleft from the pars
glandularis, the pars intermedia is not well separated from the pars
nervosa, because many of its cells extend for some distance into the lat-
ter between the neuroglial fibers. Certain of the cells in the pars inter-
media may be seen in various stages of conversion into globular hyaline
bodies, or a granular mass of material may appear in them. In either
case, the cells ultimately break down, setting free the hyaline or granular
material, which is believed to be the origin of the similar material al-
ready described as existing between the neuroglial fibers of the pars
nervosa and therefore ultimately finding its way by the infundibulum
764 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
into the third ventricle of the brain. These hyaline globules are greatly
increased after thyroidectomy. It should be mentioned, finally, that at
the margin of the intraglandular cleft the intermediary and anterior
portions of the pituitary come together, although the cells of each can
readily be distinguished on account of their staining properties. This
pars glandularis et intermedia also extends as a thin layer over part of
the pars nervosa and around the neck of the gland at the infundibulum.
These relationships are Avell shown in the accompanying diagram (Fig.
196).
Functions
Concerning the function of the pituitary, it may be said in general that
the anterior lobe has an important relationship to the nutritive con-
dition of the body during growth, especially of the skeletal structures,
and that the posterior lobe produces a very active autacoid having to do
with the physiologic activity of unstriped muscle fiber. The pars inter-
media seems to be associated with the posterior lobe in the production of
this autacoid. The function of these two parts will therefore be con-
sidered together.
Function of the Pars Glandularis. — The facts concerning the function
of the pars glandularis have been gleaned largely by observing the ef-
fects produced by partial or complete removal of the entire pituitary,
justification for ascribing to the removal of the anterior, rather than
the posterior, lobe the results that are obtained being furnished by control
experiments, in which by removal of the posterior lobe alone similar
effects are not observed.
Complete removal of the pituitary is almost invariably fatal, the con-
dition being called apituitarism. Two operative procedures have been
employed for the removal of the gland. One of these, elaborated by Gushing
and his pupils,82 consists in trephining the skull and elevating the temporal
lobe of the cerebrum so as to expose the gland. The other, elaborated
by Horsley,83 consists in approaching the gland through the orbital
cavity. Although there is some danger of injury to nervous tissues by
the intracranial method, its results are more dependable since the gland
is actually exposed to view before being removed.
Most hypophysectomized animals die within two or three days, unless
they are very young. This longer survival of young animals is ascribed
to the presence of accessory pituitary material situated in the dura mater
lining the sella turcica. The most extensive observations have been made
on dogs. On the day following the operation the animal appears about
normal, but it gradually becomes less active, refusing food and respond-
ing slowly to stimulation. It gradually gets weaker and weaker; muscu-
THE PITUITARY BODY 765
lar tremors may appear, the respiration and pulse become slow, the back
arched, the temperature subnormal; and, usually within about forty-
eight hours, coma develops and the animal dies in this condition. When
the symptoms are less acute and death does not occur so early, it is
believed by Gushing either that small portions of the gland have been
left behind or that some vicarious activity of other organs has developed
to replace that of the pituitary.
When only a part of the pituitary is removed either unintentionally
or intentionally, the symptoms are not nearly so acute, and the condition
is known as hypopituitarism. It is by a study of this condition that
most facts concerning the function of the anterior lobe have been learned.
When the operation is performed on young animals, they fail to grow
properly; the milk teeth and the lanugo are retained; the epiphyses
do not ankylose; the thyroid and thymus glands are enlarged; and the
cortex of the suprarenal and the sexual organs fails to develop. The
animal, though small, becomes very fat and may therefore increase in
weight. There is distinct evidence of mental dullness. From these
results it is concluded that the anterior lobe of the pituitary produces
autacoids having to do ivith the development of the skeletal and other
structures of the growing animal. That this autacoid is not derived from
the posterior lobe is evidenced by the fact that partial injury of this
lobe, or indeed its entire removal, is not followed by similar symptoms.
Closer examination of the metabolic function in hypophysectomized
animals has shown that there is a marked depression in the respiratory
exchange of oxygen and carbon dioxide, and that the ability to metabo-
lize carbohydrate becomes heightened; that is to say, the animal with-
out developing glycosuria can tolerate a larger quantity of sugar than
the normal animal. This effect on carbohydrate metabolism may how-
ever be associated not so much with the function of the anterior lobe as
with that of the posterior, for, as we shall see later, Gushing and his
pupils have found that extract of the posterior lobe has a marked effect
on the assimilation limit of carbohydrate.
Attempts have been made to graft the pituitary, especially the anterior
lobe, into various parts of the body. It has been found, however, that
within a few days the grafts atrophy and disappear unless there has
been complete removal of the pituitary itself, in which case the graft
may remain for a month or so and the otherwise fatal outcome of hypophy-
sectomy be warded off. Sometimes, where the graft has remained for a
longer time, it is said that a temporary increase in the growth of the
.animal has been noticed.
Other observers have investigated the effects in normal animals of
continuous oral administration of pituitary substance or of subcutaneous
766 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
injection of extract. The earlier results were indefinite and confusing,
but recently Brailsford Robertson81 has succeeded in isolating from the
anterior lobe a substance called tethelin, which accelerates growth in
young animals and is thought to have a possible value in hastening the
healing process in wounds.
Tethelin is precipitated by dry ether from an alcoholic extract of the
carefully isolated anterior lobes. It contains 1.4 per cent of phosphorus
and nitrogen in the proportion of four atoms for every atom of phos-
phorus, two of the nitrogen atoms being present as amino groups and
one in an imino group. The effects on growth of mice are in every par-
ticular like those of the administration of anterior lobes, and consist in
retardation of the first portion of the third growth cycle,* followed by
acceleration of the latter portion of this cycle. When fully grown,
tethelin-fed mice also differ from normal animals in being smaller in
size but of greater weight, with a distinct difference in the condition of
the coat. Normal animals at fourteen months of age have "shaggy,
staring and discolored coats," whereas in tethelin-fed animals they have
the glossy and silky appearance of young animals. During growth, nor-
mal animals display a greater variability in weight than tethelin-fed
animals.
Extraordinary effects have been observed by Clark85 to be produced
by feeding laying hens with pituitary gland. Thus, by giving to one-
year-old hens, in addition to their usual food, 20 milligrams of fresh
pituitary substance for four days, it was found that the average daily
number of eggs laid by a batch of 655 hens was raised from 273 during
the four days preceding the pituitary feeding to 352 during the four
days of the administration, these results being obtained at a time of
year when the natural egg-production of the hens was diminishing. It
was further observed that not only is the output of eggs greatly increased
as a result of the pituitary feeding, but likewise their fertility, for in
another experiment in which 35 hens were kept along with two cockerels
of the same breed, not only was the output of eggs increased (from 18 up
to 33), but the fertility of the eggs was greatly enhanced.
Functions of the Posterior Lobe or Pars Nervosa. — As already men-
tioned, excision of this part of the pituitary can be tolerably well with-
stood by the animal, so much so indeed that from its behavior after the
operation we can conclude little as to the function of the lobe. On the
other hand, extracts of the posterior lobe injected into normal animals
produce effects that are very striking, indicating that the main function
*Robertson has contributed valuable and very extensive data on the normal curve of growth of
white mice kept under carefully controlled conditions. Three growth cycles are present: the first
attains its maximum velocity between seven and fourteen days after birth; the second, between
twenty -one and twenty-eight days; and! the third about six weeks, after which the velocity decreases
progressively, until further growth ceases between the fiftieth and sixtieth weeks succeeding birth.
THE PITUITARY BODY 767
of this lobe is production of an autocoid. The extracts have more or less an
epinephrine-like action. Such extracts, rendered protein-free and steril-
ized, are obtainable on the market under the various names of pituitrin,
hypophysin, etc. From them a crystallizable material has been obtained,
but this is probably a mixture of various substances. In discussing the
functions of these various extracts, it must be remembered that the inter-
mediary part (pars intermedia) is included with the posterior lobe in
their preparation.
Although the effect of pituitary extract on plain muscle filer (and on
glandular tissue) appears, on first sight, to be very like that produced
by epinephrine, it has been found on closer examination that the two
substances really act in different ways. The rise in blood pressure pro-
duced by pituitary autacoid is likely to be more prolonged than that
produced by epinephrine. It stimulates increased cardiac activity, but
after the vagi have been cut or sufficient atropine administered to para-
lyze them, the pituitary autacoid continues to stimulate the strength of
the heartbeat without producing the acceleration noted with epinephrine.
Whereas epinephrine has little or no action on the coronary vessels or
on those of the lungs, pituitary autacoid usually produces constriction of
both types of vessel; and on the renal arteries the actions of the two
autacoids are entirely different, for epinephrine has a marked constric-
ing effect, while the pituitary autacoid produces dilatation.
Another striking difference in the extracts from the two glands is re-
vealed by repeating the injection after the effect of a previous one has
completely passed off. With epinephrine the original effect is repro-
duced ; with pituitrin, on the other hand, the effect of the second injec-
tion is very often the reverse of that of the first.; that is to say, the blood
pressure, instead of rising, may fall, or the rise be very much less
marked. Whether this effect of the second dose is caused by the action
of an autacoid having a chalonic rather than a hormonic influence, or
whether it is due to a reversed effect of the same hormone, it is impos-
sible at present to say. The chalonic effect in any case is much more
evanescent than the hormonic, and it is not caused by cholin, as some
have suggested. The effect of epinephrine, it will be remembered, is
abolished by ergotoxin and apocodeine. These drugs, on the other hand,
have no influence on the action of pituitrin. The difference in action
between the two autacoids is usually explained by assuming that the
epinephrine acts on the receptor substance associated in some way with
terminations of the sympathetic nerve fibers in involuntary muscle,
whereas pituitrin acts directly on the involuntary muscle fibers themselves.
Other types of involuntary fiber are also acted on by pituitrin. The
uterine contractions for example are stimulated (Fig. 197) ; so are those of
768
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
the intestine (in contrast to the inhibiting effect of epinephrme), and of the
bladder-ureter musculature. Dilatation of the pupil of the excised frog
Fig. 197. — Tracing showing the action of pituitrin on the uterine contractions and blood pressure
in a dog. Made by Harbour's method. (From Jackson.)
eye is produced. The effect of pituitrin on the muscle of the bronchioles
is shown in Fig. 198.
The glands on which the pituitrin has the most pronounced action are
THE PITUITARY BODY
769
•the mammary glands and the kidneys. The effect on the kidney is evi-
denced by the remarkable increase in the urinary flow following injection
of the pituitrin. This diuresis might of course be due merely to the
vasodilatation that we have seen such extracts produce — a vasodilatation
which is all the more marked because the vessels elsewhere in the body
undergo constriction. But pituitrin continues to cause increased urinary
outflow in the absence of any demonstrable vascular change; it also acts
after the administration of atropine, so that it is considered by most
observers to act on the excretory epithelium of the convoluted tubules
Fig. 198. — Tracing showing the constricting action of pituitrin on the bronchioles and its effect
on blood pressure in a spinal dog. (From Jackson.)
in much the same way as certain diuretics, like diuretin. This renal
hormonic action of pituitrin would appear to be analogous with that of
secretin on the epithelium of the pancreas. Another reason for believ-
ing that the secretory hormone is independent of that producing vaso-
dilatation of the renal vessels is the fact that a repeated dose of pituitrin,
although, as we have seen, it usually has a. depressor action on the blood
vessels, still produces a stimulating effect on the excretion of urine.
The value of pituitrin as a diuretic in clinical practice is now well
recognized.
The effect on milk secretion is best demonstrated by placing a cannnla
770 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
in the mammary duets so that the milk may freely flow out. By observ-'
ing the rate of outflow during the injection of pituitrin, it will be found
that a remarkable increase occurs. After this increased secretion has
ceased, however, the injection of more pituitrin has no further effect,
indicating that the influence of the first injection must have been, not so
much to stimulate the secretion of milk, as to accelerate the outflow of
that which previously had been secreted and had collected in the alveoli
and ducts. This effect explains why the pituitary galactagogue should
have very little if any effect on the total production of milk or on the
total amount of fat and other constituents contained in it. Histologic
examination of sections of a resting mammary gland and of the same
gland after administration of the pituitrin, bears out the above interpre-
tation of the action. Alveoli in the resting state will be found largely
distended with milk and the epithelium flattened against the basal mem-
brane, whereas alveoli from the gland after pituitary activity show small
shriveled-up alveoli, containing little milk, and with epithelium that is
well marked and stands out prominently from the basal membrane.
These facts taken together indicate that pituitrin stimulates the mus-
cular fibers of the ducts of the mammary glands, thus squeezing out the
milk contained in them. Muscular fibers have been described as existing
between the basal membrane and epithelial cells, much in the same way
as they do in the case of the sweat glands. At least Schafer has suc-
ceeded in demonstrating in this position rod-shaped nuclei which prob-
ably belong to muscular fibers.60 By their contraction, the milk in the
alveoli is expelled into the ducts. It has also been found that pituitrin
stimulates the secretion of cerebrospinal fluid, and that this stimulation
is independent of a rise in blood pressure.
Pituitrin has a distinct effect on carbohydrate metabolism,. After its
intravenous or subcutaneous injection, a marked lowering in the toler-
ance for sugar is observed (page 652), usually to such an extent that
glycosuria becomes established. Gushing and his pupils have concluded
that the posterior lobe contributes an autacoid which stimulates the utili-
zation of sugar in the body. Confirmatory evidence for this view is fur-
nished by the observation that mechanical stimulation of the posterior
lobe, such as is produced by puncturing it with a needle, is followed by
a temporary glycosuria, which is said to be as pronounced as that fol-
lowing puncture of the diabetic center (page 672), provided glycogen is
present in the liver. The production of this carbohydrate autacoid would
appear to be under the control of the sympathetic nervous system, for it
has been found by Gushing and others that stimulation of the superior
cervical ganglion, which has been known for many years to be fre-
quently followed by glycosuria, has this effect only provided the posterior
THE PITUITARY BODY
771
lobe of the pituitary is intact. Even surgical manipulation of the pitui-
tary may excite a hypersecretion of pituitrin, which would account for
the glycosuria often observed after experimental excision or partial
destruction of the pituitary. A similar irritation may be set up in disease
of the gland.
The glycosuria which is usually observed after partial hypophysectomy
soon passes off, to be followed by a permanent condition of increased
tolerance for sugar, because now less pituitrin is being produced. It is
said that during the stage of increased tolerance diabetes can not be pro-
duced even by excision of the pancreas. The glycosuria produced by
irritation of the posterior lobe is accompanied by a marked polyuria (dia-
betes insipidus), which may outlast the glycosuria.
A.
B.
Fig. 199. — A, To show the appearance before the onset of acromegalic symptoms; B, The ap-
pearance after seventeen years of the disease. (After Campbell Geddes.)
Clinical Characteristics
Because of their importance from a physiologic standpoint, we shall
now proceed to review briefly some of the more important facts that have
so far been brought to light by clinical observations. The pathologic
condition most frequently observed affecting the pituitary is an adenom-
atous growth particularly located in the anterior lobe. Besides pro-
ducing general symptoms of pressure, such as diminution of the visual
field and, perhaps, headache, a shadow can usually be observed when the
patient is examined by means of the x-rays. General symptoms, com-
monly ascribed to a hypersecretion of the autacoid of the anterior lobe of
the pituitary — hyperpituitarism — begin sooner or later to show them-
772
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
selves. These symptoms are almost exactly opposite in character to those
observed in animals after removal of this portion of the gland. Thus,
the bones of the extremities and of the face become stimulated to in-
creased growth, so that if the patient is young, and the epiphyses there-
fore not ossified, remarkable elongation of the long bones occurs, pro-
ducing the condition known as gigantism. On the other hand, if the dis-
ease does not develop until after ossification is complete, its, effects be-
come most marked in the bones of the face, the lower jaw becoming
Fig. 200. — Hand of a person affected with acrornegaly.
enormously hypertrophied and the supraorbital ridges very prominent.
The long bones also become enlarged at their extremities, and there may
be some increase in length of the vertebral column, although the stature
does not increase because of kyphosis (curvature of the spine). The
condition is called acromegaly. Nutritive disturbances of the skin and
hairs also become marked, causing the skin to become dry and yellowish,
and the hairs to undergo abnormal increase over the body. An early
symptom of the condition is a failure of the sexual power (Figs. 199
and 200.)
THE PITUITARY BODY 773
After a time the disease begins to affect the pars intermedia et nervosa,
and disturbances in carbohydrate metabolism come to be observed, con-
sisting usually in a diminished tolerance accompanied by glycosuria, in
the early stages of the disease, followed by increased tolerance in the
later stages. The glycosuria is usually accompanied by marked polyuria.
It should be observed that sometimes tumor of the pituitary has been
found to exist postmortem though none of the above symptoms had been
recorded during life. In these cases it is probable that the disease from
the start had been of such a nature as to produce a tendency to hypo-
pituitarism rather than hyperpituitarism, for the symptoms are very like
those observed in animals after partial or complete removal of the gland.
If the condition commences before adolescence, the body fails to grow,
although the child may continue to increase in weight because of the
remarkable deposition of fat in the tissues. Sexual development is strik-
ingly interfered with, and the secondary sexual characteristics fail to
show themselves. In boys, for example, the pubic hairs fail to extend up
to the umbilicus; and the hairs on the chin do not develop, whereas the
hair of the scalp grows profusely. The bones remain of the female type,
and a broad pelvis, rounded limbs, small feet and hands are often ob-
served. In these cases there is usually excessive tolerance for carbohy-
drates, Avhich may explain the adiposity, sugar being converted into fat.
In the light of the experimental results, the effect on carbohydrate
metabolism may be explained as due to involvement of the posterior
lobe. Mental development is retarded, and psychic derangements are
sometimes observed.
Where the hypopituitarism does not develop until after adolescence,
some of the above symptoms will of course be missed, but many will be
observed, such as dryness of the skin, loss of hair, and the tendency in
the male to adopt certain of the female characteristics, particularly with
regard to the growth of hair. Obesity and increased tolerance for sugar
are also evident, and pigmentation of the skin, something like that of
Addison's disease, is said often to be a prominent feature. Operative
interference in the early stages in many of these cases is of undoubted
benefit, as is shown by the brilliant work of Harvey Gushing, to which
the reader is referred for further information.
The Relationship of the Pituitary Gland with Other Endocrine
Organs
The relationship of the pituitary gland with other endocrine organs
seems to be an intimate one.
1. With the Thyroid and Parathyroid Glands. — That enlargement of
the pituitary occurs after thyroidectomy in man has been known for a
774 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
considerable number of years. The enlargement affects more particu-
larly the pars anterior, although changes are also described in the pars
intermedia et nervosa. Accompanying the enlargement of the anterior
lobe, vesicles containing colloid-like material often become developed in
it, but even after the hypertrophy has proceeded to a considerable de-
gree, this colloid does not contain iodine, nor does an extract have the
same physiologic effect as one of the thyroid gland. It can not replace
thyroid extract in the treatment of patients with goiter or myxedema,
or ameliorate the symptoms produced in animals by the removal of the
thyroid gland. Deposition of colloid-like material in the pars anterior
also occurs in myxedema. Histologic changes in the pars intermedia et
nervosa, although less pronounced than in the pars anterior, are never-
theless said to be perfectly distinct following thyroidectomy, and to con-
sist in an increase in the hyaline and granular masses which have already
been described as present to a certain extent in the normal gland.
Less direct evidence of an association in function between the pituitary
and the thyroid is furnished by the similarity of the effects produced on
the sexual functions and 'on the general development of young animals
by the removal of either gland. In both cases the animals fail to grow
properly; the sexual organs remain undeveloped; and the mental func-
tions are infantile in type. In hypophysial deficiency, however, extreme
adiposity is likely to be more marked than is the case in cretinism.
2. With the Sexual Organs. — That the pituitary gland has much to do
with the development of the sexual organs has already been shown. Fur-
ther evidence of a relationship between the sexual glands and the pitui-
tary is furnished by the following observations. After castration en-
largement occurs in the pituitary, and on histologic examination the
gland is found to contain a large number of oxyphile cells, particularly
in the pars anterior. This influence of the sexual glands on the pituitary
is believed to depend on the interstitial cells present in them, for it has
been found that if the ovary or testis is transplanted into other parts of
the body after the castration, the changes in the pituitary do not occur,
although, as we shall see, the transplanted gland becomes entirely
atrophied except for the interstitial cells. The enlargement of the pitui-
tary during pregnancy — an enlargement which often brings it to two or
three times its normal weight — is further .evidence of its association
with the ovary.
3. With the Suprarenals. — Association of function is suggested in this
case by the fact that extracts of suprarenal and pituitary have very much
the same effects on involuntary muscular fiber and glandular structures,
and it is said that the two extracts mutually facilitate each other's
action in this regard. It should be remembered, however, that pituitrin
THE PITUITARY BODY 775
and epinephrine do not appear to act on exactly the same peripheral
mechanism (see page 767).
4. With the Isles of Langerhans. — Since pituitrin affects carbohydrate
metabolism, which is thought to be primarily controlled by the Isles of
Langerhans, it is claimed by some observers that a relationship also
exists between the pituitary and these structures. Injections of duodenal
extracts are also said to cause a hypersecretion of pituitrin into the
cerebrospinal fluid.
CHAPTER LXXXV
THE PINEAL GLAND
This peculiar structure lies between the anterior corpora quadrigem-
ina, and weighs about two-tenths of a gram. It is largest in the early
years of life, and undergoes retrogressive changes after puberty. Micro-
scopically it consists of epithelial cells arranged loosely in trabeculse,
with large sinus-like capillaries between them; neuroglia and sometimes
muscle-fiber cells are also present. Curious globules of calcareous mat-
ter (brain-sand) are also found, especially in the pineal gland of man.
The gland is developed from an evagination of the third ventricle, and
vt is homologous with the so-called median eye of reptiles.
The functions of the pineal gland are obscure. In cases where its
extirpation has been successfully accomplished (in the fowl), it has been
found that the body growth is stimulated and the sexual characteristics
developed more quickly. This result would seem to indicate that the
clinical observation that tumors of the pineal gland are associated in
young boys Avith abnormal growth of the skeleton and with the early
development of the secondary sexual characteristics, depends on the
fact that a condition of hypopinealism is produced by the growth of a
tumor. The immediate effects of the injection of extract of pineal gland
are not characteristic, consisting merely of a fall in blood pressure, which
is,- however, obtainable when an extract of practically any cellular organ
is injected. Prolonged administration of an extract to growing animals
is said to accelerate the growth and to bring about a precocious develop-
ment of the sexual organs; but this result is somewhat difficult to inter-
pret, for, as we have just seen, similar changes occur after experimental
removal of the gland.
THE GONADS OR THE GENERATIVE ORGANS
The Generative Glands of the Male
The structures which are responsible for the well-known influence of
the testicles on the development of the male sexual characteristics are
the so-called interstitial cells of Leydig, which consist of polygonal-
shaped epithelial-like cells, with well-marked nuclei and nucleoli. Lipoid
776
THE PINEAL GLAND AND THE GONADS 777
granules, staining black with osmic acid, are also present in the cyto-
plasm. The degree of development of the interstitial cells varies in dif-
ferent animals, being marked in the cat and man and ill-marked in the
rat and rabbit. In animals which show seasonal changes in sexual activ-
ity, the cells are most prominent between the periods of sexual activity,
when the semeniferous epithelium is less evident. They also become
prominent in cases where the semeniferous epithelium is atrophied,
either as a result of disease or following ligation of the vas deferens done
in such a way that the artery and nerves to the testicles are not included
in the ligature. When the testicle or a portion of it is grafted into
another part of the body, the semeniferous epithelium degenerates, but
the interstitial cells remain alive and become quite prominent. It is
believed that the interstitial cells are responsible for the production of
an autacoid that has to do with the development of accessory sexual
characteristics.
Tlie effects of castration are not significant in animals below the verte-
brata. In all of these, however, they are very pronounced. The cas-
trated male frog fails to show development of the thumb pad, but this
development immediately ensues if portions of testis from another frog
be placed in the dorsal lymph sac. In birds the results are more pro-
nounced; in the castrated male chick the comb, spurs, wattles, etc., fail to
develop, but will usually do so if some testis from another bird is trans-
planted into its tissues. In mammals the effects are most striking in
animals that develop marked male characteristics, such as the growth
of antlers in stags. These fail to develop properly and are prematurely
shed after castration. In man also, as is well-known from a study of
eunuchs, castration has a very profound effect. Hair fails to grow on the
face; the larynx remains undeveloped; the epiphyses are a long time in
ossifying, so that the stature may become great, but at the same time
the limb bones may be more delicate than usual ; the sutures of the skull
are slow in closing ; and the whole architecture of a castrated male comes
to be very like that of the female. Confirmatory evidence of the influ-
ence of the testicles on the development of secondary sexual character-
istics is afforded by the observation that malignant tumors of the testes
in boys are associated with the premature development of the secondary
sexual characteristics, and that these may recede after the removal of
the tumor.
As a result of castration, interesting changes have also been observed
in other ductless glands. Thus, the suprarenal cortex and the thymus
become enlarged, whereas the thyroid and pituitary become atrophied.
The metabolic functions also become tardy, as is evidenced by a tendency
to the deposition of fat.
778 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
When the castration is performed on an adult man, the above changes
in the sexual characteristics are of course not so evident, although the
prostate, etc., atrophy. The effect on the metabolic functions is, how-
ever, very marked, there being a striking tendency to increased forma-
tion of fat. It is interesting that accompanying this there should usually
occur a lowering of the assimilation limit for carbohydrate, so that glyco-
suria is very readily induced. We can not assume, therefore, as Gush-
ing has done in the case of hypopituitarism, that the fat deposition is
attendant upon an improper combustion of carbohydrate.
These remarkable effects of castration have naturally prompted ob-
servers to study the influence of injection of testicular extract on the
development of sexual characteristics in different animals, but the re-
sults have in general *been considered to be negative in character.
The Female Generative Organs
It is well known that, besides their function in producing ova, the
ovaries also produce autacoids that have to do not only with the fixa-
tion of the embryo in utero, but also with the changes that occur during
pregnancy in the maternal organism. It is however at present uncertain
as to where these autacoids are produced in the ovary. The two most
likely sources are the stroma cells and the corpus luteum. In the stroma
of the ovary of certain animals, groups of cells have been described
having a different appearance from those of ordinary stroma cells.
They have been called the interstitial cells of the ovary, and are believed
to be analogous with the similar structures found in the testicle. It is
possible, however, that these interstitial cells are nothing more than
cells derived from previous corpora lutea. The latter are formed by
proliferation of the follicular epithelium which remains after extrusion
of the ovum, and by the ingrowing into the follicle of the so-called theca
cells and blood vessels. The fully developed corpus luteum in most
animals consists of cells arranged in trabeculae converging toward the.
scar which formed at the place where the follicle had burst. The luteal
cells, as they are called, are characterized by containing considerable
quantities of lipoid material.
That the ovary produces some autacoid is evidenced by both clinical
and experimental observations. Thus, if both ovaries are removed in a
young animal (oophorectomy or spaying), it is well known that not
only does the uterus fail to develop properly, but the external changes
characteristic of puberty in the female fail to materialize, although act-
ually the general effects are not so pronounced as they are in the male
after castration. Menstruation does not set in; the mammary glands fail
to develop ; and there is a tendency for the hair to grow as in the male.
THE PINEAL GLAND AND THE GONADS 779
When the operation is performed in adult life, the changes are not very
pronounced, except that menstruation ceases and the uterus and mam-
mary glands atrophy. Metabolism also becomes altered, causing a
tendency to the deposition of fat, and in the case of the human animal at
least, there is frequently evidence of mental disturbance.
Attempts to acquire more definite information regarding the physio-
logic effects of the ovarian autacoid have recently been made by Schafer
and Itagaki.60 Extracts were prepared from the corpus luteum or Graafian
follicles or from the hilum ovariae, and observations were made on the
effect produced on the behavior of the chief forms of unstriated muscle
by adding the extracts to isolated preparations of uterus or intestine
or by injecting the extracts into animals. Applied to the isolated prepa-
rations, extract of follicular tissue or of liquor folliculi was found to
increase the force and rate of the rhythmic contractions of the uterus as
well as its tone, whereas inhibition was produced when extract of the
hilum was used. Extract of corpus luteum, when injected into the
veins, was found to cause the uterus to increase its contraction or if
quiescent to begin contracting. It was further noted that extracts of
hilum caused a fall in arterial blood pressure, whereas those of corpus
luteum had little or no effect. It would appear from these observations
that the extracts contain two different autacoids, one having a hormonic
and the other a chalonic action on plain muscular fiber.
Extract of corpus luteum when intravenously injected also stimulates
the outpouring of the milk from the mammary glands, although not so
markedly so as extract of pituitary gland. This pituitary-like action is
not obtained with extracts of ovary that do not contain corpora lutea.
Besides being concerned in the outpouring of milk, corpus luteum has
also been shoAvn to be related in some way to the development of the
mammary gland during pregnancy. These glands become developed in
young virgin rabbits after the continuous administration for a month
or so of extract of corpus luteum, and they also develop in unimpreg-
nated animals when the corpus luteum is made to develop by artificial
means such as puncturing the Graafian follicle. Furthermore, destruc-
tion of the corpora lutea in a pregnant rabbit arrests development of
the mammary glands. The corpus luteum has also an important func-
tion in connection with the formation of the uterine decidua and the
fixation of the embryo. Thus, after destruction of. the corpus luteum at
an early period in pregnancy, the embryo fails to become adherent to
the uterus.
780 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
DUCTLESS GLANDS REFERENCES*
(Monographs)
ssVincent, Swale: Internal Secretions and the Ductless Glands, Ed. Arnold, London.
59Biedl: The Internal Secretory Organs, Wm. Wood & Co., 1913.
coSchafer, Sir E. A.: The Endocrine Organs, Longmans. Green & Co., New York and
London, 1916.
(Original Papers)
eiFulk, M. E., and Macleod, J. J. E.: Am. Jour. Physiol., 1916, xl, 21.
62Folin, O., Cannon, W. B., and Denis, W. : Jour. Biol* Chem., 1913, xiii, 447.
eaCannon, W. B., and Gray, EL: Am. Jour. Physiol., 1914, xxxiv, 232; also with Men-
denhall, W. L.: Ibid., 243 and 251.
e^Hartman, T. H., and others: Am. Jour. Physiol., 1915, xxxviii, 433; ibid., 1917, xliii,
311; ibid., xliv, 353; ibid., 1918, xlv.
esHoskins, E. G. : Am. Jour. Physiol., 1912, xxix, 363 ; Jour. Pharm. and Exp. Therap.,
1911, iii, 93 ; Am. Jour. Physiol., 1915, xxxvii, 471 ; ibid., 1916, xli, 513.
eeStewart, G. N., and Eogoff, J. M.: Jour. Lab. and Clin. Med., 1918, iii, 209. See full
bibliography by Eogoff in this paper.
67Elliott, T. E. : Jour. Physiol., 1912, xliv, 374.
ssStewart, G. N.: Jour. E"xp. Med., 1911, xiv, 377; ibid., 1912, xv, 547; ibid., xvi, 502.
esStewart, G. N., Eogoff, J. M., and Gibson: Jour. Pharm. and Exper. Therap., 1916,
viii, 205.
7oMeltzer, S. J. : Deutsch. med. Wchnschr., 1909, xiii.
"Stewart, G. N. : Jour. Exper. Med., 1912, xv, 547.
"Cannon, W. B., et al.: Am. Jour. Physiol.j 1911, xxviii, 64; ibid., 1914, xxxiii, 356;
also Bodily Changes in 'Hunger, Fear, and Eage, Appleton, 1915.
"Cannon, W. B., and Cattell, McKeen: Am. Jour. Physiol., 1916, xli, 74.
''^Macleod, J. J. E., and Pearce, E. G. : Am. Jour. Physiol., 1912, xxix, 419.
fs Marine, D. : Personal communication.
reMarine, D. : Jour. Exper. Med., 1914, xix, 89. ^
"Marine, D., and Lenhart, C. H.: Jour. Exper. Mea., 1910, xii, 311; ibid., 1911, xiii,
455; also Bull. Johns Hopkins Hosp., 1910, xxi, 95.
"Marine, D., and Kimball, O. P.: Jour. Lab. and Clin. Med., 1917, iii, 41.
7oKendall, E. C. : Boston Med. and Surg. Jour., 1916, 175, 557; also Proc. Am. Physiol.
Soc., Am. Jour. Physiol., 1918, xliv.
sopaton, Noel and Finlay: Quart. Jour. Exp. Physiol., 1917, x, 203. Paton, Noel,
Finlay and Watson, A. : Ibid., 233, 243, 315, and 377.
siMacCallum, W. G., etc.: Jour. Exper. Med., 1909, xi, 118; ibid., 1913, xviii, 646;
Jour. Pharm. and Exper. Therap., 1911, ii, 421.
ssCushing, Harvey: The Pituitary Body and Its Disorders, J. B. Lippincott Co., 1912.
saHorsley, V. : Brit. Med. Jour., 1885, i, 111.
»iEobertson, Brailsford, and Eay, L. A.: Jour. Biol. Chem.. 1916, xxiv, 347, 363, 385.
397, 409.
ssClark, L. N. : Jour. Biol. Chem., 1915, xxii, 485.
*The numbering is in continuation with that for metabolism.
V IX
THE CENTRAL NERVOUS SYSTEM
CHAPTER LXXXVI
THE EVOLUTION OF THE NERVOUS SYSTEM
The nervous system of the higher animals consists of the nerve cen-
ters, and the nerves with their various interconnecting tracts. The
nerve tract and centers are located mainly in the spinal cord and brain,
where, by their interlacement, they form an extremely complex struc-
ture. The exact position of the centers and the course and connections
of the tracts with the centers are problems which, under the title of
neurology, have during recent years been contributed to more particu-
larly by the anatomist and the pathologist. The information thus
gathered tells us the possible tract or tracts of nerve fibers through which
the various centers may communicate either with one another or with
the structures outside the central nervous system upon which they
act. Since each of these centers may, however, be played upon by in-
fluences coming from different regions of the body, it is evident that there
must remain, as an equally important aspect of the subject, the investi-
gation of the means by which the various available centers and tracts are
brought into communication and action at the proper time. In other
words, we must investigate the functional uses of the available paths.
We may compare the central nervous system with a telephone system,
the exchanges representing the nerve centers, and the wires the nerve
trunks. Any incoming wire may be connected by the operator with
any outgoing wire, but a knowledge of how each wire runs does not tell
us under what conditions the various wires will be connected for trans-
mission of messages. It is the same with the nervous system ; the neurolo-
gist can tell us how the tracts and centers run, but not the conditions
under which they may act together. This it is the duty of the physiologist
to ascertain.
Since it is the degree of development of the central nervous system
which determines an animal's position in the evolutionary scale, much
information concerning the relative importance of the various parts of
781
782 THE CENTRAL NERVOUS SYSTEM
it can be gleaned from a survey of the conditions under which the
nervous system makes its appearance in the lowest forms of animal
life. In the case of unicellular organisms, such as the ameba, the ap-
plication of a stimulus to the surface causes a movement, because the
protoplasm of the organism possesses, among its other properties, those
of excitability, conductivity and contractility. In the case of multicel-
lular organisms, on the other hand, some cells are set aside and spe-
cialized for the assimilation of food, others for movement, others to
receive stimuli from the outside, and yet others to compose the tougher
tissues which protect the surface of the animal from injury. This loca-
tion of specific function in specialized groups of cells makes it necessary,
for the welfare of the organism as a whole, that some means of com-
munication should be provided between the distant parts of the animal,
for otherwise the cells which are occupied in absorbing food would be
unable to move away or be protected from harm when some destructive
agency approached them, and indeed the moving (muscle) cells could
never know when the welfare of the organism as a whole demanded that
they should become active.
It is probable that, in some of the lower organisms, the messages trans-
mitted from one group of cells to the others are carried by chemical
substances present in the circulating fluid — hormones, as they are called
(page 729). For the quick adaptation that is necessary in the struggle
for existence, however, such hormones are usually too slow in bringing
about the response, and very early in the evolutionary scale we find that cer-
tain cells become differentiated for this special purpose. The cells thus
specialized constitute the nervous system, their differentiation, as would
be expected, being, however, antedated by that of. the cells that form the
muscular tissues. In the sponges, for example, muscle cells become
developed from ameboid epithelium and from a layer underneath the
external epithelium. These muscle cells contract slowly so as to cause
opening and closing of the small mouths, or oscula, on the surface of
the sponge in response to movements in the sea water. They are in-
dependent of any nervous structures.
In certain Coelenterates the muscle cells respond a little more quickly
than in the sponges, and this greater efficiency is found to be dependent
upon the appearance of a localized, very primitive nervous system1.
This nervous system consists of specially modified epithelial cells, or
receptors, sending branches from their inner ends, which either come in con-
tact with the muscle cells, or effectors, or become interlaced so as to form a
network. In the region between the receptors and the effectors the net-
work at first serves merely as a structure whereby the entire muscula-
ture of the animal can be brought into harmonious action from a single
THE EVOLUTION OF THE NERVOUS SYSTEM
783
point on the surface, as, for example, in the case of the sea anemone
(No. 2 of Fig. 201). In the jellyfish, which in contrast to the sea anemone
is a free moving animal, we find that the receptors are more highly special-
ize and, therefore, much more sensitive, and that the impulses which they
receive are transmitted to a more definite nerve network, capable not only
of conveying the excitatory process from one part of the animal to another,
Sponge
2.
Sea anemone
Simple form in
earthworm
Addition of
association neurons
in earthworm
Fig. 201. — Diagram to show gradual evolution of nervous system from an epithelial cell (<?)
and muscle fiber (m) in the sponge (/) to a specialized epithelial cell or receptor (r) and muscle
cell in the sea anemone (.2) ; then to a receptor and motor neuron joining in a ganglion (Gang.),
in simple form seen in the earthworm (j). Most of the ganglia in this and other segmented
invertebrates show also the internuncial or association neurons as indicated in 4.
but also of imprinting on the impulse a characteristic rhythmic activity
which brings about the contraction of the bell and the swimming movement
of the animal. The network now assumes the function of an adjuster
as well as a transmitter of impulses.
So far the adjuster is an extremely simple structure, and it is possible
that the effector and receptor organs are directly connected by fibers
running through it. When we come to the segmented invertebrates
784
THE CENTRAL NERVOUS SYSTEM
(such as the earthworm, crayfish, lobster, etc.,) much more definite spe-
cialization of the adjuster occurs, for now this intermediate nervous tis-
sue becomes collected into so-called ganglia, a pair existing for each
segment and the various pairs being connected by definite
nerve structures, constituting the ganglion chain. It is in
this group of animals that we have, for the first time, def-
inite evidence of the existence of the 'neuron, which may be
considered as the elementary unit of which the nervous sys-
tem of all the higher animals is built. A neuron may be
either sensory or motor, and in both cases it consists of
a cell with a nucleus, one long process, called the axon,
and several short branching processes, called the den-
drites. The axon in its course may give off a branch,
or more, at right angles, — these are sometimes called
collaterals, — and at its end it may break up into very fine
branches called a synapsis. In a sensory neuron the im-
pulse is transmitted from the end of the axon to the
nerve cell, whereas in a motor neuron it is transmitted
in the opposite direction from the cell to the end of the
axon (Fig. 203).
The simplest arrangement of sensory and motor neu-
rons to constitute the nervous system is seen in the
earthworm, in which it forms the simplest type of reflex
arc (Fig. 201, No. 3). The sensory neuron has its cell
body in the skin, and its axon proceeds to one of the
segmental ganglia, in which are large nerve cells whose
thick axons pass out from the ganglion as motor fibers
to the muscles of the body wall. The dendrites of the
motor neuron and the branching of the termination of
the sensory neuron cause a very fine interlacement of
nerve fibers in the ganglia, forming a network known
as the neuropile. The sensory impulse, on reaching the
ganglion, is transmitted by the synapsis to the den-
drites, probably without the fibers actually joining to-
gether; that is, the nerve impulses pass from the one
to the other set of branches by contact rather than by
transmission through continuous tissue.
By such an arrangement it is evident that the nervous
apparatus in each segment could cause a contraction of
the muscles of its own neighborhood, but that a stimulus applied to one re-
ceptor would be incapable of calling forth a contraction of the muscles of a
far distant segment, much less a coordinated contraction of the musculature
Fig. 202.— Dia-
gram of nervous
system of seg-
mented inverte-
brate; a, supra-
esophageal g a n-
glion; b, subeso-
phageal ganglion ;
oc, esophagus or
gullet.
THE EVOLUTION OP THE NERVOUS SYSTEM
785
of the whole animal such as would be required for locomotion. To render
this possible it is necessary that some means of communication become es-
tablished between the different segmental ganglia. This is effected by
association neurons, each of which, as the name implies, consists of a nerve
cell with its dendrites located in one ganglion and of an axon, which passes
to the next or even to some more distant ganglion, where it ends by
synapsis. The important point to note is that these association neurons
do not leave the central nervous system; they merely connect various
ganglia.
So far the ganglia of each segment are of equal importance, but if
we examine further we shall find that at the head end of the animal
several of the ganglia become fused together to form a larger ganglion,
Fig. 203. — Schema of simple reflex arc; r, receptor in an epithelial membrane; a, afferent fiber; s,
synapsis; c, nerve cell of center; e, efferent fiber; m, effector organ.
which lies just behind the gullet, and from which fibers proceed around
the gullet to unite in front of it in another large ganglion, which usually
shows three lobes. These larger ganglia receive afferent nerve fibers
from the closely adjacent primitive sense organs for sight, sound and
smell, from structures, that is, that are really highly specialized recep-
tors. The cells of the retina and ear have been made capable of reacting
to impulses of light or sound instead of those of pain, touch or tempera-
ture, to which the receptors of the integument are especially sensitized.
They are distance receptors (projicient receptors), and it is evident that
the nerve reflexes with which they are concerned are of a higher order
than those located in the segmental ganglia themselves.
Some of the neurons of the head ganglia are merely motor and act on
the muscles of the head end of the animal, but others are purely associa-
786 THE CENTRAL NERVOUS SYSTEM
tion neurons and proceed down the ganglion chain to terminate by
synapses in one or other of the segmental ganglia. These association
neurons exercise a dominating influence over the activities of the seg-
mental ganglia, so that they may determine the response of the animal
when its safety is threatened by some approaching enemy. When, for
example, the stimulus produced by some sight or sound of an approach-
ing enemy is received by the head ganglia, these will transmit impulses
down the ganglion chain which so influence the various nerve cells of
this chain as to produce in all of them a coordinated action for the pur-
pose of removing the animal from danger. Even should some local
stimulant be acting on one or more of the segments, the response may be
inhibited on account of stimuli meanwhile transmitted by way of asso-
ciation neurons from the large head ganglia; in other words, the part
controlled by the segmental ganglia becomes subservient to the whole
through the dominating control of the head ganglia.
This illustrates the beginnings of the integration of the nervous system;
and as we pass to the study of the higher animals, we shall see that this
integration becomes more and more complicated, and that, as it does so,
the nerve centers acquire the power of storing away the impressions they
receive, which they may afterwards apply to regulate the reflex response.
Thus memory and volition come to find their place in the nervous inte-
gration of the animal. The afferent stimulus arriving, let us suppose,
at nerve cells controlling the movement of the leg, may fail to cause
a response of the corresponding muscles because of impulses meanwhile
transmitted by association neurons from higher memory centers, for
the animal may have learned by experience that such a movement as the
local stimulus would in itself call forth is opposed to its own best in-
terests. This experience will have been stored away in memory nerve
centers, so that, whenever the local stimulus is repeated, impulses are
discharged from the memory centers to the local nerve centers, and
the reflex response does not occur, or is much modified in nature. For
storing away these memories and for related psychologic processes of
volition, etc., the anterior portions of the nervous system in higher ani-
mals become very highly developed so as to constitute the "brain, and
the simple chain of ganglia of the invertebrates " is replaced by t Tie
spinal cord.
As we ascend the scale of the vertebrates, the brain becomes more
and more developed, until in the higher mammalia, such as man, very
few reflex actions can occur independently of the higher centers which
are located in it. The reflex arc now involves, not one nerve center,
but several, and of these the most important are located in the brain.
There is thus no essential difference in the general nature of integra-
THE EVOLUTION OF THE NERVOUS SYSTEM 787
tion in the nervous system of the lower as compared with the higher
animals, but there is a very distinct morphologic difference: in the lower
or invertebrate animals the ganglion nerve chain is ventral to the alimen-
tary canal, whereas in the higher or vertebrate, the spinal cord, which
takes the place of the ganglia, is dorsal to the alimentary canal. In both
groups the head ganglia are dorsal to the alimentary canal, but in the
vertebrates these become much more definite in structure, and constitute
the brain. This morphologic difference between vertebrates and inverte-
brates is probably not so fundamental as at first sight it may appear to
be, for, as Gaskell has shown, it is possible that the alimentary carfal of
the invertebrates is really homologous with the central canal of the
spinal cord and the ventricles of the brain of the vertebrates. Accord-
ing to this observer, what has really happened in the latter group of
animals is that the ganglia have grown up so as to surround the alimen-
tary canal and so constitute a continuous structure, a new alimentary
canal being meanwhile provided by the enclosure of a space as a result
of ventral downgrowth of the body walls. Although this view has not
been generally accepted by biologists, there is no inherent reason why it
should not be accepted. It is no more to be wondered at than the well-
known fact that a new respiratory system becomes developed in the
passage from aquatic to land amphibians.
The fibers of the sensory neurons in vertebrates are collected together
to form the posterior roots of the spinal cord, and the cell bodies of these
neurons are located not on the surface, as in invertebrates, but in the
posterior root ganglia, the cells being connected to the fibers by T-shaped
junctions. The olfactory nerve is the only one in the higher vertebrates
which retains its primitive condition.
In the vertebrate animals the spinal member in the integration of the
central nervous system is the motor neuron, the fibers being collected in
the anterior roots. Toward the cell of this neuron impulses are transmitted,
not only from the segment in which it is itself located, but by way of as-
sociation neurons from other segments or from far distant parts of the
central nervous system. In other words, this motor neuron may transmit
impulses which cause the muscles to perform local independent move-
ments, which are coordinated with those of adjacent segments and which
may be of widely varying types. The motor neuron has therefore very
appropriately been called the final common path, and it will be one of our
main objects later to show the conditions under which several different
competing influences may obtain possession of this path.
CHAPTER LXXXVII
THE PROPERTIES OF EACH PART OF THE REFLEX ARC
Having briefly traced the physiologic development of the nervous sys-
tem, we are prepared to consider in greater detail the peculiar function
of each of the parts which enter into the formation of the reflex arc.
THE RECEPTOR
With the advance in animal organization is associated the development
of the ability to appreciate and discriminate between external phe-
nomena, special organs called receptors being evolved to receive the
stimuli which these occasion. Those receptors which are distributed
over the skin of the animal are called external or exteroceptors, and are
especially adapted to react to such stimuli as temperature, pressure,
and pain, but at the fore end of the animal certain receptors become more
highly specialized so as to react to stimuli coming from a distance-^
that is, to stimuli that are not produced by contact of external objects
with the surface of the animal. These specialized receptors — sometimes
called projicient — include the eye, the ear, and the olfactory epithelium.
Receptors are also provided in the interior of the organism for the pur-
pose of receiving stimuli dependent upon the activities of the organism
itself. They may be called internal receptors, and we may further dis-
tinguish two groups of them — namely, those which come from the sur-
faces of the mucous membranes and those which come from the sub-
stance of the various organs and tissues themselves, as, for example,
from the substance of muscle or tendon.
A receptor may be defined in a general way as a mechanism in which
some particular kind of stimulus produces changes that result in the
excitation of the nerve fiber with which the receptor is connected, al-
though the stimulus in itself is incapable of exciting the nerve fiber. In
other words, as Sherrington puts it, the receptor has the threshold of
its excitability raised to- every kind of stimulus save one, toward which
it is lowered. A nerve fiber, for instance, responds to every kind of
stimulus approximately equally; a receptor will also respond to these
same stimuli, but with great inequality, since each receptor is specialized
tp react to one kind of stimulus and to others only when these are very
strong.
788
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 789
It is often a difficult matter to determine just exactly what it is in
the nature of the stimulus that makes it capable of affecting one receptor
and not another; for example, it is often merely a question of the rate
of vibration of the stimulus. Light and heat rays are both due to
vibration of the ether which fills space. When these vibrations are
slow, they stimulate receptors that have been specialized for apprecia-
tion of temperature, but when they are rapid and exist as rays of light,
they no longer affect the temperature receptors but only the highly spe-
cialized receptors of the retina. Similar vibrations of the air in place
of the ether cause sound and stimulate the auditory receptors. It is
quite likely that the receptors in different groups of animals are attuned
to react to different rates of vibration. For example, a cat can hear
higher pitched notes than man, and it is possible that the retinas of
some animals respond to rays vibrating with a different frequency from
those to which the retina of man is adapted. In this connection it is of in-
terest to note that the touch receptors of the skin respond so promptly
to stimulation that one hundred vibrations of a tuning fork per second
can be felt as separate stimuli, whereas to the ear at this frequency the
fork emits a continuous note. The receptors of touch are therefore more
prompt in their response than the receptors of the auditory nerve.
When once the receptor has been stimulated, the impulse passes and
is transmitted to the nerve centers, where it is translated into a par-
ticular sensation. The conditions are really not unlike those which ob-
tain in" the case of the various physical instruments used to receive and
convert into the electric current stimuli of heat, light, chemical energy,
etc. The receiver required to bring about this transformation must be
especially constructed in each case, that for light being the actinometer,
that for motion the dynamo, that for heat the thermopile, and that for
chemical energy the concentration cell. Each of these physical instru-
ments may be considered as a specialized receptor for the purpose of
producing an electric current out of other forms of energy.
In accepting the above analogy we must not fail to bear in mind
that very feeble stimuli are often able to set in operation nerve impulses
that are as potent as those produced by much stronger stimuli. Here
again, we have a physical analogue in the case of relay currents, in
which a feeble electric current may operate to complete the circuit from
independent sources of electric discharge and thus set in motion a much
larger amount of energy.
These general considerations of the nature of a receptor naturally
lead us to the law of the specific properties of nerve, which is to the
effect that, however excited, each nerve of special sense gives rise to
its own peculiar sensation. Thus, in whatever way the chorda tympani
790 THE CENTRAL NERVOUS SYSTEM
nerve is stimulated (chemically, mechanically or electrically) during its
passage across the tympanum, the sensation evoked is that of taste.
And so with the receptor; whatever the means by which it is excited,
whether by the particular kind of stimulus for which it is adapted or by
excessive intensities of other stimuli, excitation always evokes the same
sensation. If the optic nerve or retina is mechanically stimulated, as
by pressure against the outer canthus of the eye or by an electric cur-
rent, the sensation is that of light. Applying these facts to less well-
known receptors, such as those of heat and cold, it is interesting to note
that stimulation of a "cold spot" by extreme heat or by mechanical
or electrical stimuli brings out the sensation of cold.
Properties of Epicritic and Protopathic Receptors
A valuable grouping of receptors of the skin has 'been demonstrated by
Head and his pupils by experiments on himself. Head found after sec-
tion of the skin nerves — of the radial nerve, for example — that deep
pressure and pain were still present in the area supplied by the nerve,
indicating that these deep sensations are carried by the sensory fibers
present in the muscular nerves. In such a paralyzed sensory region the
power of general localization is fairly good, although light, touch, tem-
perature and superficial pain are entirely absent in the overlying skin.
In the case of the fingers the nerves of deep sensibility run in the ten-
dons of the finger muscles, so that after severance of the cutaneous
nerves and tendons of the hand, all sensibility is gone.
During the regeneration of the cut nerve the cutaneous sensations re-
appear at two periods: one group, called the protopathic, begins to ap-
pear in from seven to twenty-six weeks, whereas the other, called epicritic,
does not fully appear for one or two years.2 The protophatic sensations
are of a distinctly lower order than the epicritic. When they alone are
present, there is the sensation of pain, but not that of fine touch; tem-
perature sensations are felt when extreme degrees of heat or cold — above
38° C. or below 20° C. — are applied to the skin, but not for slight de-
grees; the power of discriminating between two points is almost entirely
absent ; and the sense of localization is very imperfect. For example, the
person will often refer the point that has actually been stimulated to a
neighboring normal portion of skin. Protopathic sensibility is more or
less distributed in spots, and it is strongly "affective" in character, caus-
ing an intense subjective sensation. A stimulus that causes only moderate
pain under normal conditions produces in a "protopathic area" a pain
that may be intense.
The epicritic sensation, as will be inferred from the foregoing, responds
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 791
to finer grades of stimulation. By it we can feel the lightest touch and
can discriminate the finest grades of temperature between 26° and 37° C.
The power of localization of the stimulus and the ability to discriminate
between two points also return with epicritic regeneration.
In the spinal cord the nerve fibers carrying one kind of sensation
are grouped together, in the sense that pain sensations, whether deep or
protopathic, run in the same column in the cord. Likewise temperature
sensations, whether protopathic or epicritic, run together.
The Peculiarities of Each of the Separate Sensations
Temperature. — The receptors for temperature are arranged in groups,
some being sensitized for heat, others for cold. These groups of receptors
are called heat and cold spots. They can be very easily detected on an
Fig. 204. — Thermoesthesiometer.
area of skin by means of a pointed hollow vessel, through which water is
made to flow at a temperature a little below or a little above that of the
skin. The instrument is called a thermo-esthesiometer. On a part of the
skin where there are no heat and cold spots, the thermo-esthesiometer will
elicit no sensation either of heat or of cold. This is charted on an outline
drawing of the part as a neutral spot. At other places it will call forth
a sensation of heat, indicating the presence' of heat spots, or at others a
sensation of cold, indicating the presence of cold spots. It will be noted
that certain of the spots are much more reactive than others, and that those
of cold are much the more numerous (see Fig. 205). Both heat and cold
spots are most frequent at the nipples ; then, in order, come the chest, the
nose, the anterior surface of the arm, and the abdomen. They are least
marked on the exposed surface of the skin, such as the face, and they are
792
THE CENTRAL NERVOUS SYSTEM
also very infrequent in the scalp. They are almost absent from the mucous
membranes, which explains why one is able to swallow a liquid that is too
hot for the hand.
The acuteness of the temperature sensation, as with all the other cu-
taneous sensations, depends very much on the condition of the skin,
being most sensitive when this is at the ordinary temperature, but very
imperfect when it is either very hot or very cold. There is also very
marked adaptation of the sense. This can be very well shown by the simple
experiment of taking three vessels of water, one at a moderate tempera-
ture, one very hot and one very cold. If a finger of one hand is placed
in the hot water and a finger of the other in the cold, and they are left
there for a short time, until the skin has assumed the same temperature
as the water, and then transferred to the lukewarm water, the finger
A.
Fig. 205. — Cold spots (A) and heat spots (B) of an area of skin of the right hand. In each
case the most intense sensations were experienced in the black areas, less intense in the lined,
and least in the dotted. The blank areas represent parts where no special sensation of either
kind was experienced. (From Goldseheider.)
transferred from the cold water will feel hot, and that transferred from
the hot water will feel cold. Temperature sensation also produces a
marked positive after-effect. Thus, if a cold coin is placed on the fore-
head and then removed, the cold sensation will persist for some time in
the area of skin on which the coin was laid.
That the receptors for 'heat and cold respond only to one kind of
stimulus, or if to others, only when these are excessive, can be well il-
lustrated by the experiment of touching a cold spot with a very hot ob-
ject: the sensation will be that of cold. The hot object has so pronounced
a power of stimulation that it has overstepped the threshold for heat
of the cold-adapted receptors. The sensation of cold is elicited more
promptly than that of warmth. The distinction between a warm and a
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 793
hot bath may really depend on the fact that in the latter the cold spots
are stimulated as well as those of heat. It is at least interesting to note
that the physiologic reflexes stimulated by either a cold or a very hot
bath are the same ; thus, a rise of blood pressure and a contraction of the
muscles of the skin occur in both cases.
The Touch Sense. — In order to investigate the touch sense accurately,
von Frey has devised a method of using hairs of different thickness each
mounted on a different handle. The hair which produces a sensation
of touch when pressed on the skin so that it just bends is then similarly
pressed on one scale pan of a balance, and the weight required in the
other scale pan to hold the beam horizontal when the hair just bends, is
ascertained. From the diameter of the hair one can then calculate how
many grams per square millimeter are necessary to elicit the sensation
of touch. The following quantitative results have been obtained by ap-
plying von Frey's method to different parts of the body:
Gm. per sq. mm.
Tongue and nose 2
Lip 2.5
Finger tip and forehead 3
Back of finger 5
Palm 7
Forearm 8
Back of hand 12
Calf, shoulder li>
Abdomen 26
Outside of thigh 2(5
Shin and sole 28
Back of forearm 33
Loin 48
That the sense of touch is located in spots — touch spots — can best be
demonstrated on the calf of the leg. If this is shaved and then carefully
explored with a fairly stiff hair, it will be found that there are only
some twelve to fifteen spots in an area of a square centimeter at which
the hair can be felt. Between these spots there is no sensation of touch.
That these spots are composed of specialized receptors can be very clearly
shown by pressing a fine needle into one of them, when no pain will be
experienced but only a peculiar shotty sense of pressure.
Careful examination of the position of the touch spots will further
show that they are grouped around hair follicles, particularly on the side
from which the hair extends — the windward side, we may call it. This
fact explains the well-known experience that an object may be felt more
acutely on a hairy surface than after that surface has been shaved. The
hairs bend slightly when the object comes in contact with them, thus
794 THE CENTRAL NERVOUS SYSTEM
causing pressure to be exerted on the hair follicles, so that the touch
corpuscles in the neighborhood of the follicles, or perhaps the fine nerve
plexus which surrounds them, becomes excited. The influence of hairs
in increasing the touch sensation can be demonstrated by the von Frey
method; for example, in one experiment over an area of 9 square mil-
limeters of skin with hairs present, 2 milligrams were found to produce the
sensation, whereas after the hairs had been removed, it required 36 milli-
grams.
The frequency of touch corpuscles differs very much in different parts
of the body. They are most plentiful on the fingers, relatively infrequent
over the skin of the back, and very scarce in the skin directly over bony
surfaces. They are entirely absent from the cornea, the conjunctiva
of the upper lid, and the glans penis. The adequate stimulus for touch
is evidently deformation of the surface. Pressure exerted over all the
touch corpuscles of a portion of skin is not felt. This can be demon-
strated by dipping the finger into mercury. The pressure of the mercury
is felt on the surface but not in the submerged portion of the finger.
Touch is the most responsive of all the sensations. Thus, as. has already
been noted, a tuning fork can be felt vibrating by the finger when to
the ear its note is a continuous one, and the stimuli produced by a re-
volving serrated wheel can be felt by the fingers as separate even up
to a rate of five or six hundred stimuli per second. Adaptation is also
a marked feature of the touch sense, as is the experience of every one
who has worn flannel underclothing or a plate of false teeth.
Closely related to the tactile sense is the power of discrimination be-
tween two points. This is tested by finding at what distance the two
points of a pair of calipers stand in order to be distinguished as separate.
The result in any given part of the body varies a little according to
whether the points rest on touch corpuscles and according to the rela-
tionship of the calipers to the hair follicles. On an average, however,
we may take the following distances in millimeters as being those at
which the two points can be distinguished over different areas of the
body:
mm.
Tip of tongue 1.1
• Volar surface of finger tip 2.3
Dorsal of first phalanx 6.8
Palm of hand 11.3
Back of hand 31.6
Back of neck 64.0
Middle of back, upper arm and side 67.1
It is clear from this list that the power of discrimination tends to
dimmish in proportion to the lessening mobility of the part. It is greatest
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 795
at the tip of the tongue and the tip of the fingers; it is least on the
relatively immobile skin of the back. These distances are much less when
the points rest on two touch corpuscles. Under these conditions, for in-
stance, the distance for the volar side of the finger tip or even for the
palm of the hand may be only one-tenth of a millimeter; and for the
arm and back it may become reduced to half a millimeter.
Localization of touch is a very accurate process, at least in the most
sensitive parts of the skin, but nevertheless it is very probably a mat-
ter of education. An evidence of this is the fact that in the much more
highly specialized retina the power of localization of objects in the visual
field is a process of education and experience. For this reason a person
from whom a congenital cataract has been removed, can not locate the
objects which he sees until after he has learned by his experience of touch,
taste, etc., to associate the portion of the retina stimulated with a certain
part of the visual field. If this is true for the retina, it is also probably
true for touch. The famous experiment of Aristotle is explicable on the
same basis. If the fingers are crossed and a marble placed between the
crossed fingers, it will be felt as double, since now it touches two skin
surfaces which have not been accustomed to touch the same object, but
educated to feel different objects. Experience associates those two skin
areas with different objects, not with the same object.
The Pain Sense. — It was at one time thought that the sensation of pain
was due to overstinralation of any kind of receptor, but it is now known that
for this, as for^ other skin sensations, there exist special receptors. Thus,
it is found that in certain parts of the body, such as the cornea, and to a
certain extent in the glans penis, pain receptors alone are present, and
in disease the sense of pain may be entirely abolished, whereas that of
touch remains, this condition being called analgesia. Overstimulation of
a touch spot does not, as we have seen, cause pain but only a sense of
pressure. Although pain is appreciated by special receptors, the charac-
ter of the pain is dependent on the other sense receptors simultaneously
excited; for example, a throbbing pain is due to the simultaneous pres-
sure produced by dilated blood vessels, etc. A sensation of pain accom-
panies certain reflexes of a protective nature (nociceptive reflexes, page
825) , and when the reflex is absent the part is likedy to suffer damage. On
this account the pain nerves may be regarded as trophic nerves. The
sense of pain may also occur in structures which are devoid of ordinary
sensibility, such as the intestine and the ureter.
CHAPTER LXXXVIII
THE NERVE NETWORK
In all animals above the Celenterates, no direct protoplasmic contiiiuitj-
exists between the various neurons, the transmission of the nerve impulse
depending on contiguity rather than continuity of the elements that con-
stitute the reflex arc. This transmission may be effected through a syn-
apsis coming in contact either with dendrites or with nerve cells. It is
extremely difficult to know whether there is really any anatomic con-
tinuity between the various fibers which form the network in the gray
matter of the central nervous system. We shall not attempt to discuss
this vexed question here, but in order that we may learn something of the
possible functions of a nerve network, we may consider that present in
the walls of the intestine (plexus of Auerbach and Meissner.) This plexus
seems to have an important function to- perform in connection with the
myenteric reflex (see page 466). At least it has been shown by Meek3 that
after transsection of the intestine the muscular and epithelial structures be-
come regenerated considerably earlier than the nervous plexus, but that
the myenteric reflex, which, it will be remembered, is characterized by a
wave of inhibition preceding one of contraction does not occur until after
the plexus has been regenerated.
NETWORK ON SKIN NERVES
A very important type of nerve network, from the medical viewpoint,
is that which is produced close to their receptor endings by the branch-
ing of the afferent fibers of the skin. Through these branches the vas-
cular reactions following the application of an irritant to the sensory
surface take place without the intervention of any nerve cells. It used
to be thought that such reflex vasodilatation depended upon the trans-
mission of an impulse along an afferent neuron to an efferent vaso-
dilator neuron, a view strictly in consonance with the neuron hypothesis.
That such is not the case, however, is shown by the fact observed 'by
Ninian Bruce* that irritants such as mustard oil applied to the skin
or cornea continue to produce their usual reaction for some time after
796
THE PROPERTIES OF EACH PART OF THE REFLEX ARC
797
section of the posterior roots of the spinal cord, but fail to do so if
the nerve fibers are cut and allowed to degenerate, or if the stimuli are
blocked by applying cocaine to the skin. What actually happens is
evidently that the impulse set up by the irritant as it travels up the
afferent fiber passes on to one of the branches above referred to, along
which it then proceeds to the blood vessels, which it causes to dilate.
That such vasodilator impulses may be transmitted down the fibers of
an afferent nerve has been confirmed by Bayliss, who found that vaso-
dilatation occurred in the hind limb when the posterior spinal roots
were stimulated (see page 234).
Post, roof
gang.-
Fig. 206. — Diagram to show axon reflex of sensory nerve fiber of skin. A stimulus applied to
the skin is transmitted by the sensory fiber (AT), part of it going to the spinal cord (SC), and
part of it passing by the collateral (C) to the arteriole (A), which it causes to dilate.
In this peripheral branching of the afferent fibers of the skin, we
have therefore a sort of neuropile which, like that of certain forms of
Celenterates (see page 782), is capable of serving as a pathway for the
transmission of a sensory impulse to an effector organ without the in-
tervention of nerve cells. Such a reflex is known as an axon reflex, and
it is evident that it may occur through any fiber which gives off branches,
one traveling to a sensory surface, the other to some effector organ, as
occurs in the hypogastric nerves to the bladder (see page 883).
THE SYNAPSIS
At the point of contact between a branch of one neuron and a nerve
cell of the next, we have seen that there exists a structure known as
the synapsis. Although this is described by histologists as a tuft-like
798
branching of the end of the axon (Fig. 207), it may really consist of a
sort of membrane — the synaptic membrane. It permits the nerve im-
pulse to pass in one direction only, from synapsis to cell. Of what this
membrane may be composed, we do not know, but there can be no
doubt as to its great functional importance in connection with the in-
tegration of the central nervous system; for example, failure of an im-
pulse to pass between two neurons may be due to retraction of the
synaptic membrane from the cell, or to alteration in its permeability to-
wards the nerve impulse, perhaps as a consequence of changes in surface
Fig. 207. — Arborization of collaterals from the posterior root fibers around the cells of the
posterior horn. A, ascending fiber in posterior columns; B, collaterals; C, cells of posterior horn;
E, synapsis. (From Ramon y Cajal.)
tension. Similar changes may also be brought about by the action of
electrolytes or by chloroform, strychnine, and other, drugs. As we shall
see when we come to study the reflexes of the higher animals, there can
be little doubt that it is in the synaptic membrane that many of the
peculiarities reside which characterize conduction in a reflex arc as
compared with that in a nerve trunk. The phenomena of summation,
of reciprocal inhibition, of facilitation, etc., are undoubtedly depend-
ent upon such alterations. The synapsis is also almost certainly the
seat of fatigue in the central nervous system, and it is possibly the
structure whose physiologic activity becomes upset in surgical shock.
THE PROPERTIES OF EACH PART OF THE REFLEX ARC
799
THE NERVE CELL
Aside from being a meeting place of fibers coming from various
sources, the nerve cell may have other functions, such as that of rein-
forcing impulses, just as a relay may reinforce an electric current. It
is also responsible for maintaining the nutrition of the axon with which
it is connected. In the case of the posterior root fibers of higher ani-
Fig. 208. — Normal cell from the anterior horn, stained to show Nissl's granules, a, the axon.
(From Howell.)
mals, this function is probably the most important which the cell per-
forms, for it has been found by separating the ganglia from their blood
supply in the frog that, although the cells degenerate in about two
weeks, sensory impulses continue to be transmitted through the gan-
glia. Similar observations have been made in the case of the crab, in
which the cell bodies of the neurons lie on the surface of the ganglion
800
THE CENTRAL NERVOUS SYSTEM
mass, from which they can be separated, leaving merely the neuropile,
through which, however, the reflex continues to be conveyed. After a
time, of course in this case also the reflex disappears, because
an axon can not live indefinitely after it has been separated from its
nerve cell.
These facts regarding the general function of the nerve cell arouse
our curiosity as to its morphologic structure. When nerve cells are
fixed and stained in various ways they show certain elements in the
Fig. 209. — Part of an anterior cornual cell from, the calf's spinal cord, stained to show neurofibrils.
ax, axon; a, b, c, dendrites. (From Bethe.)
cytoplasm — namely, (1) large granules or masses, which stain deeply
with basic dyes .and are called Nissl bodies (Fig. 208), and (2) a fine
network of fibrils passing through the cell substance from one process or
clendrite to another — neurofibrils (Fig. 209). These appearances in fixed
and stained preparations are possibly, however, entirely artificial ; for when
nerve cells are preserved in a living state — by being suspended in some of
the animal's own lymph or blood serum — it is found, when they are ex-
amined by the aid of the ultramicroscope (see page 52), that the cytoplasm
THE PROPERTIES OP EACH PART OF THE REFLEX ARC 801
is composed of a viscous fluid full of extremely minute granules, each of
which apparently consists of a colloidal solution surrounded by a lipoid
envelope (Fig. 210). When the temperature is raised, the granules dis-
appear, and when the cells are deprived of oxygen, the cytoplasm and
nucleus become swollen. A similar swelling of the cell and nucleus super-
venes upon section of the axon; and in stained specimens the Nissl
granules disappear and the protoplasm stains diffusely (chromatolysis).
In embryonic life the processes of the nerve cells appear to be capa-
ble of undergoing a certain amount of ameboid movement, and fibers
grow out from them, indicating, therefore, that in the development
of the nervous system the nerve cells appear first, and the nerves sub-
sequently grow out from them to their proper destination. Prolifera-
tion of isolated tissue cells in vitro has been observed for many other
Fig. 210. — Living nerve cells (from the dorsal root ganglia of a dog three days old) examined
by the ultramicroscope. There are no Nissl bodies or neurofibrils, only fine particles, present in
the protoplasm. (From Marinesco.)
tissues, such as cardiac muscle, renal epithelium and connective tis-
sue. Its occurrence indicates that the therapeutic principle that the
aim of treatment should be to give the diseased organ a rest so that by
cell regeneration it may recover its lost function, is one which may ap-
ply to the nerve tissues of young animals. Whether adult nerve cells
may regenerate is as yet not certain.
This growing out of nerve fibers from their cells is the essential na-
ture of the development of the nervous system in the developing animal.
At birth, unlike the cells of other tissues, those of the central nervous
system are already provided. No new ones are added during postnatal
life. The axons gradually develop from this inherited stock of nerve
cells, and by connecting with other neurons serve to bring about the
integration which is the important characteristic of the adult nervous
802 THE CENTRAL NERVOUS SYSTEM
system. The more complex the integration, the higher the intelligence
of the animal.
Besides performing these functions the nerve cells serve as store-
houses for memory impressions, certain types of them being especially
adapted for this function. The differences observed in the relative thick-
ness of the cell layers composing the cerebral cortex are more or less
associated with the function which it can be shown the different areas
of this possess. Nerve cells are extraordinarily sensitive to deficiency
in oxygen supply, and yet little evidence of oxygen consumption by
the brain can be revealed by the usual methods of investigation (page
396) .
THE INTERMEDIATE OR INTERNUNCIAL NEURON
It would be profitless at this stage to consider the possible influences
that, the intermediate neuron may have on the impulses passing along
the reflex arc. Before doing so we must see how the problem can be
approached, for it is plain that the neuron in the case of the simpler
reflexes is too short to make any investigation of its peculiar functions
a possibility. We must study the characteristics of some type of re-
flex in which this neuron is drawn out, such as the scratch reflex, in
which, as we shall see, it extends from- the shoulder area of the cord
to the lumbar region.
SPINAL SHOCK
Having become familiar with the peculiar properties of each of the
structures which go to make up the reflex arc, we may now proceed to
consider the function of the arc as a whole. It may be well first of all
to consider briefly the experimental method by which such studies may
be made. The object aimed at is to simplify the conditions as
much as possible, for it will be evident that, in the intact nervous sys-
tem, with 'the brain exercising a dominating influence over the great
majority of all the reflexes, it would be impossible by applying a given
stimulus, to predict exactly what kind of reflex response it might call
forth. The reflex will be conditioned upon the accompanying influence
Avhich the brain exercises on the reflex involved.
In order to render the reflex unconditioned, we must remove the in-
fluence of higher centers. This can be done experimentally for the re-
flexes of a great part of the body by cutting the spinal cord above the
level of the segment in which the reflex under investigation resides.
Some of the reflexes elicitable from the cord isolated in this way in-
volve only one or two neighboring segments, whereas others spread
over several. The reflexes which have been most extensively employed
are those which involve the musculature of the hind limbs. Since some
of the receptors concerned come from the skin of the flank and shoul-
der areas, the section is usually made at the upper end of the thoracic
region of the spinal cord.
Spinal Shock in Laboratory Animals
Immediately after the operation a profound condition of depression sets
in, involving all the reflex arcs in the separated portion of cord. This
condition is known as spinal shock. It supervenes in all classes of ani-
mals having a spinal cord, but is much more profound in the higher
than in the lower animals. As a result of this depression, the part of
the body below the section exists in a limp and flaccid condition, and the
application of even very strong stimuli to the skin will evoke no form
of reflex movement. In the case of the lower animals, such as the frog,
803
804 THE CENTRAL NERVOUS SYSTEM
the condition begins to pass off in from twenty minutes to half an hour,
after which a stimulus applied to the skin of the foot is followed by a
typical flexion movement at knee and hip, the so-called flexion reflex.
In the rabbit very little reflex response is elicitable for several hours
after the operation, but in a few days the reflexes return completely
below the level of the section. In the dog, on which a great deal of
work has been done, the involved regions of the body are profoundly
paralyzed. The skin is in a more or less unhealthy, unnatural condi-
tion, the surface cold, the hairs ruffled; and if care is not taken, the
slightest abrasion of the surface may result in a nasty ulceration. On
account of the paralysis of the centers of micturition and defecation,
there is also incontinence of urine and of feces.
The Reflexes in the Spinal Animal
With reasonable attention, however, the dog makes a wonderful re-
covery. After an interval of two weeks the hind limbs, although com-
pletely paralyzed so far as voluntary movement is concerned, begin to
show considerable signs of improvement. The first reflexes to return
are those concerned with the deeper structures, such as the vascular
reflexes, thus bringing the skin back to its normal temperature and
condition. The reflexes of micturition and defecation also soon return,
so that the animal no longer suffers from the continuous discharge of
urine and feces. About the same time the knee-jerk becomes elicitable.
This reflex is obtained by tapping the tendon which connects the patella
with the tibia, the response being a smart contraction of the extensor
muscles of the knee joint. The flexion reflex also begins to reappear.
This is elicited by applying a pinprick or other hurtful stimulus to
the skin of a lower extremity, and when fully developed consists in a
flexion of the knee and hip joints. The evident object of this move-
ment is that the stimulated parts may be removed from the source of
stimulation, and it is plain that all stimuli that produce the flexion
reflex are such as would cause in the intact animal a sensation of pain.
Such stimuli are thus classified as nocuous, and the reflex is styled a
nociceptive reflex. Accompanying flexion of the stimulated limb the op-
posite or contralateral limb usually undergoes a definite extension,
called the crossed extension reflex. The occurrence of this together with
the flexion of the stimulated limb is an important thing to remember
in testing the reflexes in man. Malingerers who attempt to make it ap-
pear that they have some lesion of the spinal cord may know that if
such lesion exists no movement of the leg occurs when the skin is
stimulated, but they are unlikely to know that under these conditions
the opposite leg also fails to show a simultaneous extension.
REFLEXES IN THE SPINAL ANIMAL AND SPINAL SHOCK 805
That the nociceptive reflexes should be among the first to return after
spinal transection is of considerable interest as indicating their im-
portance in the protection of the animal from injury. They are the
essential reflexes of defense, and it is considerably later in the recovery
of the animal before reflexes dependent upon stimulation of other tac-
tile receptors begin to show themselves. The most important of this
latter group of more special reflex movements include the so-called
scratch reflex and the extensor thrust. The scratch reflex, as its name
implies, is the scratching movement of flexion and extension of the hind
limb at a rate of about four contractions per second that occurs when
a mechanical stimulus is applied to the flank and shoulder area of the
animal. For example, if we gently draw a pencil or the fingers back-
ward and forward among the hairs on this region of the spinal animal,
the corresponding hind limb will be brought up so that the claws are
approximately at the place stimulated, and the limb thus directed will
undergo a series of flexions and extensions, designed evidently for the
purpose of scratching the area of skin that has been stimulated. If the
stimulus is a weak one, only the initial stages of the movement may
occur, such as the preliminary flexion of the leg. As we have already
stated, the receptive stimulus calling forth this reflex is ve"ry specific
in nature. A pinprick or rough friction of the reflex area will not produce
it, nor will the application of heat or of a single electric shock. The
most adequate stimulus is one simulating as nearly as possible the con-
dition which would be produced by the movement on the flank of the
animal of some insect. This more or less complicated scratch reflex can
of course also be elicited in animals whose spinal cord has not been cut,
but we can not predict in such cases whether the reflex will occur. The
brain may inhibit the reflex arc and -prevent the movement. In a spinal
animal, however, the reflex always occurs provided an adequate stimulus
is applied. The great importance of the scratch reflex in the study of
the physiology of the spinal cord rests in the fact that a large stretch
of cord is involved in the reflex path. The afferent impulses must enter
at a much higher level than the efferent impulses leave, and between
these two points there must exist a long intraspinal neuron (see- page
813). This permits us to study many conditions influencing reflex action
which otherwise in a reflex located in one segment only it would be im-
possible to investigate.4*
The extensor thrust is elicited by applying pressure to the pad of the
paw or the sole of the foot. It consists of a quick extension movement
of the corresponding limb usually with a flexion of the opposite limb.
After complete recovery from shock, the paralyzed parts of the body
are capable of performing even more complex movements than those al-
806 THE CENTRAL NERVOUS SYSTEM
ready mentioned. For example, if the animal is held up with the hind
legs hanging down, these will often exhibit rhythmic flexion and exten-
sion movements, with the two limbs acting alternately, as they would
in walking or running. This is sometimes called the mark-time reflex.
Another complicated movement may be produced by placing the animal
in water, when it may make the movements of swimming, but its swim-
ming will not be sufficient to keep it on the surface. These swimming
movements are more perfect in the spinal frog.
After complete recovery from spinal shock, the hind limbs are more
or less in a condition of extension contracture; the vascular and other
visceral reflexes are in perfect condition, and a marked rise in blood
pressure occurs when one of the sensory nerves of the hind limb is
stimulated — an experiment which can be performed in such animals
without the administration of any anesthetic, since the animal feels
no pain. In female spinal animals impregnation may occur and preg-
nancy proceed in normal fashion accompanied by the usual secretion
of milk. The significance" of this fact will be dwelt upon later.
SPINAL SHOCK IN MAN
As we ascend the animal scale we find that recovery from spinal shock
takes longer and longer to occur and becomes less and less perfect. In
the case of man, recovery is never complete, for a permanent condition,
M-hich has been called "isolation dystrophy," supervenes before the
symptoms of shock have been recovered from. The tendon jerks are
permanently abolished in complete lesions of the cord in man, and even
when the lesion involves only one lateral half of the cord, this reflex
is either entirely absent or very feeble on the corresponding side, though
normal on the other (Holmes5). Severe lesions above the lower dorsal
region practically always leave the legs in a permanently flaccid con-
dition, with accompanying atrophy, but sometimes automatic movements
of flexion and extension, like those of the mark-time reflex, may set in.
When the injury of the cord is less severe, the limb musculature is
flaccid and toneless for some time, the tendon jerk and the abdominal
and cremasteric skin reflexes being entirely absent. After some time,
however — it may be as early as ten days — the muscles begin to reac-
quire some tone, and a little later the tendon jerk becomes elicitable.
Regarding the behavior of the flexion reflex after spinal injuries in
man, it has been found that the part of it known as the Bakinski reflex
is not elicitable after severe lesions, but in those that are less severe
a flexion of the great toe may occur on stimulation of the sole. Later
this movement may be accompanied by contraction of the hamstrings,
and later still, in favorable cases, by flexion at knee and hip. In these
REFLEXES IN THE SPINAL ANIMAL AND SPINAL SHOCK 807
cases also the Babinski reflex changes from a flexion to an extension
of the great toe. It is important to note in connection with the above
association of movements, that the sensory area of the sole is connected
with the same segment of the spinal cord that furnishes the motor
fibers to the flexors of the toes and the hamstrings (first sacral.) The
recovery after shock therefore sets in earlier for unisegmental reflex
areas than for those involving several segments.
The Cause of Spinal Shock
The relationship of the profundity of spinal shock to the phylogenetic
position of the animal indicates that the shock must be due to the
isolation of the lower spinal segments from the higher centers (Pike6).
It has been suggested that the spinal section in the higher but not in
the lower animals breaks a nervous pathway in which normally the
reflex impulses travel. According to this view, the afferent impulse,
when it enters the spinal cord in the lower animals, chooses the shortest
possible route to the effector neuron of the same or closely adjacent seg-
ments bv the collateral branches springing from the sensory neuron.
In the higher animals, however, it would appear that, although this local
spinal pathway is present and mav be taken, yet it is usually passed
bv and the impulse travels up to the higher centers, from which it is
then transmitted bv the pyramidal tracts to the motor neurons con-
cerned. This would appear to be the pathway for nervous reflex im-
pulses in higher animals — the beaten track. When the spinal cord is
severed, therefore, the condition of shock supervenes because impulses
have not yet learned -that they may find a shorter road to the motor
neuron by the collateral than by the pathway which they usually travel.
Thev learn this only after some time, which explains the slow re-
covery of the reflexes from shock.
It is obviously a difficult matter to supply direct proof in support of
the above hypothesis of the cause of spinal shock, but besides the in-
direct evidence furnished by observations on the degree to which this
condition supervenes in different groups of anmials, the hypothesis
also conforms well with all the other facts which we know regarding
the condition. For example, it is well known that the portion of the
body above the transection of the spinal cord in no way suffers from
the shock. Sherrington has described a monkey the cord of which was
cut below the cervical region, and which immediately after the opera-
tion amused itself by catching flies with the anterior extremities, whereas
the posterior extremities were in a condition of the profoundest shock.
Such experiments further indicate that the shock can not be dependent
808 THE CENTRAL NERVOUS SYSTEM
upon the lowering of arterial blood pressure which a section of the
cord higher than the mid-dorsal region must entail. The poor nutritive
condition of the skin which we have seen to exist in the hind limbs
in shock, shows that the blood vessels in them are profoundly dilated,
but evidently the fall in blood pressure has nothing to do with the
faulty conduction through the spinal cord, for such a fall would affect
the centers for the fore limbs as well as those for the hind, and yet
the former show no symptoms of shock.
Exactly similar shock is obtained by any section of the spinal cord
as high up as the medulla. Of course as the section is made higher and
higher up, the resulting paralysis becomes more and more marked, and
may reach such a degree of severity that recovery of the animal be-
comes an impossiblity.
When we come to consider the functions of the various parts of the
brain, we shall have occasion to study the effects following section at
higher levels of the cerebrospinal axis. Meanwhile, however, it is im-
portant to note that when a section is made across the crura cerebri, so
that the cerebral hemispheres alone are isolated from the rest of the
nervous system, a condition of contracture of all of the extensor muscles
occurs. This condition is known as decerebrate rigidity.
CHAPTER XC
PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC
We may now proceed to study the properties of reflex action occur-
ring through the isolated spinal centers of a spinal animal. There are
two aspects of the question to be considered: (1) the properties of a
single reflex arc, and (2) the action or influence of one reflex arc on
another. The importance of the latter will be evident when it is re-
membered that complicated muscular movements depend for their proper
coordination entirely on the interaction between the various reflex arcs
which compose the nervous system. This interaction, as already ex-
plained, has been called by Sherrington the integration of the nervous
system.
Probably the simplest way to study the physiologic properties of
the simple reflex is to compare the mode of conduction of a
nerve impulse through it with conduction along a simple nerve trunk.
By comparing the two modes of conduction we shall be better able' to
appreciate the modifications to which the impulse is subjected by con-
duction through the reflex arc. The important points are these:
1. The Latent Period. — The latent period, or period which intervenes be-
tween the moment of application of the stimulus and the response, is
very short in the case of a nerve trunk, and under normal conditions
always the same, but is quite variable and sometimes very long in the
case of a reflex arc. Thus, in the case of the conjunctival reflex, which
is produced by applying a stimulus to the corneal conjunctiva (causing
a closing of the eyelids), the reflex time is very short and invariable,
whereas in the case of the scratch reflex it may vary from two and a
half to three and a half seconds, according to the strength of the stimu-
lus. The seat of delay in the reflex arc is probably in the synapse, but
its cause is obscure.
2. Grading of Intensity. — In a nerve trunk the intensity of the im-
pulse is more or less proportional to the strength of the stimulus. This
can be judged by observing either the action current in the nerve by
means of a galvanometer or the response of the end organ; e. g., muscle,
attached to the nerve. In the case of a reflex arc, on the other hand,
there is by no means so evident a parallelism between stimulus and
response. Reflexes, however, vary considerably in this regard. The
conjunctival reflex and the extensor thrust behave according to the so-called
"all or nothing principle;" i. e., the intensity of the response is more or
less independent of the strength of the stimulus. In other reflexes, such
as the flexion reflex and the scratch reflex, the intensity of the response
809
810 THE CENTRAL NERVOUS SYSTEM
is much more nearly proportional to the strength of the stimulus. Thus,
a feeble stimulus applied to the flank calls forth only a slight flexion
of the hind limb of the same side, whereas a stronger stimulus sets
going a typical scratching movement.
3. After-effect. — When a stimulus is removed from a nerve, the effect
which it produces, as judged, for example, by the action current, im-
mediately disappears. There is no after-response. In reflex arcs, how-
ever, such a phenomenon is usually well marked. Particularly is this
the case in the flexion and scratch reflexes of the spinal dog. A mo-
mentary stimulus of optimal strength applied to the scratch skin-area
may produce no immediate response, but after its removal a violent
scratching movement may set in. This after-discharge, in cases in which
the stimulus is strong, may indeed, as in the flexion reflex, be more
marked than the response during the time of application of the stimulus.
In this particular reflex, the after-discharge often takes the form of a
clonus, with a rate of contraction of from seven and a half to twelve
per second. The crossed extension reflex also has a very pronounced
after-discharge, which may outlast the stimulus for from ten to fifteen
seconds. Regarding the phenomenon of after-discharge, Sherrington
has stated that there is "no feature of the conduction of a reflex arc
which distinguishes its mechanism more universally from that of a*
nerve fiber, tract or trunk than lengthy after-discharge."
4. Summation. — When a subliminal stimulus — that is, one that has
in itself no visible effect — is frequently repeated in the case of a nerve,
no response occurs. In the case of a reflex arc, however, such repeti-
tion of subliminal stimuli ultimately calls forth response. This sum-
mation is very evident in the case of the scratch reflex; e. g., one or
two electrical stimuli applied to the scratch field-area call forth, as a
rule, no movement of the corresponding hind leg, but if these same
stimuli are frequently repeated, the typical reflex scratching movement
will occur. Evidently, then, in a reflex arc there is a considerable
amount of resistance towards a single stimulus, which resistance is
overcome by a succession of stimuli. In other words, the threshold of
the excitability of the reflex mechanism becomes lowered as a result
of its previous stimulation. Each stimulus excites the sensory surface
so that it responds more easily to the succeeding stimulus.
5. IrreveTsibility of the Direction of Conduction. — This is well illus-
trated in the so-called Bell-Ma gendie law of conduction in the spinal
nerve roots. A motor impulse travels out of the cord by the anterior
roots, while a sensory impulse travels in by the posterior. This direc-
tive influence can not depend on the nerve trunks or the nerve cells, for
nerve trunks conduct equally in both directions, and so also must the
nerve cell. The irreversibility must therefore depend on the synaptic
PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC 811
connections. It can be demonstrated by observing the action cur-
rent produced in the spinal cord by stimulating the anterior or posterior
spinal roots. In the former case no action current is observed, but it is
very evident in the latter case.
6. The Refractory Period. — This has been well denned by Sherrington
as being "a state during which apart from fatigue the mechanism shows
less than its full excitability." "We are already familiar with the re-
fractory period in the cases of the heart muscle and the musculature of
the esophagus and intestine. For example, the application of a stimu-
lus to the quiescent frog heart while it is contracting in response to an im-
mediately preceding stimulus fails to produce any further effect. The re-
fractory period is extremely brief (one thousandth of a second) in a
nerve trunk, but is much longer in a reflex arc, being probably longest
in the case of the scratch reflex, in which it is demonstrated by the
fact that, however frequently we apply suitable stimuli to the sensory
surface, the rhythm of response of the contracting limb is always the
same. After each stimulus, therefore, a refractory period must become
developed during which a repetition of the stimulus has no effect. It
is evident that the existence of the refractory period is the factor
responsible for the rhythm of the movements.
It is interesting to consider the exact structure of the reflex arc that
is responsible for the existence of the refractory phase. It obviously
can not be a function of the motor neuron, for through the same motor
neuron may be discharged, at one time, impulses which bring about the
scratching movement and, at another, those causing a tonic flexion of
the same muscles. Nor can the seat of the refractory period be in the
sensory area of the skin or the afferent neuron, for if a scratch move-
ment is elicited by stimulation at a point A in the proper skin area,
the rhythm of response which it calls forth will not in any
way be altered by the application of a second stimulus applied at B
at some distance from A and having a different frequency (Fig. 211).
There is evidently, therefore, some part of the reflex arc that is common to
impulses starting both at A and at B, for if in each of these spots a refrac-
tory phase occurred, then there would be interference before the two im-
pulses had reached the centers of the spinal cord. By exclusion, there-
fore, "the seat of the refractory phase seems to lie somewhere central
to the receptive neuron in the afferent arc "-—(Sherrington18).
Many other types of reflex activity illustrate rhythm due to the re-
fractory phase. Two laboratory examples may be given: (1) When
the central end of an afferent root is stimulated in the lumbar region
of the spinal cord, the movement produced is distinctly rhythmic .in
character. (2) Upon stimulating the central end of the sciatic nerve
in a frog whose spinal cord has been cut some days previously, a clonic
812 THE CENTRAL NERVOUS SYSTEM
action of the contralateral foot occurs, and the rate of the rhythm, is
not affected by variation in the frequency of the stimulus.
In all the above cases the refractory period may be held responsible
for the rhythmic nature of the contraction. In other reflexes it exists
for another purpose. In the case of the extensor thrust, which it will
be remembered is elicited by pressure applied to the pads of the plantar
aspect of the foot, the momentary extension of the leg lasts only for a
little less than two-tenths of a second, but is followed by a refractory
Fig. 211. — Tracing from the hind limb of a spinal dog during the scratching movements pro-
duced by applying stimuli at two skin points (A and B), the application of the stimuli beJng in-
dicated by the signals. Not only were the stimuli applied at different points, but at B they
were of much greater frequency than at A. Although there is a slight change in "local sign," it
will be observed that there is no alteration in rhythm, indicating that this property must be a
function of the final common path. (From Sherrington.)
period lasting nearly a whole second, during which a second stimulus
elicits no response. The object of this long refractory period is no doubt
that opportunity may be given for the flexor muscles to perform the
contraction that would naturally ensue during the normal occurrence
of the extensor thrust, as in the act of walking. When the animal
places his foot on the ground, the sudden pressure exerted on the pad
of the foot immediately calls forth the extensor thrust, by means of
PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC 813
which, the weight of the body is temporarily removed from the ground,
and the muscles perform the contractions necessary to produce flexion
of the limb. Although the refractory period is unaffected by the strength
of the stimulus it is very dependent upon the internal condition of the
nerve reflex arc, such as that caused by changes in blood supply or by
narcosis.
Reflex conduction is much less resistant than nerve conduction to various
conditions affecting the nutritive condition of the conducting pathway.
For example, deprivation of oxygen causes but slight interference with
the conduction along a nerve trunk, but very soon abolishes the spinal
reflexes. Even in the frog, reflex movements entirely disappear in thirty
to forty-five minutes after the centers have been rendered completely
anemic, and in mammals they disappear in a few minutes. In the case
of drugs such as chloroform, 0.3 per cent of the drug may be required to
abolish conduction in a nerve, whereas a much lower percentage is suffi-
cient to abolish it in a reflex arc.
Prom the above differences in conduction in a nerve trunk and a re-
flex arc, we learn many facts concerning the importance of the latter,
and we further see that the differences are due very largely to the
synaptic connection.
SUCCESSIVE DEGENERATION
Before concluding the subject, it may be of interest to consider briefly
the method of successive degeneration, by which Sherrington succeeded
in demonstrating the exact tracts in the white matter of the spinal cord
along which the intraspinal neurons travel from one segment to another.
This was worked out in the case of the scratch reflex in the following
manner: The spinal cord was first of all cut in the upper thoracic region,
so that degeneration occurred in all the descending tracts below the
level of the section. In about a year's time these degenerated tracts had
entirely disappeared, and the debris of the degenerated fibers had been
replaced by cicatricial tissue, so that a section of the cord revealed noth-
ing but healthy nervous tissue with cicatrices where the degenerated
tracts had existed. When at this stage a second cut was made across
the cord a little lower than the first one, further degeneration occurred
involving those fibers whose centers were located between the two cuts —
that is, the fibers coming from the intraspinal neurons, with the cells of
which the afferent nerve fibers coming from the skin of the scratch re-
flex area were connected. A section of the cord, stained appropriately
for degenerated fibers, at this time demonstrated these fibers to exist
in the lateral column of white matter, those that travel a short distance—
i. e., between neighboring segments — being near the gray matter, and
those traveling greater distances, towards the outside.
CHAPTER XCI
RECIPROCAL INNERVATION
Reciprocal Inhibition. — It might appear that to bend a joint or to
move the eyeball the only muscular action required would be contrac-
tion of the muscles which flex the joint or rotate the eyeball, and that
the antagonistic muscles would merely become passively elongated.
When we remember, however, that all the muscles of the body are or-
dinarily in a condition of slight contraction, or tone, and that this tends
to become increased. when the muscles are passively stretched, then we
see that for efficient movement there must be inhibition of the tone of
the muscles which oppose those that are contracting. This reciprocal
inhibition, as it is called, is a very widespread function throughout the
animal body. Sometimes it is purely peripheral in origin, as in the claw
of the crayfish, where stimulation of the nerve causes an opening of the
claw due to the contraction of one set of muscles and the simultaneous
inhibition of their antagonists. Instances of peripheral reciprocal in-
hibition in the higher animals are not so common, but are illustrated in
the case of the myenteric reflex, where it will be remembered a contraction of
the intestine over a bolus of food is accompanied by inhibition in front of
the bolus. The reciprocal action in this case is probably dependent on
the myenteric plexus.
On the other hand, reciprocal inhibition of central origin is very com-
mon in the higher mammalia. Thus, in the case of the lateral movement
of the eyes, if we cut the third and fourth nerves to one eye, say, the
left, the external rectus of that eye will alone be under the control
of the nervous system, through the sixth nerve; nevertheless, if we after-
ward cause the animal to look toward the right, as by holding some ob-
ject in that direction, it will be found that the left eye as well as the
right follows the object. Obviously there must be an inhibition of the
external rectus muscle of the left eye, an inhibition which is pronounced
enough to bring about a movement of the eyeball, and which exactly cor-
responds in point of time with the contraction of the external rectus of
the right eye. This movement, due to the atonicity of the external rec-
tus, does not however succeed in causing the eye to rotate beyond the
midline of the field of vision. This is an instance of a willed reciprocal
inhibition ; i. e., a reciprocal inhibition brought about by stimuli coming
814
RECIPROCAL INNERVATION 815
from the volitional center in the cerebrum. The same result may be
obtained by electric stimulation of the center for eye movements on the
cerebral cortex.
The most important details concerning the mechanism of reciprocal
innervation have been obtained by studying the flexion reflex in a spinal
animal which has completely recovered from shock. In such an animal
the tonus of the extensor muscles of the knees is well marked. This
tonus is maintained by afferent impulses transmitted to the spinal cord
from receptors situated in the muscles, and its degree of intensity can
be estimated by the briskness of the knee-jerk, which, it will be remem-
Fig. 212. — Record from myograph connected with the extensor muscle of the knee. During
the time marked by the lower signal, the skin of the opposite foot was stimulated, thus causing
the crossed extension reflex. While still maintaining this stimulation, faradic shocks were ap-
plied to the skin of the foot of the same side (as indicated by the upper signal), with the result
that immediate inhibition of the contracted extensor occurred. (From Sherrington.)
bered, is elicited by tapping the patellar tendon, and consists of a sud-
den extension movement at the knee joint. By observing the briskness
of the knee-jerk we are therefore enabled to form an estimate of the
tonicity of the extensor muscles; and if after doing so we throw the
flexors which are their antagonists into activity by eliciting the flexion
reflex, the knee-jerk will be found much less active. If we prevent the
flexors from acting on the knee joint and the leg is held in an extended
position, irritation of the skin of the leg will cause the flexion of the-
816 THE CENTRAL NERVOUS SYSTEM
disconnected hamstring muscles simultaneously with, a visible relaxation
of the extensors (Fig. 212). If the leg is held properly, this relaxation may
be marked enough to cause a slight flexion at the joint; and in any case,
if the knee-jerk is regularly elicited by equal taps applied to the patellar
tendon, it will be found that, while the flexion is being produced, the
knee-jerk is very much less than normal, if not entirely absent, thus in-
dicating that the tone of the extensor muscles is diminished. This ex-
periment is very striking when performed on a decerebrate animal, in
which, as we shall see, the extensor muscles of the limb are in a per-
manent state of hypertonicity (Fig. 213).
Before it is permissible to conclude that this reciprocal inhibition is a
necessary event in the movement of a joint, we must however show that
it occurs at exactly the same time as the flexion of the antagonist. Sher-
rington has succeeded in doing this in a considerable variety of experi-
,<Ant. Crural N.
(Femoralis)
Sciatic N.
(Ischiddicus)
Fig. 213. — Diagram showing the muscles and nerves concerned in reciprocal innervation. (After
Sherrington.)
ments, one of which we may cite here. If, in a spinal dog, the tendons
of the flexor muscles of the knee joint of one hind limb and the ex-
tensor tendons of the opposite limb are cut, then the former limb will
be unable to flex properly, but will nevertheless exhibit reciprocal
inhibition of the intact extensor muscle, while the latter limb will flex,
but require passive extension to bring it back to its old position. If
suitable stimuli are simultaneously applied to the skin of both legs and
the movements of the isolated muscles recorded, the onset of inhibition
of the intact extensor of the one leg and the contraction of the flexors
of the opposite leg will be found to agree with regard to latent periods,
strength of required stimulus, summation and indeed all the other phys-
iologic properties of reflex action.
Reciprocal innervation can also be demonstrated by stimulating the
central end of suitable afferent nerves — that is, certain afferent nerves
RECIPROCAL INNERVATION
817
Fig. 214. — Reciprocal innervation. Tracings made by myographs connected with E, an ex-
tensor muscle (vastus crureus), and F, a flexor muscle (semitendinosus), of a decerebrate cat.
At signal / the contralateral peroneal nerve was excited, causing contraction of the flexors and in-
hibition of the tone of the extensors. At signal // the flexors were again thrown into contraction by
exciting the contralateral peroneal nerve, and (without removing this stimulus) the homolateral
peroneal nerve was excited (as shown in the lower signal), with the result that the contraction
of the flexors was inhibited at the same time that the extensors contracted. On removal of the
latter stimulus, the former one reasserted its influence. This experiment demonstrates very clearly
the accurate coincidence of the reciprocal action. (From Sherrington.)
818
THE CENTRAL NERVOUS SYSTEM
acting on the same groups of neurons will produce a flexion reflex, others
an extension reflex; thus,- stimulation of the contralateral peroneal nerve
produces a flexion reflex of the hind limb (excitatory for flexors, in-
hibitory for extensors), whereas stimulation of the homolateral peroneal
nerve produces an extension (inhibitory for flexors, excitatory for ex-
tensors). By taking advantage of these facts further proof may be
supplied that inhibition and contraction occur simultaneously, as shown
in Fig. 214.
It is impossible to demonstrate any trace of inhibition of the skeletal
Fig. 215. — Sherringtoii's diagram illustrating the mechanism 'of reciprocal innervation. The
afferent fibers (S) from the skin of the leg and (5') from the flexor muscles of the knee (in
hamstring nerve) pass to the spinal cord, where each gives off a branch which divides into two
others, of which one in each case goes to a motor neuron of the extensor muscles (£) and the
other to a _motor neuron (8) of the flexor muscles (F). Branches also pass across the median
line to similar motor neurons on the opposite side of the cord. As indicated by the plus and
minus signs, the afferent stimuli either stimulate or inhibit the activities of the motor neurons,
the determination of the exact effect being a function of the synapsis. (From Sherrington.)
muscles by stimulation of their motor nerves, thus indicating that in-
hibition is dependent upon the nerve center. Furthermore, since inhibition
occurs along with flexion of the antagonistic muscle, we must assume
that the afferent impulse on entering the spinal cord divides into
two branches, one going to one motor neuron so as to excite it, the other
to another neuron so as to inhibit the tonic stimuli which it is con-
stantly sending to the muscles (Fig. 215).
Since the seat of the inhibition is in the nerve center, it is to be ex-
pected that impulses- transmitted from other parts of the nervous system
RECIPROCAL INNERVATION 819
than the particular level of that reflex, will also be able to induce the
inhibition. In the case of the decerebrate cat this can be demonstrated
by stimulation of the lateral columns of the spinal cord; inhibition of
the extensor muscles of the elbow joint occurs, which is all the more
marked because in such a preparation these muscles are in a state of
hypertonicity. We shall see later also that through the pyramidal tract
impulses may descend from the cerebrum which exercise a marked in-
hibitory influence over the reflex activities of the cord. Similarly the
inhibition itself may be terminated by impulses from other sources, and
the motor neuron thus thrown from a state of inhibition into one of ex-
citation. This fact can perhaps best be demonstrated by exciting the
central end of the homolateral peroneal nerve (which produces a reflex
extension of the leg) while the leg is being held in a flexed position by
stimulation of the contralateral peroneal nerve. This will be clear from a
study of Fig. 214.
Such alternating excitation and inhibition of an active motor neuron
serve to make it possible for rhythmic discharges to occur through the
neuron, as in the action of the muscles of the leg in walking or during
the scratching movement. In order to insure that the same final com-
mon path may be occupied at one time by but one kind of stimulus, either
inhibitory or excitatory, it is further of importance that the after-dis-
charge (see page 810) of the first stimulus should be capable of imme-
diate inhibition ; otherwise, while one reflex was in progress, it would be
impossible to start another of a different type employing the same motor
neuron without confusion of movement. That this occurs can be demon-
strated in the case of the after-discharge of the flexion reflex by stimula-
tion of the proper afferent nerve.
In view of all these facts it is probable that the seat of the reciprocal
innervation is at or about the synapsis. In other words, the synapsis at
the termination of one collateral will allow a stimulating impulse to pass
to the cells of one motor neuron, whereas that at the end of another col-
lateral of the same afferent fiber will allow an inhibiting impulse to pass
to an antagonistic motor neuron, these conditions being, however, readily
interchangeable and thus making even rapid rhythmic contraction and
relaxation a possibility.
The Action of Strychnine and Tetanus Toxin on Reciprocal Inhibition
Under certain conditions reciprocal action may fail to occur, as, for
example, at certain stages of strychnine poisoning and during the action
of tetanus toxin. In order to demonstrate this failure of reciprocal ac-
tion, it is necessary to examine muscles which act on one joint only, and
820 THE CENTRAL NERVOUS SYSTEM
to observe their behavior when an afferent nerve is stimulated which un-
der ordinary conditions would throw them into inhibition. Such a
preparation can be obtained in the hind limb of a dog by cutting all the
muscles that act on the knee joint except the vastus crureus, which in a
normal animal invariably undergoes inhibition when the central end of
the internal saphenous nerve is stimulated. If a suitable dose of strych-
nine is injected, it will be found that stimulation of the internal saphenous
nerve, in place of inhibition, causes contraction of the vastus crureus
muscle. The same result is obtained by injection of tetanus toxin.
The failure of the reflex inhibition explains the symptoms produced
by these substances. It explains, for example, the well-known rigidly
extended condition of the limbs in strychnine poisoning, and the dis-
tressing symptom of lockjaw in tetanus infection. In this latter con-
dition the sufferer* is subjected to extreme torture with every endeavor
that he makes to open the jaw for the purpose of taking food or drink.
Firmer closure is the result because the normal inhibition of the temporal
and masseter muscles does not occur, but instead they become excited
and the jaw all the more firmly closed. Not only does the inhibition fail
to occur, but the above muscles are usually in a state of constant hy-
perexcitability, which it is impossible for the patient to restrain ; indeed,
whenever he attempts to do so the opposite occurs and the excitation
becomes heightened. Chloroform acts on reciprocal innervation in an
opposite way from strychnine and tetanus ; namely, it paralyzes the ex-
citation of the contracting muscles.
Finally, it must be pointed out that this mechanism of reciprocal in-
nervation is by no means confined to the voluntary muscles. We have
already seen that it occurs in the case of the myenteric reflex. It is also
a most important function in the innervation of the blood vessels, dilata-
tion in one vascular area being accompanied by constriction in another.
These facts have been already sufficiently dwelt upon elsewhere (page
243). Sometimes also we may have reciprocal action between differently
acting nervous mechanisms, as for example in the case of the submaxil-
lary glands, which respond to stimulation of the chorda tympani nerve
by dilatation of the blood vessels, an inhibition of their tone occurring
along with stimulation of the activity of the gland cells.
CHAPTER XCII
INTERACTION AMONG REFLEXES
A single reflex acting independently of the rest of the central nervous
system does not really occur. An afferent impulse on entering the cord
spreads so as to involve a large variety of motor neurons, each of which
may, however, be excited through other afferent fibers arriving either
from other receptors or from higher nerve centers. The motor neuron
itself may therefore be a pathway occupied at different times by very
different types of nerve impulse. Hence it is appropriately called the
final common path, and its activity at any moment must depend on the
nature of the various afferent impulses that are transmitted to it through
the synapses. In other words, an entering afferent fiber must communi-
cate in the cord with internuncial paths which are available in various
degrees to other afferent fibers. Since it is through internuncial paths
that the impulse is transmitted to the final common path, it is obvious
that, if afferent impulses in several of these paths were competing at
the same time for the possession of the final common path, confusion of
movement would result unless some provision were made whereby only
one kind of stimulus could be transmitted at one time. "One kind of
stimulus must be inhibited and the other facilitated in its occupancy of
the final common path."
To understand the nature of th'is integration of the central nervous
system, it is therefore necessary for us to consider the factors which de-
termine which of two competing afferent impulses shall obtain possession
of the final common path. Let us take the competition between the flexion
reflex and the scratch reflex of the spinal dog. If we elicit the scratch
reflex and, while it is in progress, apply some nocuous stimulus to the
skin of the hind leg and thus induce the flexion reflex, it will be found
that the scratching movement subsides and the flexion movement comes
on without any overlapping or confusion. If, however, the stimulus
responsible for the scratching movement is a strong one, and that ap-
plied to the skin of the hind leg a feeble one, then the displacement may
not occur (see Fig. 216).
In considering this integration of reflexes, as it is called, we must dis-
tinguish between those that are allied and those that are antagonistic,
and we must further distinguish between reflexes that are simultane-
821
822
THE CENTRAL NERVOUS SYSTEM
ously competing for the same final common path and those which occupy
it successively.
INTEGRATION OF ALLIED REFLEXES
Perhaps the simplest experiment to show this is performed by using
the scratch reflex. The skin area from which this reflex can be elicited
is very widespread (see Fig. 217), the type of reflex produced from any
given area being in general the same, although "the local sign" — that
is, the point at which the animal scratches — will vary according to the
point stimulated. If then we take point A in the reflex scratch area
and apply to it a stimulus which is just inadequate to produce any reflex
at all, and then, while this stimulus is still in progress, apply a similar
subliminal stimulus to point B a little removed from it, the two sub-
Fig. 216. — Diagram showing the reflex arcs involved in the scratch reflex. Ra and R/3 represent
the afferent neurons connected with hairs on the skin of the back and flank. The afferent im-
pulses are transmitted by these fibers, and on entering the corresponding segments of the spinal
cord terminate by synapses on cells of the internuncial neurons, whose arrows Pa and P0 travel
down in the lateral columns to terminate similarly around the cells of the motor neurons that
innervate the muscles of the hind limb. Since afferent impulses coming from elsewhere, par-
ticularly from the skin of the leg (R and L), also terminate on these neurons and may excite
them to a different type of action, the motor neuron is called the final common path (.F.C.).
(From Sherrington.)
liminal stimuli will become effective and produce a typical scratching
movement. In other words, the subliminal stimulus of point A be-
comes added on the final common path with the subliminal stimulus of
point B; the one has reinforced the other and produced, therefore, a
simultaneous integration of allied reflexes.
The receptors from which these mutually reinforcing impulses are re-
ceived need not, as in the above example, be of the same kind, similar
results being obtained by stimulation of receptors of widely different
kinds, such as exteroceptors and proprioceptors (see page 788). For ex-
ample, if a stimulus inadequate to elicit a flexion reflex is applied to the
skin of the leg, and another stimulus, itself also inadequate, is ap-
plied to the central end of some deep afferent nerve in the same leg,
then the two subliminal stimuli will become effective in producing a
INTERACTION AMONG REFLEXES 823
flexion movement. Nevertheless, the more closely allied the receptors
are to one another, the more easily does summation occur.
The mutual reinforcement of allied reflexes lasts for a short time after
the stimulation has been removed, the phenomenon being now known as
successive integration of allied reflexes. It can be illustrated also in the
case of the scratch reflex. If point A on the skin area is excited with a
stimulus that in itself would be inadequate, immediately after an effec-
tive stimulus has been discontinued at point B, then the scratch move-
ment will be kept up smoothly although it will of course become modi-
fied in local sign. For the same reason, a moving stimulus applied
to the scratch area is far more effective than a stationary stimulus ap-
plied over the same extent of area. In such a case the stimulus that
excites a reflex tends by its occupancy of the nervous pathway to faeili-
Fig. 217. — Showing region of body of dog from which the scratch reflex can be elicited. (From
Sherrington.)
tate the spread along the same pathway of succeeding allied stimuli;
towards such it lowers the threshold of excitability of the reflex arc.
This phenomenon is also often called immediate induction, and it is
by no means confined to the spinal cord. It is well illustrated, for ex-
ample, in the case of vision. If a thin line drawn on a white card be
looked at so that it falls on the edge of the receptive field of the retina,
it will not be seen so well as a dot of similar width which is moved
through the same distance as the line.
From these facts we see, therefore, that, when two allied impulses are
being transmitted to the final common path, the one is likely to reinforce
the other, and that this tendency to reinforce the allied impulse is main-
tained for a brief period of time after the impulse has been removed. We
may now proceed to consider the factors which will become operative
in determining to which of two competing or antagonistic reflexes the
final common path will become available.
824 THE CENTRAL NERVOUS SYSTEM
Integration of Antagonistic Reflexes. — Although the phenomenon of
immediate induction encourages integration of allied reflexes, yet it is
frequently succeeded by one of successive induction, in which just the
opposite conditions occur; the resistance in the reflex pathway becomes
lowered for a type of movement antagonistic to that which first occu-
pied the reflex. To understand clearly what relationship this bears to
immediate induction, it may be well to take the instances in which these
phenomena apply in the case of vision. "WJtien the eye, after darkness,
is suddenly directed to a light and then closed, there remains a bright
image (positive after-effect) of the light; but if the light is looked at
for some time, then on closing the eyes it will be seen as a dark pat-
tern (negative after-effect). In the former instance we have an exam-
ple of immediate induction, in the latter, one of successive induction.
In the spinal animal successive induction is demonstrated with equal
ease by using two reflexes that are of a more or less antagonistic charac-
ter— for example, the flexion reflex and the knee-jerk, or better still
the crossed extension reflex and the flexion reflex. If we elicit the knee-
jerk in a spinal dog at regular intervals, with stimuli of equal intensity,
the extension movements (the kicks) will be approximately equal. If
now we apply a nocuous stimulus to the skin of the foot and so throw
the leg into flexion, it will be found, after the flexion movement has dis-
appeared, that the knee-jerk is much more pronounced than previously.
Similarly, if we elicit the crossed extension reflex by nocuous stimuli
of equal intensity applied to the opposite limb, the extension movements
will be approximately equal. By now throwing the limb exhibiting them
into the flexion reflex, the extensor movements will of course disappear,
but after the flexion has been discontinued, they will reappear with
marked intensity.
These facts show us, then, that after the final common path has been
occupied by a reflex of one type, it becomes more available to a reflex
of an opposite type. In other words, it is evident that if the two op-
posite reflexes are constantly competing with each other for possession
of the final common path, they will tend alternately to occupy it, thus
bringing about a rhythmic movement. Such is the mechanism involved
in walking: the leg is lifted from the ground (flexion reflex) ; it is then
brought on the ground, and the mechanical push given to the plantar
surface of the foot brings out the extensor thrust, the appearance of
which is greatly facilitated by the fact that immediately before the flexion
reflex occupied the final common path.
Other Factors Which Determine the Occupancy of the Final Common
Path. — Besides immediate and successive induction, several other fac-
tors affect the relative availability of the reflexes to afferent stimulation.
INTERACTION AMONG REFLEXES 825
Important among these is fatigue of the reflex arc for a particular kind
of stimulus. Many characteristics differentiate reflex fatigue from fatigue
of a nerve as observed in an isolated nerve-muscle preparation. The
most important of these distinguishing features are as follows: (1)
The fatigue comes on intermittently; thus, when the flexion reflex is
persistently elicited, the first sign of fatigue is an irregular decline in
the flexion movement followed by its entire disappearance for a short
time. These lapses become more and more frequent, until at last com-
plete fatigue sets in and no flexion occurs. (2) Reflex fatigue soon
passes off. (3) It appears earlier for weak than for strong stimuli. (4)
The movement produced by the reflex action may also change in character
during reflex fatigue; thus, the beat of the scratch reflex may become
slower and less steady and the foot be less accurately directed to the
spot stimulated. The locus of the fatigue in the reflex arc can not
be the motor neuron itself, for, after this has been completely fatigued
by stimulation of the scratch area, the same muscles may quite readily
be thrown into a perfectly normal flexion reflex by stimulation of the
skin of the hind leg.
It is evident that, when two reflexes are competing with each other
for possession of the same final common path, the one that becomes fa-
tigued will be mastered by the other, especially since at the same time
successive induction will be well developed. Thus, ordinarily the scratch
reflex is much less readily elicited than the flexion reflex, and if both
are excited at the same time the latter will prevail; but if the flexion re-
flex is kept up until it shows signs of fatigue, then by simultaneous
excitation of both reflexes the scratch reflex will obtain the mastery.
Another important factor is the relative strength of the competing
impulses. This depends partly on the nature of the reflex and partly on the
intensity of the stimulus. Regarding the nature of the reflex, it is important
to remember that crossed reflexes are usually less easily obtained than homo-
lateral ones, but ' of still greater importance is the species of reflex —
that is, whether flexion, scratch, extension, etc. The reflex movements
produced by nocuous stimuli (nociceptive reflexes) always take precedence
of those produced by other kinds of stimuli; or, to put it in other
words, "nociceptive reflexes are prepotent in their occupancy of the
final common path" — (Sherrington18).
The best known example of a nociceptive reflex is the flexion reflex.
Its movement is one produced with the intention of removing the stimu-
lated portion of 'the body from the source of the stimulus, all stimuli which
produce it being such as would elicit pain in an intact animal, or if per-
sisted in cause some damage to the skin. In contrast to such nociceptive
reflexes we may take those which are concerned in maintaining the cen-
826 THE CENTRAL NERVOUS SYSTEM
ter of gravity of the body — postural reflexes, as they are called. The
best type of this reflex is the knee-jerk, another good example being the
extensor thrust. The scratch reflex contains a certain element of the
nociceptive in it, and of the simpler reflexes it comes second in its claim
on the final common path. In brief, then, in reflexes which in an intact
animal would cause the sensation of pain and probably some reflex ac-
tivity of the vocal organs, we get in the spinal animal a reflex flexion
movement of the part stimulated with the evident object of removing
that part from the stimulating agency. This reflex flexion secures pos-
session of the final common path whatever other reflex may at the time
be occupying it. Thus, if the animal is scratching itself and something
occurs to hurt its foot, then immediately the scratching movement will
give place to one of flexion, and so on.
Some integration between distant reflex arcs in the nervous system
is to a certain extent an application of the principle of reciprocal in-
hibition of the muscles moving a joint. In this broader integration the
inhibition affects more removed fields of reflex activity so as to harmonize
the activities of one part of the animal with those of every other part.
The manner in which the stimulation may spread along the various
available pathways also depends on the strength of the afferent im-
pulses. If a very feeble stimulus is applied to the skin of the leg in a
spinal animal, the reflex will be represented only by a slight contraction
of the inner ends of the hamstring muscles. As the stimulus is increased
in strength the reaction will spread, until at last it involves all the
flexors in contraction and the antagonistic extensors in inhibition. If it is
still further increased, the flexion movement will be accompanied by an
extension of the muscles of the opposite hind limb — the crossed exten-
sion reflex. Further increase of the stimulus will cause the reflex move-
ment to spread to the anterior extremities, involving, first of all, the
fore limb of the same side (extension at the elbow and contraction at
the shoulder), and then that of the opposite side (flexion at the elbow
and extension at the wrist). A very powerful stimulus applied to the
hind limb will even spread to other more distant muscular groups, such
as those of the neck, causing a turning of the head to the side stimu-
lated, opening the mouth, etc.
This spread or irradiation of the reflex in the spinal cord can not be
entirely explained on anatomic grounds, and must depend, therefore,
upon varying resistance to the flow of the afferent impulse to different
motor neurons, some of which it excites while others it inhibits.
The necessity for adjustable resistance to the transmission of different
afferent stimuli on to the final common path becomes evident when we
remember that, not only are there about five times as many fibers en-
INTERACTION AMONG REFLEXES 827
tering the cord as motor fibers leaving it, but also that each afferent
fiber, after its entry to the cord, gives off several collaterals, each of
which runs to some nerve center in the cord (see Fig. 207).
Certain conditions may break down the path along which the impulse
passes; for example, at a certain stage in the action of strychnine all
pathways become opened up, so that the reflexes which ordinarily do not
occur together, act simultaneously, with the result that a typical convul-
sive movement is produced. Strychnine, as we have already seen, also
interferes with the sorting out of the impulses into inhibitory and ex-
citatory, so that no reciprocal action occurs.
THE TENDON JEEKS; SENSORY PATHWAYS IN
SPINAL COED
Certain responses are of importance largely because of their clinical
application. Of greatest interest in this connection are the tendon jerks.
The location of the sensory pathways in the spinal cord also demands at-
tention.
The Tendon Jerks. — One of the most important reflexes for diagnostic
purposes is that known as the knee-jerk, which is elicited in man by ap-
plying a smart tap to the patellar tendon of a person who is sitting on
a high stool or table so that the joint is passively flexed and the leg
hangs loosely from the knee joint. In this position the extensor muscles
are put slightly on the stretch, and when the patellar tendon is struck,
these muscles contract and cause the leg to become extended as in kick-
ing. This reflex, as we have seen, is also readily elicited in spinal
animals. Its importance from a clinical standpoint depends on the
fact that it may be altered not only in various general conditions of the
body, but also when any pathologic condition disturbs the continuity of
the reflex arc concerned in maintaining the tonicity of the extensor mus-
cles of the thigh. The centers involved in this arc are situated about
the third or fourth lumbar segment, and the afferent impulses come
partly from the antagonistic flexor muscles and partly from the extensor
muscle itself. Abolition of the reflexes may therefore be produced either
by neuritis involving the afferent fibers or myelitis affecting the gray
matter of the cord. That certain of the afferent impulses come from the
hamstring muscles is shown by the fact that when the central end of the
cut motor nerve of the extensor muscles is stimulated electrically, the
knee-jerk becomes much less evident, a result which is also obtained by
massaging the muscles.
Although such facts show clearly that the knee-jerk is of reflex na-
ture, yet there are difficulties in explaining the exact mechanism by
which the tap to the tendon produces the muscular contraction. The
chief difficulty is in accounting for the promptness with which the contrac-
tion occurs, the latent period being very much shorter than that of such
reflexes as the flexion or even the conjunctival. The tot&l latent period
of the knee-jerk, as judged by the time elapsing between applying a tap
828
TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 829
to the tendon and the electrical response observed in the vastus internus
muscle by the string galvanometer, was found by Jolly in the spinal cat
to be 0.0055 of a second, whereas measured in the same way the latent
period of the flexion reflex was found to be just twice as long ; i. e., 0.0106 of
a second. These differences were explained by Jolly as indicating that
the knee-jerk is a simple reflex, involving but two neurons, whereas the
flexion reflex involves three and therefore has twice as long a latent
period. By subtracting from the total latent period the time occupied
in the transmission of the impulse along the nerves and the time lost at
the afferent and efferent nerve endings, we secure a figure giving the time
lost in .the .synapses between the neurons. This synapse time, as it is
called, was found by Jolly to be 0.0021 of a second for the knee-jerk
and 0.0043 of a second for the flexion reflex.7 Snyder obtained somewhat
similar results in man by the same method.
Some authors, particularly Gowers, do not, however, believe that the
knee-jerk is of the nature of a simple reflex, but explain it as being due
to a contraction of the extensor muscles brought about by direct
mechanical stimulation while the muscle is in a hyperexcitable condition
as a result of a reflex increase in its tonicity. Gowers believes that by
putting the extensor muscles on the stretch and the hamstring muscles
in the relaxed condition, afferent impulses are transmitted to the cord
which excite the efferent neurons of the extensor muscles, so as to throw
them into a hypertonic condition, during which the tapping of the ten-
don directly excites a contraction. Of course this hypothesis would ac-
count once and for all for the remarkably short latency of the knee-
jerk, but on the other hand it leaves us many difficulties to explain;
such, for example, as the fact that, although tapping the tendon produces
the jerk, similar tapping of the muscle itself has no effect.
The effective stimulus of the jerk is a slight passive increase of the
tension to which the extensor muscle itself is subjected, and not a stimu-
lation of receptors in the tendon, for it still occurs after the tendon has
been denervated. The importance of the relationship of the hamstring
nerve to the knee-jerk becomes evident in connection with reciprocal
action; thus, when the flexor is contracted, as in the flexion reflex, the
knee-jerk disappears (page 814), whereas when the hamstring nerves are
cut, it is augmented.
Whatever its nature may be, the knee-jerk is of value because of the
ease with which it can be altered not only by conditions affecting the
reflex arc concerned, but also by changes occurring elsewhere in the
central nervous system. The best known of these conditions is that
known as reinforcement. This is brought about by having the patient
make some voluntary muscular effort at the moment that the tap is ap-
830 THE CENTRAL NERVOUS SYSTEM
plied to the tendon. If this voluntary effort coincides in time with the
tapping of the tendon, the knee-jerk will be found much augmented; but
if the two events do not accurately coincide, we may find instead that
the knee-jerk is diminished; that is to say, we may have positive fol-
lowed by negative reinforcement. The most usual way of having the
patient make this voluntary effort is to ask him to lock the fingers of his
two hands together and then at a given signal try to pull the locked
arms apart.
Similar reinforcement may also be produced by the application of a
strong sensory stimulus in some distant part of the nervous system, as,
for example, by pulling the hair or pinching the ear. Accurate work
on the time relationship between the reinforcing act and the tap on the
tendon has shown that the knee-jerk is most marked when the tap ac-
curately corresponds with the voluntary effort or sensory stimulation.
It then quickly declines and an inhibitory influence appears in about
0.3 to 0.6 of a second, immediately after which it becomes pronounced
again, gradually fading off to be no longer evident in about 1.7 of a
second; that is, no change from the normal will be found in the knee-jerk
in about 1.5 of a second after the reinforcing act (Lombard8).
Many explanations have been offered of the mechanism involved in
this reinforcement. The most commonly accepted is that it is due to
the overflow of impulses from other parts of the nervous system, par-
ticularly the cerebrum, upon the reflex arc concerned in the knee-jerk.
During voluntary effort the cerebral impulses discharged down the spinal
cord pass not only to the neuron for which they are intended, but ir-
radiate or spread to other, even far distant, neurons, thus adding their
effect to that of the afferent impulse entering the cord locally. The suc-
ceeding inhibition may be assumed to be due to successive induction (see
page 824). It is difficult to offer direct experimental proof in support of
the explanation, but indirect evidence is furnished, in so far at least as
the augmentation is concerned, by the results of the experiments which
we have already described concerning the integration of allied reflexes
(page 822). To these might be added -the well-known fact that the simul-
taneous application of two subliminal stimuli, one to the cerebral cortex
and the other to the skin of the corresponding body area, may call forth
a contraction of certain groups of muscles.
AFFERENT SPINAL PATHWAYS
The nature of the impulses transmitted by the various afferent path-
ways in the spinal cord. We have seen that the sensory impulses travel-
ing from the periphery to the spinal cord group themselves into three
TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 831
classes: protopathic, epicritic, and deep or muscular. It is important
now for us to consider what becomes of each of these impulses after
entering the spinal cord, for there is abundant evidence that they travel
up to the brain by different pathways. This evidence is furnished partly
by examination of the cord of patients who during life exhibited per-
versions of the skin sensations, and partly by producing experimental
lesions affecting different parts of the spinal cord in animals. In the
disease syringomyelia, for example, enlargement of the central canal
of the spinal cord causes rupture of certain of the tracts and a conse-
quent disintegration of the skin sensations; that is, the sensations of pain
and temperature disappear, whereas those of touch and deep muscular
sensation remain. Or, from the experimental side, if we make a lateral
hemisection of the spinal cord, then after recovery, so far as we can
study it in a dumb animal, we shall be able to show that certain sen-
sations have disappeared, whereas others remain. It is evident, how-
ever, that we must judge by objective and not by subjective phenomena
in these experiments, and our results are only approximate and very
liable to misinterpretation. Important contributions to this subject have
recently been made, particularly by Holmes5 and by Collier,9 on sol-
diers \vounded in the spinal cord.
Summing up the results obtained by the earlier investigators, Brown-
Sequard some sixty years ago stated that hemisection of the cord on one
side produced the following results: (1) paralysis of voluntary motion
of the same side; (2) paralysis of vasomotor control on the same side,
so that the limb is hotter than normal; (3) anesthesia for all kinds of sen-
sation, except muscular sense on the side opposite to that of the lesion;
(4) a condition of heightened skin sensitivity (called hyperesthesia) on
the same side as the lesion, with the exception of a narrow strip of skin
corresponding to the segment at which the cord is cut, which is anesthetic.
These results indicate that in general the skin sensations of pain, touch,
and temperature cross over to the other side shortly after their entry
into the cord, but that the deep muscular sensations remain in large
part uncrossed. More recent experimental and clinical investigations
do not support Brown-Sequard's conclusions.
Ransom has recently shown that the afferent roots of the spinal cord
contain both medullated and nonmedullated nerve fibers, and he be-
lieves that the former transmit the epicritic sensations, and the latter
the protopathic. By tracing those different kinds of fibers into the
spinal cord, he found that the nonmedullated lie in Lissauer's tract for
one or two segments and then pass into the substantia gelatinosa Eo-
landi, which, therefore, appears to be the nucleus for the reception of
the protopathic impulses.
832
Among the reflex activities which become excited by these nociceptive
impulses are those causing a rise in blood pressure — pressor impulses.
This correlation between nociceptive impulses and those affecting the
vascular reflexes has prompted Ranson and von Hess10 to make a care-
ful study in cats of the vascular reflexes that could be elicited from
various lesions in the spinal cord. Two kinds of vascular reflexes were
studied, pressor and depressor, the former being elicited by strong
and the latter by very feeble stimulation of the central end of the
sciatic and brachial nerves. They found that the pathways for pres-
sor and depressor afferent impulses were quite Different. Thus, after
lateral hemisection of the cord, the depressor reflex obtained by weak
stimulation of the sciatic on the same side as the lesion was normal,
whereas it was greatly reduced when the sciatic nerve on the opposite
side from the lesion was stimulated. On the other hand, the pressor
reactions that were most markedly diminished were those from the
sciatic on the same side as the lesion. The depressor fibers evidently
cross in the cord, whereas the pressor do so only to a limited degree.
Further it was found, after cutting ' across the posterior part of the
cord, that the pressor reflexes were interfered with but not the de-
pressor, thus indicating that the former are transmitted either by the
posterior columns of white matter or by the gray matter of the posterior
horns. To determine which, experiments were also performed in which
the posterior columns were alone destroyed and the results compared
with others in which the tip of the posterior horn was included. Since
it was only in the latter experiment that any interference with pressor re-
flexes was found to occur, it was concluded that the posterior horn alone
is concerned in the transmission of pressor impulses.
Regarding conduction of the afferent impulses which in consciousness
produce pain and of those concerned in the reflex changes in respiration,
it was found that the posterior horn of gray matter is not concerned,
from which it is inferred that such impulses are conducted by the same
afferent path that is involved in the depressor reflex; that is to say, as
we have indicated above, the impulses cross in the cord to the opposite
side and ascend in the lateral funiculus. The pathway of the epicritic
and pressor sensations in the cord is not well known. It is believed,
however, that impulses of touch pass up the posterior column on the
same side of the cord for four or five segments, and then gradually pass
to the anterior column of the opposite side.
But for obvious reasons it is mainly from clinical observations and
accurate postmortem location of the spinal damage that the problem
must finally be solved. By these methods it has been shown that sen-
sations of pain and temperature pass through the opposite lateral col-
TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 833
umns, muscle sense through the homolateral dorsal column, while tactile
sensations pass partly ~by the uncrossed fibers of the dorsal column and
partly by the opposite lateral columns. It is interesting that of these
two paths for tactile impulses the crossed one is alone closely associated
with the tract that carries pain (Holmes).
Head and Thompson11 have also found that the sensations are grouped
to the extent that those of one kind travel together, whether they ars
from deep or superficial, from protopathic or epicritic receptors. When
the appreciation of cutaneous pain is lost, so also is that produced by
deep pressure; light touch and heavy touch are also lost simultane-
ously. The appreciation of all degrees of temperature is abolished at the
same time. The ability to discriminate between two points, the apprecia-
tion of weight, the recognition of the vibrations of a heavy tuning fork
applied to the skin — all depend on impulses conducted through the
homolateral dorsal columns.
Because the crossing in the cord of sensory fibers carrying certain sen-
sations occurs more promptly than that of those carrying others, and for
other less clearly understood reasons, the clinical findings are often difficult
of interpretation, especially when the lesions are only partial. The
senses of pain and temperature are undoubtedly lost much more readily
than those of cutaneous sensibility, though sometimes the reverse con-
ditions are found. If a partial lesion of one-half of the cord occurs
about the level of the twelfth dorsal segment, a very common symptom
is loss of power and temperature on the opposite side, but not of touch
even when strong stimuli are applied. This crossed relation does not,
however, occur when the lesion is below the twelfth dorsal.
Regarding the number of segments necessary for the decussation of
each kind of sense fiber, observations on cases in which there is unilat-
eral injury of the cord are being collected, so that the upper limit of the
anesthetic area may be compared with the segmental level of the injury.
It appears that pain and thermal impulses cross quickly (i. e., within a
segment or two) in the middorsal region, but that those of touch cross
somewhat more gradually. In the upper segments the obliquity of
crossing of both kinds of fibers is greater, and in the cervical region
it may require five or six segments for the crossing of pain impulses.
With this increasing obliquity, a distinction appears in the crossing levels
of pain and temperature, for the latter cross a little more quickly.
This conforms with the clinical observation that thermal appreciation
may be disturbed without that of pain. Even the thermal impulses do
not all decussate at the same level, for anesthesia to heat may reach
higher up on the skin area than that to cold.
When recovery occurs, the sensations gradually reappear caudalwards.
834 THE CENTRAL NERVOUS SYSTEM
Sometimes in high lesions of the cord there is anesthesia at the cor-
responding level, but the area supplied by the lower spinal roots, espe-
cially the skin in the region of the anus, is sensitive to one or other
kind of stimulation. In recovery, too, there may be an early reap-
pearance of sensations in isolated caudal areas. The explanation given
for these results is that the fibers carrying different kinds of sensation
have a lamellar arrangement in the cord, the longest fibers being on the
outside (see page 813). When a partial lesion affects the mesial fibers
more than the lateral, there will accordingly be recovery of the caudal
skin areas before those higher up.
CHAPTER XCIV
EFFECTS OF EXPERIMENTAL LESIONS OF VARIOUS PARTS
OF THE NERVOUS SYSTEM
Having learned the main characteristics of reflex action, we shall now
proceed to study the peculiar function of each part of 'the cerebrospinal
system by noting the effects which follow destruction or stimulation of
its different parts.
THE ANTERIOR ROOT
Section of an anterior root produces a limited degree of paralysis in-
volving several muscles having no functional relationships to one an-
other. If several anterior roots are cut, the paralysis becomes much
more extended, and is followed very soon by an evident atrophy of the
muscles concerned. Reflex actions from these muscles are of course im-
possible. Stimulation of the peripheral end of a cut motor root. causes
partial contraction of several muscles, no definite joint movement, how-
ever, being the result, because the affected muscles are not functionally re-
lated and there is no reciprocal inhibition. Flexor and extensor, adductor
and abductor may contract at the same time, thus causing the joint on which
th'ey act to become muscle-bound. It is in the plexus that the nerve
fibers of the roots become sorted out, according to function, into motor
and sensory nerve trunks. The distribution of the anterior root fibers
according to segments in man for the cervical and lumbosacral regions
is as follows:
C5 Deltoid, biceps, brachialis, supinators, rhomboids. Occasionally
radial extensors. Earely pronator radii teres.
C6 Pronators, radial extensors, pectoralis major (clavicular libers),
serratus anticus.
C7 Triceps, extensor carpi ulnaris, extensors of fingers, pectoralis
major.
C8 Flexors of wrist and fingers.
Tl Intrinsic muscles of hand.
S3, 4 Levator ani, sphincter ani, perineal muscles.
S2 Glutei, biceps, semitendinosus and semimembranosus,
SI Intrinsic muscles of foot, tibialis posticus, and muscles of calf.
L5 Muscles of ventrolateral leg (except tibialis anticus).
L4 Extensors of leg and tibialis anticus.
835
836 THE CENTRAL NERVOUS SYSTEM
The knowledge of the segmental innervation of the limb muscles, as
furnished in the above table, is of value in the localization of spinal
lesions. Paralysis of the extension movements of the wrist and fingers,
along with the triceps, for example, usually indicates a lesion of the
seventh cervical. It is more particularly in the trunk, however, that
the segmental innervation of the muscles is evident. The innervation
of the intercostal muscles being unsegmental, one may diagnose the
level of a lesion of the upper thoracic region of the cord by observing their
behavior during deep inspiration. If the fingers are placed in the in-
tercostal spaces, the paralyzed muscles will feel limp and the fingers
sink into the space during the act.
Localization may also be shown by studying the paralyses of the
abdominal muscles when the lesion involves one of the lower six thoracic
segments. When the patient with a lesion of the eleventh thoracic raises
his head from the bed or coughs, the "rectus contracts, but the iliac re-
gions bulge owing to paralysis of the lower portions of the obliques. Under
the same conditions, when the ninth segment is involved the rectus contracts
from about one inch above the umbilicus, whereas below this level it remains
uncontracted, so that the umbilicus is pulled up.
Besides muscular movement, stimulation of the anterior roots in lightly
anesthetized animals sometimes causes evidence of general reflex re-
sponse and of pain. The explanation is that there are present in the an-
terior root certain sensory fibers which are derived from the posterior root
but recur in the anterior, so as to reach the membranes of the spinal
cord where they terminate. The stimulation of the peripheral end of the
motor root must produce, therefore, the same reflex responses as stimu-
lation of the central end of the sensory root. Stimulation of the cen-
tral end of a motor root has of course no effect.
THE POSTERIOR ROOT
The posterior root is the pathway by which impulses of the various
receptors enter the spinal cord. Section of any considerable number of
posterior roots causes therefore, anesthesia of the corresponding skin
and muscle areas, but such a result does not become evident when one
root alone is cut, because the sensory area supplied by each root over-
laps at least half of that supplied by the neighboring roots. Although
it is often difficult to distinguish the segmental distribution in the
ramification of the fibers of the motor roots by finding what muscles
they influence, this is more evident in the case of the sensory roots. On
the trunk itself this segmental arrangement is very plain, but in the
EFFECTS OF EXPERIMENTAL LESIONS
837
extremities it is not at first sight so clear, although it can be accurately
worked out, as is indicated in the accompanying diagram (Fig. 218).
In attempting to determine the level of a lesion from the sensory paraly-
sis, some confusion often arises on account of the oblique course of the
decussation of the sensory fibers in the spinal cord, fibers for the different
sensations not crossing at the same levels. For example, the appreciation
of moderate temperature is often lost slightly higher than that of pain.
The appreciation of the vibrations caused by drawing the base of a
Fig. 218. — Diagram showing the segniental arrangement of the sensory nerves. (From Purves
Stewart.)
heavy tuning fork over the skin is often very useful in locating the
lesion, particularly in the abdomen. When this method is used on the
thorax, however, the skin should be pulled up in folds before the fork
is applied, since otherwise the thorax will act as a resonator and spread
the sensation. Section of two or more sensory roots produces a very
definite area of anesthesia, involving all the skin sensations as well as
several of those of deep sensation.
// the severed roots include all of those going to one of the extremities,
there is not only an entire absence of sensation, but a marked interference
838 THE CENTRAL NERVOUS SYSTEM
with the usefulness of the limb, the condition being called apesthesia.
The exact results depend somewhat on the type of animal. If all the
posterior roots of the anterior extremity are cut in a monkey, the
corresponding limb will not be used in climbing or for other purposes.
It will appear to be completely paralyzed, unless when the opposite
normal limb is in vigorous activity, when the apesthetic arm may be
moved in association.
On careful examination, however, it will be found that marked dif-
ferences exist in the types of paralysis produced by the section of the
anterior and the posterior roots. When a motor root is cut no reflexes
are possible either from the skin or from the cerebral cortex, and the
muscles undergo atrophy. After section of the posterior root, on the
other hand, although reflexes from the skin area affected are impos-
sible, yet movements may be elicited by artificial stimulation of the
cerebral cortex, and the muscles do not atrophy to the same extent.
If only one sensory root of an extremity is left uncut — for example,
the last cervical — so that the skin of the hand is still supplied with
sensation but all the deep receptors are severed, then the limb may be
used to a modified degree. It may be used by the monkey to pick up
nuts, but the movement will be distinctly clumsy and ataxic in nature.
Instead of neatly picking up the nuts, he will 'make wild movements
and often miss them.
The apesthesia is not so profound in lower animals. After section of
all the sensory roots to both hind limbs in the dog, there may be a
certain attempt at walking on the part of the affected limb; that is to
say, when the animal tries to progress, the 'hind limbs, although at first
merely dragged along the ground, afterwards begin to execute walking
movements, which however are very jerky or ataxic in nature and con-
tribute little to the forward progression of the animal, although he
may succeed to a certain extent in supporting the body by the hind limbs.
The importance of the sensory root in controlling the contraction of
the muscles is further illustrated by comparing the contraction curve of
a muscle produced by stimulating its uncut motor nerve with that pro-
duced by stimulating the peripheral end of the cut nerve. In the former
case, the curve is more prolonged and shows a gradual relaxation,
whereas when the peripheral end of the cut nerve is stimulated, the con-
traction is brief and the relaxation is followed by a distinct rebound
or "inertia swing," as it is called. That this difference depends on
afferent impulses is indicated by the fact that, after section of the
posterior roots, stimulation of the uncut nerve in the limb will produce
the same effect as occurs when the cut nerve is stimulated. These re-
sults can be very clearly obtained in the case of the frog, in which
EFFECTS OF EXPERIMENTAL LESIONS 839
also it will be noted that after section of the posterior roots of one side,
the corresponding limb hangs lower than its fellow because its muscles
are toneless.
Stimulation of the central end of a cut afferent root produces, as has
already been indicated, a contraction of the muscles accompanied by a
reciprocal inhibition of their antagonists, so that some definite move-
ment of the joint takes place. This movement is, however, merely a
flexion or extension or rotation, but with no very evident object in view.
In this regard it is quite different from the purposeful movement which
results from stimulation of a skin area, indicating, therefore, that the
receptor apparatus itself must contribute to the nerve impulse some-
thing which causes it to bring about a more perfectly integrated move-
ment of the musculature than is the case when the nerve trunk is di-
rectly stimulated.
Besides the movements of the musculature innervated from segments
which are beside those of the stimulated afferent root, there is a general
reflex response through other centers, for example, the respiratory and
the vasomotor; and, in animals which are not deeply anesthetized, there
is also evidence of pain. Stimulation of the peripheral end of the sen-
sory root has of course no effect.
THE SPINAL CORD AND BRAIN STEM
The results of transsection of the cord have been already sufficiently de-
scribed. It remains to discuss the effect of total ablation or removal of
portions of the cord. As would be expected, there is a marked degree
of shock for some weeks after ablation. During this shock the tone
of the sphincters and vessels is greatly depressed, so that congestion
and edema of the feet, diarrhea and retentio urinae are marked, and
ulceration of the skin is practically unavoidable. After a few weeks,
however, recovery becomes evident in so far as the blood vessels and
sphincters are concerned, but the skeletal musculature atrophies very
extensively and comes to resemble connective tissue. If the spinal
ablation involves the thoracic region, for example, the affected in-
tercostal muscles become stiff and parchment-like; the bones also get
brittle, and visible perspiration can not be produced. On the other
hand, after some time the sphincters functionate more or less normally,
the hair is shed and renewed in normal fashion, and the application of
cold to the skin causes the usual vascular reaction. It is of interest
that in female animals whose lumbar spinal cord has been removed,
pregnancy may take place normally, followed by lactation.
Section Just Above the Medulla. — After such an operation, the ani-
840 THE CENTRAL NERVOUS SYSTEM
mal — bulbospinal, as it is called — shows a greater integration of re-
flexes than is possible when the section is between the medulla and the
spinal cord. Its reflex responses are more broadly integrated, but the
extremities are incapable of executing movements that are of any value
in progression. Movements like those of progression may occur, but they
are quite ineffective. Such animals show marked superiority over strictly
spinal ones on account of the fact that in the medulla are located so
many of the important centers which control circulation, respiration
and the anterior openings of the body; that is, the mechanisms which
accompany the first stages in the digestion of food.
Section Just Behind the Posterior Corpora Quadrigemina. — A very
distinct improvement becomes noticeable in the responses of the animal.
This condition has been studied most carefully in the case of the frog,
which after such a section can walk, spring and swim apparently like a
normal animal, and croaks when the side of the body is stroked. In
the mammal a similar increase in the complexity of movement is evident,
but there is not yet any power of progression.
Section in Front of the Anterior Corpora Quadrigemina. — When the
medulla, pons and mesencephalon are present, as well as the spinal cord,
the condition known as decerebrate rigidity supervenes. This is most
marked in mammals, but is also present to a certain extent in much
lower animals, as, for example, in frogs. It consists of a tonic condition
of the postural musculature of the body, mainly of the extensor mus-
cles; the elbows and knees are extended and they resist passive flexing;
the tail is stiff and straight ; the neck and head are retracted. The con-
dition is undoubtedly due to overactivity of the reflex tonic function of
the spinal centers, for it disappears when the posterior spinal roots are
cut. The reflexes that depend on the tone of the musculature — for example,
the knee-jerk and extensor thrust — are very pronounced in such an ani-
mal, and, on account of the higher integration present, reflexes appear
that are absent in animals having the cerebrospinal axis, cut lower down.
For example, although such an animal can not feel, yet when a stimulus
is applied that in a normal animal would cause pain, the vocal apparatus
may be excited so that a sound or cry of pain is produced. The rigidity
does not affect the respiratory muscles. After such an operation, how-
ever, normal respiration is much more likely to be maintained if the
section is in front of the anterior corpora quadrigemina than behind it.
Removal of the Cerebral Hemispheres.— This furnishes us with what
is known as a decerebrate preparation — that is, one in which the animal
retains everythirig from the basal ganglia downward. The operation
produces a condition which varies according to the habits of the animal.
Thus, in such fish as the Elasmobranchs, which depend for their impressions
EFFECTS OF EXPERIMENTAL LESIONS 841
very largely on the sense of smell, we find that decerebration causes the ani-
mal to become completely immobile. It can not seek food because the
sense of smell, upon which it ordinarily solely depends, has been de-
stroyed. In a bony fish, on the other hand, decerebration causes very
little difference in the behavior of the animal, provided the thalami
and optic lobes have been left intact. It continually swims about and is
able to distinguish edible from iionedible material.
In the frog the result depends very largely upon whether the optic
thalami have been simultaneously removed. Even when these structures
have been removed along with the cerebrum, the animal at first appears
very little different from the normal frog. It springs away when touched,
it climbs up an inclined plane, and when thrown in water it swims. It
is, however, quite incapable of producing any spontaneous movement,
and is in short nothing more than an extremely complex machine, re-
acting always in exactly the same way to the same kind of stimulus.
When the optic thalami are also intact, spontaneous movements are said
to be occasionally observed. Such a frog is said indeed to react on the
approach of winter as normal frogs do by preparing itself for hiberna-
tion, and with spring, to resume its activity and feed itself by catching
insects.
In the bird, in which the operation of removing the cerebral hemi-
spheres is a very easy one, the movements after decerebration may be
quite complicated, particularly if the optic lobes are intact. Such a
bird is more active than usual during daylight, but becomes perfectly
still in the dark. It is, however, unable to distinguish friends from ene-
mies, and it shows no fear.
As we ascend further in the animal scale, the operation of decere-
bration becomes very difficult. Goltz, however, succeeded some years
ago in removing practically all of the cerebrum from a dog by perform-
ing the operation in three stages separated by considerable intervals of
time. The animal lived eighteen months after the last operation, and
during this time it behaved exactly like an automatic machine. All its
reflexes were perfectly normal. It could not -distinguish objects, but a
bright light caused it to close its eyes. During daytime it walked con-
tinuously up and down its cage, whereas at night it would sleep and
remain perfectly quiet. When food was placed in the mouth, the dog
would masticate and swallow in a perfectly normal fashion, and would
reject unpalatable food. While asleep, a very loud sound might awaken it,
and when a harmful stimulus was applied to the skin, the animal would
snarl and growl and attempt to fight the offending object. There were
absolutely no signs of pleasure or of recognition of the person that fed
it or of fear.
842 THE CENTRAL NERVOUS SYSTEM
From these results it is in general clear that the brain stem is able
to adjust the motor and the visceral reactions of the animal to changes
in the immediate environment, but that no power of spontaneous move-
ment is possible. Although in the higher apes and in man removal of
any considerable part of the cerebral cortex is impossible, yet we may
infer, from the results which have just been considered, that in the
higher animals more and more of the action becomes shifted to the
motor centers of the cerebrum. Reflexes which in the lower animals
involve only a spinal or a bulbo-spinal tract, also involve in the higher
forms a cerebral path which is laid down only as the result of experience
and education. The newly born infant is able to perform fewer move-
ments than is the case in the lower forms of animal life, but his power
of learning new movements is incomparably greater. He inherits less
in the way of stereotyped reflexes, but in place of these he possesses
•
innumerable nerve tracts leading through cerebral neurons, through
which new reflex responses may be laid down as a result of education.
In connection with these experiments it is interesting to note that
in lower animals it can readily be demonstrated that the general in-
fluence of the higher on the spinal centers is of an inhibitory nature.
Thus, the latent time of the flexion reflex in the decerebrate frog, as
judged by the Turck method,* is very much prolonged when a stimulus,
such as that produced by a crystal of common salt, is applied to the
optic lobes just posterior to the cerebrum. In general, the influence
which the cerebrum exercises on the spinal centers is an inhibitory one,
whereas that of the cerebellum is augmentatory.
*Turck's method consists in measuring with a metronome the time that elapses between dipping
the foot into weak acid solution and the reflex flexion of the leg.
CHAPTER XCV
CEREBRAL LOCALIZATION
Of much greater practical importance than the experiments in which
the entire cerebrum is removed, as described in the last chapter, are
those in which various parts of it are destroyed or stimulated. From
the results conclusions may be drawn regarding the important subject
of cerebral localization. The effects produced by removal or stimulation of
different parts of the cerebral cortex vary considerably, some parts of the
cortex being set apart for the control of the motor mechanism of the
body, others for the reception and interpretation of afferent stimuli,
while others, and these by far the most extensive, are concerned in the
correlation or association of the sensory and motor centers. It may
be stated in general that: (1) The precentral region of the cerebrum
contains the centers of higher thought. (2) The ascending frontal con-
volution immediately in front of the precentral sulcus contains the
chief motor centers, a center being distinguishable for each muscular
grouping of the body. (3) The postcentral convolution has to do with
the centers for the immediate reception of sensory stimuli, the so-called
sensory centers. (4) A large area occupying most of the parietal lobe
and part of the occipital is undoubtedly associational in its function,
since from it no response can be obtained by .stimulation, etc. (5) Be-
'hind this, in the occipital lobe, there is a center having to do with the
reception of visual impulses. (6) In the upper convolution of the tem-
poro-sphenoidal lobe, is a similar center for hearing.
These centers have been differentiated from one another by anatomical,
experimental and clinical research. At present we shall confine ourselves
to the experimental results. These are obtained by ablation and stimula-
tion, and in considering the results it will be convenient to divide the
centers into motor, sensory, and nonreactive.
ABLATION OF THE MOTOR CENTERS
Removal of the cortex from the area which controls the movements of
a definite part of the body — say, the arm — will be found to produce an
immediate and profound muscular paralysis. The animal does not use
the paralyzed extremity for any purpose whatsoever, and yet the mus-
843
844 THE CENTRAL NERVOUS SYSTEM
cles do not undergo any more atrophy than can be accounted for by
disuse. The extremity does not suffer from any of the nutritional dis-
turbances which we saw supervene upon destruction of the motor cen-
ter in the cord; and likewise local reflex actions elicited by stimulation
of the local receptors are perfectly normal. A pinprick, for instance,
causes the usual flexion reflex.
After some weeks the limb begins to recover and can be used in
volitional movement. Eecovery rapidly progresses until, in the case of
the higher apes, it becomes almost complete in a little over four months.
It occurs earlier in the lower animals. When a center is destroyed on
the cerebral cortex in the case of man, only partial recovery takes place.
So that in general we may say that the higher the animal in the animal
scale, the less complete will be recovery from the paralysis produced by
cerebral ablation.
Regarding the nature of the recovery, several possibilities exist: either
the nerve centers become regenerated in the destroyed area, or the cor-
responding area of the opposite hemisphere or some other part of the same
hemisphere or the basal ganglia assume the function. Evidence has been
furnished by Sherrington and Graham Brown12 tending to show that
the last of these is the most likely cause for the recovery. Thus, it was
found, in working on the arm centers on the brain of the chimpanzee,
that after complete recovery of the paralysis produced by removal of
the center on one side, stimulation of the area that had been removed
caused no movements, indicating that no regeneration had occurred, and
that removal of the corresponding center of the opposite hemisphere,
although followed by paralysis of the arm to which it corresponded, still
did not cause any paralysis of the limb which had recovered from the
previous operation. To see whether some other part of the gray cortex
might have assumed the lost function, the postcentral convolution was
removed two months after the removal of the arm centers. Although
a temporary weakness of both arms resulted, the voluntary movements
were soon as good as before. These results are of course exactly what
we should expect from the experiment on the dog, already described
in which the cerebral cortex had been entirely removed, and the conclusion
that we must draw is that the basal ganglia assume the function of the
lost cerebral cortex.
STIMULATION OF THE MOTOR CENTERS
To investigate the effects of stimulation, it is found that the stimulus is
best applied by the electrical method, one pointed electrode, called the stim-
ulating, being applied to the area under investigation, and the other,
CEREBRAL LOCALIZATION 845
called the indifferent electrode and consisting of a flat plate, being
placed on some other part of the body, such as the skin of the back. This
unipolar method gives much finer results than when the ordinary bipolar
electrodes are employed.
Before we describe the results which have been obtained by the use
of this method, a question arises which it may be well to consider
briefly ; namely, how do we know that the electric current is really stimu-
lating the center present in the gray matter of the cortex, and not
the numerous nerve fibers that constitute the white matter of the brain
and along which, between the two electrodes, it is plain some of the
electric current must pass ? The evidence that we are really stimulating
centers is as follows: (1) The latent period for a response produced by
stimulating the centers is much longer than that which follows upon di-
rect stimulation of ,the white matter. (2) Under deep narcosis, as that
produced by chloral or morphine, the effect of stimulation of the gray
ma1 ler is greatly delayed and altered in type ; whereas1 stimulation of the
white matter gives the usual response. (3) A weaker current suffices to
stimulate the gray matter than that required for the exposed white
matter.
In order to demonstrate the movements which follow stimulation of
the cerebral cortex, it is necessary, as will be inferred from the pre-
ce.ding remarks, that the animal be not too deeply anesthetized. Fur-
thermore, it is necessary to be very careful in adjusting the strength
of stimulation employed, for the results vary considerably accordingly.
When the stimulus is of the proper intensity, the movements are located
in some particular group of muscles, — for example, those of the thumb
or of the hand, — whereas, if the stimulation is strong, the movements
spread over much larger areas. As a result of feeble or moderate stimu-
lation, it is found that the muscles which move are those of the opposite
side of the body, and that the localization is finer the higher the position
of the animal in the scale of development. The movements are perfectly
coordinate and purposeful in character, and reciprocal innervation is
evident.
There is, however, a marked difference in the reactions obtained by
stimulation of the motor cortex and those obtained by eliciting spinal
reflexes. For example, the movements produced by stimulation of the
cortex are much more readily modified by slight variations in the con-
dition of the animal, the blood supply, the degree of narcosis, etc., than
are those elicited by stimulation of receptor neurons. A careful study
of this difference has been made in recent years by Brown and Sher-
rington.13 They observed the behavior of two antagonistic muscles
acting on the elbow when the respective cortical centers were stimu-
846 THE CENTRAL NERVOUS SYSTEM
lated, and found that the latent periods were very variable, the after-
effects indefinite, and inhibition more prominent than excitation. More-
over, the inhibition was more or less independent of the simultaneous
excitation of the antagonistic muscle, in which respect it therefore dif-
fered from the type exhibited in the reciprocal innervation of the spinal
reflexes (see page 814). Nor were the results obtained from a given
cortical center always the same; thus, if a point giving a certain re-
sponse Avas stimulated immediately after previous stimulation, the re-
sult was often reversed; if it was inhibition in the first instance, it
might be excitation immediately afterward. But if sufficient time was
allowed, then the response Avas always of the same kind.
By comparing the effect of simultaneous stimulation of an afferent
spinal root and of a flexion or extension point on the cortex, it was
found that the stimulation of the afferent root when a flexion point
was being stimulated augmented the flexion, but when an extension
point was stimulated, stimulation of the afferent root might change the
response to flexion, the exact result depending considerably on the rel-
ative strength of the two stimuli. The general conclusion that may be
drawn from these results is that the special function of the cortex is to
reverse the centers of purely spinal reflexes when such reversal is de-
sirable or necessary. The cortex dominates the spinal reflexes, and in
general it may be said that its main effect is inhibitory in nature.
It is particularly by the use of the method of moderate electrical stimu-
lation that exact localization has been worked out on the cerebral cor-
tex. As would be expected, this localization is much less defined and
definite in the loAver as compared with the higher animals. In the
higher apes, it has been found that the motor centers are limited to a
narroAv strip of cortex immediately in front of the Rolandic fissure —
the precentral area, as it is called. (Fig. 219.)
From the accompanying figure, it Avill be noted that the centers are
arranged from beloAV upward, in the reverse order to that in which the
muscular groups occur in the body; that is to say, the face, neck, etc.,
are located lowest on the cortex, and the leg highest up. It Avill further
be noted that the extent of the centers for the neck and tongue is very
much greater than for the body or leg, that for the arm being interme-
diate. It is not, therefore, the extent of the muscular tissue that de-
termines the size of the cortical area controlling its movements, but
the type or complexity of the movements that the muscles perform.
The complex movements of the tongue and the vocal cords evidently
require greater cortical representation than do the coarser movements
of the large mass of muscular tissue of the trunk. The centers extend
someAvhat upon the mesial aspect of the brain, but occupy here only a
CEREBRAL LOCALIZATION
847
very small part of the superficial gray matter. They extend also into
the fissure of Rolando and the other fissures, and the extent of the ex-
citable area which is thus buried away in the fissures may exceed that
on the free surface of the hemispheres.
It will be noted that there are two centers for the movements of the
eyes, one in the frontal lobe isolated from the motor area, and the
other at the tip of the occipital lobe. The former is the motor center
for the conjugated movements of the two eyeballs, whereas the latter
functionates in association with the so-called visual center, which re-
ceives the visual impressions and transmits them to other parts of the
Anus & Vagina Sulcus centralis
Toes,?' !
Ankle
Knee,
Abdomen
^Chest
Shoulder-
YES
Sulcus centralis
Mastic
Fig 219.— Outer aspect of the brain of the chimpanzee, showing the position of the centers.
Electric stimulation at the parts indicated causes coordinate movements of the corresponding mus-
cle groups. (After Sherrington.)
brain to be interpreted and correlated. Excitation of the center for
eye movements in the frontal lobe, say, of the right side, causes con-
jugate deviation of both eyes to the opposite side, that is, to the left;
and it can readily be shown that this movement of the eyeballs is the
result of reciprocal innervatioii of the extraocular muscles (page 814).
Even at the risk of repetition AVC will again describe the fundamental
experiment that demonstrates this. When the eyes, as in the above
experiment, move to the left, it means that the internal rectus of the
right eye and the external rectus of the left are contracting, whereas
the external rectus of the former and the internal rectus of the latter
THE CENTRAL NERVOUS SYSTEM
are becoming reciprocally inhibited, the other muscles participating
to a slight degree. If all the nerves to the extraocular muscles of the
right eye are cut except the sixth, which supplies the external rectus,
it will be found that this eye looks permanently toward the right side ;
that is, an external strabismus is produced. If now the right cortex is
stimulated, both eyes will, as before, move to the left, although the
right eye will not move farther than the middle line. Its movement as
far as this, however, must evidently be due to an active inhibition of the.
external rectus muscle, for none of the other muscles can act since the
nerves are cut.
The experiment of conjugate deviation brings out another point re-
garding cerebral localization — namely, that the muscles which ordinarily
act along with muscles on opposite sides of the body, are innervated
from both sides of the cerebral cortex. This applies not only to the
movements of the eyes, but to the respiratory and other movements of
the neck and trunk. Destruction of the trunk center of the cerebral
cortex on one side does not produce any paralysis, while stimulation in
this region produces an equal movement on both sides. "We may say
therefore that bilaterally acting muscles are innervated from both sides
of the cerebral cortex.
The movements produced by stronger stimulation of the cerebrum do
not remain localized, and they persist for some time after the stimulus
has been removed. Still further increase in the strength of the stimulus
may cause the contraction to spread until it affects all parts of the
body, giving rise to a convulsion. There are two types of contraction
during this convulsion, the first being a strong tonic contraction, which
outlasts the stimulus for some time and then gives way to a series of
clonic contractions, occurring at intervals of from six to ten per second,
and gradually getting slower as the fit dies away. The convulsion al-
ways starts in the muscle group represented by the cortical center that
is being stimulated. Thus, if the hand area is the seat of stimulation,
the convulsions begin in the muscles of the hand; then they spread to
the muscles of the forearm and shoulder on the same side, and then to
the face, the trunk, and the leg; and if the stimulus is strong enough,
they may spread to the opposite side and thus involve the whole body.
This "march" of the convulsion depends upon the overflow of the stimu-
lus on to the various centers of the brain, and the pathways through which
it occurs seem to be located in the subcortical centers, for the spread
is not prevented by isolating the cortical centers from one another by
cuts encircling them, or by division of the corpus callosum. Never-
theless, the centers do in some way become involved in the spread, as
is indicated by the fact that the complete excision of one of them
CEREBRAL LOCALIZATION 849
will exclude the corresponding muscular area from participation in
the fit.
CLINICAL OBSERVATIONS
The foregoing results obtained by experimental stimulation in animals,
are very similar to the symptoms observed in man when the cerebral
cortex is stimulated by the pressure on it of a meningeal tumor or a
spicule of bone. Such stimulation causes contraction in the correspond-
ing muscular area; the contraction then spreads to neighboring groups
of muscles, and may ultimately involve the whole musculature of the
body in a convulsive fit, like that produced in animals. This is known
as Jacksonian epilepsy, and it is to be distinguished from ordinary
epilepsy by the fact that the patient does not become unconscious dur-
ing the fit. Like ordinary epilepsy, however, the Jacksonian' type is
usually preceded by a peculiar sensation of numbness or tingling in the
area that is to show the first contraction. One of the greatest achieve-
ments of modern brain surgery is the cure of Jacksonian epilepsy, by
trephining the skull over the affected center and removing the meningeal
tumor or spicule of bone which is responsible for the stimulation. To
enable the surgeon to locate exactly the position of the irritating body,
it is necessary to examine the patient very closely as to the muscular
group which is initially affected during the convulsions, and then to
examine an outline map of the cerebral hemisphere indicating the po-
sition of the various motor and sensory areas as deduced mainly from
experiments on the higher monkeys and verified by the experience
gained by previous operations. Topographic maps indicating the sur-
face markings corresponding to the various convolutions of the cerebrum
must also be used. In such operations the surgeon often has the op-
portunity of experimentally verifying the position of various centers.
CEREBRAL LOCALIZATION (Cont'd)
SENSORY CENTERS
That the motor centers are located in the areas which we have just
described does not indicate that the nerve cells of the centers normally
dominate the reflex movements which their stimulation elicits. The motor
centers, strictly speaking, are the anterior horn cells of the spinal cord ;
and the so-called motor centers of the cerebral cortex must really repre-
sent nothing more than internuncial neurons between the entering and
leaving paths concerned in reflex movements. They are only links in
the long cerebral chain — way-houses on the reflex cerebral pathway.
According to this view we should expect that these centers would be
the ultimate recipients of sensation, as well as the distributors of motor
impulses; sensorimotor, they have been called. Such, however, is not
the case, for Sherrington has shown that the centers most directly con-
cerned in the reception of sensory impulses are not located in front of
the Rolandic fissure but immediately behind it in the ascending parietal
or postcentral convolution. Electrical stimulation in this region does
not evoke any immediate response, at least if the stimulus is not too
strong. A movement indirectly due to the receipt of a sensation may
be elicited by a strong stimulus, just as is the case when the visual cen-
ter in the occipital lobe is strongly stimulated, producing secondary
movements of the eyes.
Histologic, experimental and clinical evidence has been furnished to
support this location of the chief sensory center. The clinical evidence
was furnished by Harvey Gushing,14 who induced two patients in whom
this part in the brain was exposed to allow him to stimulate it while they
were in a conscious state. As the result of the stimulation of the post-
central convolution definite sensory impressions were experienced, consist-
ing of a sensation of numbness or deadness to tactual impressions, but
no muscular groups under Avent movement unless the precentral con-
volution was stimulated. During these movements, moreover, no sen-
sations were experienced by the patient except those which accompanied
the change in the position of the part that was moved. The sensations
which are thus represented on the cortex are those of touch discrimina-
850
CEREBRAL LOCALIZATION 851
tion and those relating to the position and movements of the muscles.
Pain and temperature sensations do not seem to have cortical represen-
tation.
There is of course a close association between sensory and motor cen-
ters, as is illustrated in the experiment described elsewhere under the
head of apesthesia (page 838), in which it will be remembered that the
complete section of all the posterior roots of an extremity renders the
part as effectively paralyzed for volitional movement as it would have
been had the motor roots themselves been cut. Afferent impulses are
therefore necessary for the efficient volitional control of the muscular
movements.
SENSE CENTERS
Attempts to locate exactly the position on the cerebral cortex where
impressions of the projicient sensations — vision, hearing, etc. — are re-
ceived are of course more or less difficult because of the fact that the
experiments have to be performed on dumb animals. Nevertheless some
information can be gleaned from the results of ablation and stimulation
of various parts of the cortex, ablation causing, for example, definite
evidence either of blindness or of deafness, and stimulation causing
movements of the eyes or ears similar to those ordinarily observed when
these organs are stimulated in the usual way.
The auditory center is located in the back part of the superior temporal
convolution. Stimulation of this area in animals causes a pricking up
of the ear on the opposite side as if the animal heard a sound. Clinical
observation has confirmed this conclusion.
The visual center is located in the occipital lobe. It is important to re-
peat again that there are two centers on the cerebral cortex concerned in
vision: the frontal visual center, located as we have seen in the frontal
lobe, and the so-called visual center itself, located in the occipital lobe.
Stimulation of the frontal visual center produces a prompter movement
of the eyes than does stimulation of the occipital center, indicating that
the frontal center has the immediate control of the muscular movements,
whereas the occipital lobe is probably concerned in the adjustment of
the muscular reactions which are necessary in controlling the eye move-
ments, so that the objects may be properly viewed and judgments
formed, by the extent of the movements, of its distance, position, etc.
The actual response to stimulation of the occipital centers shows that
the lobe on one side is connected with the corresponding half of each
retina, the fovea centralis being, however, connected with both lobes.
852
THE CENTRAL NERVOUS SYSTEM
ASSOCIATION AREAS
The brilliant outcome of the researches conducted by the experimental
method in enabling us to locate the chief motor and sensory areas of
Motor leg area
•f*t I*
4
foVjS*A
»VKA*tf«
Visuosensory
Visuopsychic
Fig. 220. — Three sections through different parts of the cerebral cortex. For description see
content. (Redrawn from Starling.)
the cerebral cortex leaves yet uncharted those vast areas lying between
them which do not respond in any definite way to artificial stimulation,
and the ablation of which results only in more or less indefinite symp-
CEREBRAL LOCALIZATION
853
toms. In order to discover the exact function of these areas, it has been
necessary to employ an entirely different method— that of histologic
and embryologic examination. When the patterns of the gray cortex
are compared with the habits of the animals, in different groups of
animals (phylogenetic study), or even in different individuals of the
Fig. 221. — The location of the chief motor and sensory areas on the outer (A) and mesial (B)
aspects of the human brain, as determined by the microscopic structure of the cortex. These
maps are only approximately accurate, but they indicate in a general way how the cortex is
structurally composed. (From Starling after Campbell.)
same group (ontogenetic study), much useful knowledge concerning
cerebral localization can also be gained. In the human animal much
progress is being made by comparing the structural pattern of the cor-
854 THE CENTRAL NERVOUS SYSTEM
tex in different parts of the normal brain with that found in the brain
of psychopathic individuals whose mental symptoms have been care-
fully studied before death.*
For the purpose of this work it is necessary to recognize several
laminae or layers of nerve cells and nerve fibers composing the cortex.
The most practical division is represented in Fig. 220, and is as follows:
(1) a narrow superficial layer of nerve fibers, with a few scattered cells —
the outer fiber lamina or molecular layer; (2) a much wider layer of small,
medium and large pyramidal cells — the outer or pyramidal cell lamina ;
(3) a layer of granules — the middle cell lamina; (4) an inner layer
of nerve fibers, sometimes containing large solitary cells (Betz cells) —
the inner fiber lamina; (5) a layer of polymorphic cells — the inner cell
lamina. This five-layer type undergoes structural modifications in the
different regions of the cortex, and these modifications possess a dis-
tinct functional significance. The only part of the brain in Avhich they
can not be recognized is the hippocampus and the pyriform lobe. Based
on the relative thickness of these layers maps of the cerebral cortex
have been produced. The most important are those of Brodmann and
Campbell, of which the latter is reproduced in Fig. 221. Two re-
gions can be very definitely located; namely, the precentral or Betz-
cell area, and the visual or visuosensory area of Campbell. Surrounding
the visuosensory area is a definite visuopsychic area, and similarly,
bordering on the precentral is the so-called intermediate precentral
area. At the very front of the frontal lobe is the prefrontal area. Post-
central and intermediate postcentral areas are indicated, but the re-
mainder of the center is undefined.
Reasoning from phylogenetic and ontogenetic data, we can assign to
each of these laminae a functional significance, which according to Bol-
ton is as follows: The outer cell lamina (pyramidal cell lamina) proba-
bly constitutes the "higher level" basis for the carrying on of the higher
or psychic cerebral functions. It is a prominent, feature of the human
cortex, the last cell layer to be evolved, and the one which undergoes
retrogression most readily. During development it rapidly attains ma-
turity in the visuosensory area, next in the visuopsychic, and only later
in the prefrontal region. In the visuopsychic area it is practically of
the same depth as in the visuosensory, whereas in the prefrontal region it
develops according to the mental capacity of the animal. In patients ex-
hibiting symptoms of dementia this layer of cells is distinctly deficient.
These facts indicate that the outer or pyramidal cell lamina "subserves
the psychic or associational functions of the cerebrum" — (Bolton).
*An excellent account of the physiologic basis for such work is given by Bolton in Leonard Hill's
Further Advances in Physiology. We have made free use of this article in the present review anrl
would strongly recommend its perusal by any who may desire further information.19
CEREBRAL LOCALIZATION 855
The middle cell lamina is much hypertrophied in the so-called projec-
tion areas of the cerebrum — for example, in the visuosensory area (see
Fig. 20), where it is so thick that it is usually described as being divided
into two parts by a narrow fiber band (the line of Gennari). Diminution
in the layer occurs in the visuosensory area in long-standing cases of
atrophy. "It seems therefore primarily to subserve the function of re-
ceiving afferent impressions whether these arrive directly from the lower
sensory neurons or indirectly through other regions of the cerebrum."
The fifth or inner cell lamina is the first to become differentiated, and
it is of extremely constant depth in the adult. It is not much affected in
amentia, unless when this is very severe, as in patients who are unable
to carry on the ordinary animal functions. In short, "it subserves the lower
voluntary and instinctive activities of the animal economy" — (Bolton).
Taking the results as a whole, it appears that the region of the cortex
behind the Rolandic fissure consists of sensory areas and association
areas which may be immediately connected with them (visuopsychic and
intermediate postcentral) or more removed (in parietal lobe). The por-
tion in front of the Rolandic fissure, on the other hand, contains the
efferent areas, of which the precentral may be regarded as of lowest
grade. The motor discharges from it are conditioned upon impulses
coming partly from the adjacent intermediate precentral area, in which
again are elaborated those received from the sensory areas, and partly
from those coming from the prefrontal region, which is the most com-
plex zone of association. This last is indeed the supreme dominating
area. It coordinates or integrates the activities of the other association
areas and may be considered as the seat of the intellect. The evidence
for this high evolution of the prefrontal area is very strong. It is the
last portion of the cortex to be evolved and the first to undergo retro-
gression. In idiots and imbeciles the thickness of the pyramidal cell
layer in this region is directly proportional to the mental power, and
in dementia degrees of retrogression occur that vary directly with the
existing grade of dementia. In normal brains this layer is the very
one which varies in depth in different individuals. Along with its high
development in the brain of man, as compared with that of other ani-
mals, goes hand in hand a great increase in the other association areas.
Thought is the product of integration between these various associa-
tion areas, and articulate and written language the outward manifesta-
tion of the process. It is owing to the relatively great extent and com-
plexity and constant development of the prefrontal lobe that man so far
excels even the highest apes in his intellectual activity, and it is owing
to the relative functional development of this lobe that individuals dif-
fer from one another in their mental powers.
CHAPTER XCVII
CONDITIONED AND UNCONDITIONED EEFLEXES .
In studying the reflexes in the spinal animal, we have seen that the
effect of a given stimulus or of different stimuli is predictable with
absolute certainty. There is a fatality in the responses. When the
higher centers are included in the reflex arc, the reflexes become modi-
fied or inhibited by events occurring in other parts of the central ner-
vous system and the results come to be more and more unpredictable.
The reflex comes to be a conditioned reflex (Pavlov). Studies of the
circumstances affecting these conditioned reflexes really constitute a
study of the function of the higher centers in the brain. Since such
experiments must be performed on the lower animals, we are limited in
the investigation to motor responses, for we have no way whatever of
studying the subjective sensations produced. The motor phenomena by
which the animal may express its sensations can be interpreted by us
only in terms of psychologic ideas that in large part are derived from
our own experiences. Obviously the conclusions that can be drawn
are subject to very great sources of error, and it must be considered as
one of the greatest advances of modern physiology that Pavlov and
others should have succeeded in evolving methods by which we may ar-
rive at conclusions regarding the nature of certain of the integrations
that occur in the higher centers of the nervous system preceding the
motor manifestations by which the animal expresses its sensations.
The methods employed for the study of these higher integrations of
the central nervous system all depend on the reactions of the animal
that are associated with the taking of food. When the food is actu-
ally placed in the mouth, it excites a secretion of saliva, whatever the
circumstances may be. This is an unconditioned reflex. Suppose, how-
ever, that every time food is given a particular sound is made; after
some time it will be found that the occurrence of the sound alone is
sufficient to cause a secretion of saliva. In other words, a conditioned
reflex has been formed. Similarly, sight or smell or any other type of
sensation may be made the excitant for the conditioned reflex. The
secretion now becomes psychic instead of merely physiologic. To quote
Bayliss: "Any phenomenon of the outer world for which the animal in
question possesses appropriate receptors can be drawn into temporary
856
CONDITIONED AND UNCONDITIONED REFLEXES 857
association with salivary secretion, so that it becomes an exciter of se-
cretion if only it has been frequently presented at the same time with
the unconditioned reflex stimulus, food in the mouth."
Work along lines similar to that devised by Pavlov has more recently
been undertaken by students of animal behavior, who have utilized the
acquired habits of an animal in searching for its food in order to study the
influence of conditioning circumstances on its procedure. The advantage
of this method depends mainly on the fact that it can be applied to all
groups of animals. In carrying out such an observation, the animal is
placed in one compartment of a cage, from which it is then released to
a second compartment, the end of which is divided into two passage-
ways, one leading to food, the other leading to some compartment in
which the animal is punished for its mistake as by receiving an electric
shock. Objects such as colored lights are placed in the different pas-
sageways, and the animal by repeated trial comes ultimately to learn
which particular colored light signifies the passage along which he
will receive food. A reflex has therefore become established conditioned
on the particular colored light.
On account of the unavailability of his publications, it is impossible
at present to give any complete account of Pavlov's discoveries. A few
facts, however, are of such importance that it is necessary for us to
state them here as far as we know them. (See Bayliss, Physiology.} Two
mechanisms seem to be concerned in the conditioned reflexes: (1) that
of temporary association, and (2) that of analysis. Temporary associa-
tion is well illustrated in the above experiment in which the secretion of
saliva is induced by a sound. Temporary association of the sound with
the secretion of the saliva may readily be inhibited by all kinds of ex-
ternal phenomena; thus, if the dog's attention becomes diverted while
the conditioned reflex is being stimulated, the response does not occur.
In a dog that had been trained to secrete saliva to the sound of a par-
ticular metronome beat, inhibition occurred one day because, just as
the dog was being presented with the food, the laboratory servant made
a noise outside of the building which diverted the animal's attention.
The conditioned reflex may also be interfered with by internal inhibi-
tion, which is illustrated by experiments in which, after a dog has been
trained to respond to a given conditional reflex, several occasions follow
when food is not given to the animal after the particular sensation to which
it has been trained to respond. The condition — for example, a sound — loses
its effect. This is internal inhibition, but it is a temporary condition
since the reflex returns of itself after a period of rest.
These experiments illustrate what is meant by the formation of tem-
porary associations occurring in conditioned reflexes, but in order that
858 THE CENTRAL NERVOUS SYSTEM
there may be a fine discrimination between those stimuli which shall
and those which shall not serve to call forth the conditioned reflex, an-
other mechanism becomes involved — that of analysis. This is performed
by a sense organ the function of which is to separate and distinguish
the complicated phenomena of the outer world. For example, it has
been proved that small differences in the pitch of a musical note may
determine whether or not a conditioned reflex will be excited or in-
hibited, as in the case of one animal that was trained to respond by
the secretion of saliva to a tuning fork vibrating at 100 per second. It
was found that no secretion was produced by a tuning fork vibrating
at 104 or at 96. Much work has also been done with the skin receptors.
Thus, when a given spot of skin is stimulated every time that food is
presented, this becomes an active spot for the conditioned reflex. At
the same time another spot may be stimulated so as to be associated by
the animal Avith the nonpresentation of food; it is a conditioned reflex
for no food, and is associated with the absence of salivary secretion.
By comparing the responses from active and inactive spots when both
are stimulated either simultaneously or at close intervals, much can
be learned concerning the delicacy of appreciation for external stimuli
and the influence of the inhibitory on the excitatory process. Bayliss
cites the following experiment, Along a series of spots on the skin
of the leg five devices are arranged for producing equal mechanical
stimulations of the skin. The four uppermost of these are made active
spots for the salivary reflex, and the lowest one inactive — that is, when-
ever it is stimulated no food is presented. Let us suppose that upon
administering mechanical stimuli of equal intensity to each of the active
four spots, a certain amount of saliva is produced in a certain time; if
now the inactive spot is stimulated and then thirty seconds later one
of the uppermost spots, there will be no secretion. The previous stimu-
lation of the inactive spot must evidently have caused an inhibition to be
set up in the nerve centers concerned in the reflex. This inhibition only
gradually passes away, disappearing first in the spot farthest removed
from that made inactive, but it may take several minutes before all the
active spots have reacquired their original sensitivity.
The persistence of the inhibition produced by stimulating the inac-
tive spot in the above experiment indicates an important factor in con-
nection with the production of conditioned reflexes. For example, an
animal can be trained to know that in a certain number of minutes after
the sound of a given bell food will be presented to him; the condi-
tioned reflex will become established so that he salivates at exactly
the same time after the bell is sounded. Something must be going on
in the centers during this time — something inhibiting the reflex. If
CONDITIONED AND UNCONDITIONED REFLEXES 859
during this interval of inhibition some other sensory stimulus is applied,
it will be likely to cut short the inhibition ; in other words, it produces
an inhibition of inhibition, so that the secretion of saliva occurs.
Another most curious combination of conditioned stimuli is illustrated
in the following experiment. Suppose, for example, that a given light
and sound are each separately made a stimulus for a conditioned reflex,
but that when they occur together there is no reflex. Suppose now that
while one of these active stimuli is being presented, the other stimulus
is also presented; the result will be that the secretion produced by the
one stimulus will stop. Evidently, although each is in itself a stimulus,
acting together they cause inhibition.
By studying the conditioned reflexes after a certain part of the cere-
bral cortex has been removed, it has been found that the power of estab-
lishing certain kinds of conditioned reflexes becomes abolished, while
that for others is retained.
CHAPTER XCVIII
THE HIGHER FUNCTIONS OF THE CEREBRUM IN MAN;
APHASIA
The study of the higher functions of the cerebrum leads us to the border-
land between physiology and psychology, but into this vast and relatively
unexplored field we can not venture here, unless just far enough to gain a
suitable vantage point from which to understand the pathology of the
condition known as aphasia.* As we have seen from our studies on cerebral
localization, the cerebrum must be regarded as a great sensorimotor gan-
glion, whose functional activities are indicated by various movements.
These movements may, in general, be classified as objective indications
either of feeling and emotion or of intelligence. Although both classes are
evident in all animals, it is particularly in the case of man that the evi-
dences of intelligent activity are especially prominent, since they include
gesticulation and the muscular activities required in spoken and written
language. The movements that express emotional conditions are 'evolved
earlier and from lower planes than those of intellectual activity. Thus,
very young infants "make faces" when there is reason to believe they
feel pain, and, as they develop, their power of expressing emotion is
evolved long before they present evidence of intelligent motor activity,
and still longer before they can articulate words.
The phenomenon of hunian psychic activity which is of greatest im-
portance is that of language, and to understand the nature of the cerebral
integration required to produce it, we must briefly consider the cerebral
processes involved in the intellectual development of the infant. The
first step in this development is the storing away in projection centers of
memories of the sensations which these centers have received. For ex-
ample, when the child looks at a bell, there is stored in the visual center a
memory of the shape of the bell, and when the bell moves so as to produce
sound, this also is stored as a sound impression in the auditory center.
Likewise, when he touches the bell impressions of its hardness and smooth-
ness and temperature are stored in the centers for cutaneous sensations.
At first each of these memory impressions occupies an isolated position ;
but later, association tracts open up between them, so that the calling
*Free use of Bolton's article is made in this chapter.
860
HIGHER FUNCTIONS OF THE CEREBRUM IN MAN; APHASIA 861
forth of one memory impression is associated with others, and the child
comes to be able to associate the appearance or image of the bell with a
certain sound and with certain sensations of hardness, rotundity, etc. This
preliminary use of observation is known as perception. It involves the
fusion of direct sensations as well as their correlation with memory im-
pressions of former sensations. The number and variety of the latter
called into activity by a particular sensation will obviously vary at dif-
ferent times. On seeing a bell, for example, a child may associate it with
sound on one occasion, and on the next with the feeling of the bell. On
account of this difference in the detail of the method of association, it is
evident that perception must be a product of cerebral integration rather
than one depending on memory impressions stored in the isolated centers.
It is a complicated process with an infinite variety of possibilites as to the
exact way in which it is integrated on each occasion.
The act of perception, however, becomes considerably simplified in the
higher animals by the laying down of short-cut paths of association.
These are formed first of all with the auditory center, in which the memory
impression of an articulated sound representing the object — for example,
the word "bell" — is stored away. The child comes to learn that this par-
ticular word is to be associated with the memory impressions it has stored
away of the sound, the sight, and the feeling of the bell. Similar short-cut
paths later become developed in connection with the visual centers, where
a certain symbol, like the word "bell," is presented to the child as signi-
fying all the other attributes of bell. In its most highly developed form,
therefore, perception may be described as the act of calling up one or
more sensorimemorial images when a name is seen or heard.
Having acquired the ability to integrate sensorimemorial impressions
in the above described manner, the child next learns to integrate the motor
centers concerned in the control of the articulatory apparatus so as to
produce a sound. This sound is the word indicating the object involved
in the integrating process. It is the integration necessary to produce the
sound which symbolizes the particular object.
When the power of understanding and producing language has been
acquired, the crowning process of intellectual development — the forma-
tion of a concept, or general notion — becomes evolved. Thus, the evolu-
tion of a general name will include a number of particular objects or acts.
"This process of conception involves the revivification of numerous sen-
sorimemorial images which present common points of similarity" — (Bol-
ton). It is relatively a simple process for such general objects as animal,
man, building, but becomes very complex for such abstract concepts as
heaviness, beauty, etc. It is obviously a process to which no one cerebral
862 THE CENTRAL NERVOUS SYSTEM
center can be assigned. The outward manifestation of the conception is
spoken or written language.
Language consists, therefore, in an extremely complex symbolic system,
involving various centers and association tracts in the cerebrum, and
capable of an almost infinite degree of development by the laying down
of new symbolic systems. Language, indeed, becomes the instrument of
thought, practically all of the higher intellectual processes being dependent
on its evolution. In this connection it is interesting to note that a great
number of individuals, especially those who do not read, depend on the
sense of hearing for the acquisition of the impressions required for their
psychic development, while others depend on the sense of sight for the
same purpose. .
At least four different types of center are involved in the integration
of language; namely, auditory, visual, chirographic, and articulatory. We
may call these "word centers," and we must assume that they lie near to
the auditory, visual and general sensory projection areas of the cortex.
To understand and to be able to produce spoken and written language, it
is necessary that all these four word centers participate through associa-
tion tracts, although the meaning of a word may be perceived without all
of them being involved.
PSYCHOPATHOLOGICAL APPLICATIONS
In the study of mental diseases the most important conclusion which
we can draw from the above facts is that language is essentially a sym-
bolic mechanism for the integration of sensorimemorial images. It is
therefore the symbolic system of the integrated processes of the brain; it
is the servant of thought. "When, as is often the case, language is used
without the proper exercise of thought, it becomes merely an automatic
affair. A practical deduction from these facts is that any considerable
derangement of the language mechanism must necessarily involve some
interference with the complicated processes of association that go to make
up the psychic function.
These considerations naturally lead us to the subject of aphasia. It has
been usual to distinguish three varieties of this; namely, motor aphasia,
sensory aphasia, and anarthria. In motor aphasia the patient, although
he understands what is said to him, is unable to speak, and the intellectual
powers are little, if at all, impaired. In sensory aphasia speech is possible
in a more or less intelligible manner, but there is a distinct impairment of
intelligence. In anarthria, or subcortical aphasia, the only disability is the
loss or impairment of the power of articulate speech because of some lesion
existing in the center coordinating the lower neurons concerned in the
HIGHER FUNCTIONS OF THE CEREBRUM IN MAN; APHASIA 863
movements of the laryngeal and tongue muscles. Pierre Marie, as a
result of very extensive experience in Paris, has shown that this classifi-
cation is unjustified. • He maintains that there is only one true form of
aphasia, and that such a thing as pure motor aphasia as above defined
does not exist, the condition being invariably accompanied by intellectual
impairment.
Marie points out that the various claims that aphasia may exist without
intellectual impairment have been made without sufficient investigation of
the intellectual status of the patient. He shows that many patients suf-
fering from aphasia if asked to do ordinary things, such as cough or spit
or raise the hand, can do them as well as a normal individual, but that
these after all are very crude acts in the ordinary performances of a normal
individual. To test the intellectual powers it is necessary to require the
patient to perform acts which entail a considerable amount of cerebral
integration. We must ask him to perform some sequence of events such
as walking several times in one direction, then in another, touching cer-
tain objects, etc., or better still we should observe the patient closely in his
business transactions and everyday routine of life to see whether he does
things exactly as he did them before. It is always possible by such tests
to show that in aphasia the mental powers have become distinctly de-
preciated.
The portion of the cerebral cortex affected in aphasia is always in the
neighborhood of the so-called area of Wernicke, which is closely related to
the visual and auditory centers. In making this sweeping conclusion,
Marie admits that cases of pure word-blindness but not of word-deafness
may exist ; that is, a patient still retaining his intellectual powers may lose
his ability to interpret correctly what he sees, although he can still interpret
accurately what he hears.
This conclusion conforms exactly with those of the psychophysiologists
regarding the difference in the language mechanisms of educated and un-
educated persons. Language is learned through the sense of hearing, and
it is only by later education that more is learned by the sense of sight;
that is to say, a person learns to read only after he has learned to under-
stand spoken language. Word-blindness may therefore occur as a pure
symptom, and is less likely than word-deafness to be associated with ab-
normal integrative functions of the cerebrum. Word-deafness however de-
pends upon a lesion involving the auditory center; it necessarily means
disturbance in the association functions of the cerebrum, and is always
accompained by a certain amount of mental derangement.
In corroboration of these facts may be cited the well-known fact that
a deaf-mute is mentally far inferior to one that is congenitally blind.
Loss of hearing leads to more serious cerebral disability than loss of sight.
864 THE CENTRAL NEEVOUS SYSTEM
To quote Bolton again, ' ' In such cases deafness is therefore a more serious
deprivation than blindness, as, for the evolution of the functional activity
of the cerebrum, an entirely new development of associational spheres to
replace those normally employed for auditory and spoken language has
to be acquired. In the case of congenital or early-acquired blindness, on
the other hand, the complex sphere of language, with all its psychic com-
ponents, can be employed in a perfectly normal manner and almost- ex-
actly as it is brought into use in the case of persons who neither read nor
write."
It would be beyond the scope of this work to go into the clinical and
pathological evidence upon which Marie bases his far-reaching conclusions.
Suffice it to say that it is definitely shown that the old contention of Broca,
that a special speech center exists, is entirely unjustified by the facts of
clinical and pathological experience. Broca, it will be remembered, con-
tended that motor aphasia is always due to destructive processes occurring
in the lower portion of the ascending frontal convolution on the left side,
and he concluded that this portion of the cerebrum represents the speech
center. Marie has shown, however, that a patient may show distinct evi-
dence of aphasia without any lesion involving this so-called Broca area,
and, on the other hand, that cases not infrequently occur in which this is
completely destroyed without any evidence of aphasia. Important though
this discovery of the inaccuracy of Broca 's conclusion is, by far the most
important conclusion which we may draw from Marie's work is that, since
language is a product of an extended integration of impressions and
memories stored in different parts of the cerebrum^ it is not so likely to be
interfered with by destruction of any one of the centers as it is by destruc-
tion of the paths which connect the centers with one another. As a matter
of fact, Marie has shown that in cases of aphasia the lesion is. nearly
always located in the course of the pathway connecting the visual and
auditory centers with the other centers of the cerebrum ; it lies around the
upper end of the fissure of Sylvius in the region which in previous years
had been considered particularly associated with the condition known as
sensory aphasia. Those interested in this subject should consult Bolton 's
article.
CHAPTER XCIX
FUNCTIONS OF THE CEREBELLUM
In our discussion of reflex action we have so far considered only those
receptors coming from the exterior of the body, although we have recog-
nized that a considerable number of the afferent nerve roots contain fibers
coming from receptors situated in the muscles, the tendons and the joints,
and called proprioceptors because they respond .not to changes in the
environment but to alterations in the body itself. "We have seen that the
proprioceptors consist structurally of muscle spindles and of the nerve
endings in the tendons and ligaments and synovial membranes. They are
receptors that are attuned to respond to differences in tension caused either
by bulging of the muscles or by stretching of the fibers of tendons and
ligaments.
The impulses are transmitted in the spinal cord, either by the posterior
columns or by the lateral cerebellar tracts. Those traveling by the pos-
terior columns are sent mainly to the cerebral tfortex of the opposite side,
whereas those in the cerebellar tracts enter the cerebellum by the inferior
peduncles of the same side. The cerebral impulses connect with neurons,
which transmit the impulse back again to the cerebellum of the opposite
side, so that ultimately the cerebellar cortex is connected with the spinal
cord of the same side either directly or indirectly through the cerebral
cortex. These anatomic facts indicate in a general way that we may
expect the function of the cerebellum to be that of the chief nerve center
concerned in the integration of the proprioceptive impulses originated by
the condition of contraction or relaxation of the different groups of
muscles in the body, and by the amount of tension existing in the various
tendons, ligaments and other membranes surrounding the joints.
Experimental investigation has justified these expectations. The re-
moval of the entire cerebellum — an operation which has usually been
performed on birds, particularly pigeons, because of the ease with which
it can be done in these animals — leads immediately to a condition in which
muscular activity is entirely uncontrolled. A pigeon after this operation
flies about in an incoordinate way, turning summersaults, dashing itself
against the walls of its chamber, and ultimately after constant futile move-
ments, exhausting itself. If one cerebellar lobe is removed, the body when
at rest is curved toward the side of the lesion, and the movements of the
865
866
THE CENTRAL NERVOUS SYSTEM
animal cause it to fall in that direction. A similar experiment with
dogs yields like results, but the operation is of course considerably more
difficult. In man, a destructive tumor of the cerebellum produces a con-
dition known as "cerebellar ataxy," in which the patient moves his
limbs in a very incoordinate fashion; he staggers, is uncertain in his
gait, and behaves in general very like a drunken man.
Although these immediate effects of cerebellar extirpation indicate
Fig. 222. — Footprints after destruction of the cerebellum in a dog: a, before the operation;
b, four days after; c, five days after; d, a month after; e, two months after. (From Luciani.)
clearly that this organ has to do with the control of muscular movements,
yet the results are probably not primarily dependent on the ablation,
but rather on the conditions of irritation which are set up as a result of
the operation, and which probably affect the cerebellar peduncles. At
least, such is the view that Luciani, one of the greatest investigators in this
field, has adopted because of the fact that, if the animal is kept alive for
sufficient time so that the symptoms of irritation disappear, they become
FUNCTIONS OF THE CEREBELLUM 867
replaced by those of an entirely different nature. The pigeon may reac-
quire the power of flying straight, or — and this is particularly important —
the dog may reacquire the power of apparently normal progression, al-
though, if its muscular movements are carefully examined by physiologic
methods, it will be found that at least three changes have developed as a
late result of the extirpation ; namely, a weakness of contraction, a tremor
during the contraction, and a want of tone when at rest. These condi-
tions have been called asthenia, atonia and astasm, respectively. On su-
perficial examination it may often be difficult to make out these three con-
ditions, but they can readily be observed in animals in which the cerebellar
extirpation has been performed on one side, so that the abnormal may be
compared with the normal side. In a dog that has had one cerebellar
hemisphere removed some time previously, the muscles on the correspond-
ing side are so much weaker than those on the opposite side that the
animal, in order to retain his equilibrium, has to prop himself up either
by leaning against whatever object may be convenient, or by extending
his legs so as to increase his base of support. In other words, he constantly
tends to fall to the side of the lesion, but tries to prevent this either by
increased muscular effort or by taking advantage of artificial support.
The effect which this weakening has on his gait can be very clearly demon-
strated by comparing the footprints produced by the normal with those of
the abnormal side, these footprints being obtained by making the animal
trot along a piece of glazed paper blackened with a carbon deposit as in
taking tracings (Fig. 222).
Localization of Function in the Cerebellum
Although these facts in themselves would tend to indicate a certain de-
gree of localization of function in the cerebellum, or at least that certain
parts of the cerebellar cortex have to do with certain groups of muscles,
yet for many years it was considered that the cerebellum did not show in
any marked degree the same kind of localization that we find in the cere-
bral cortex. One cause for the backward state of our knowledge concern-
ing cerebellar localization is that, unlike the cerebrum, its cortex is
practically inexcitable. In recent years, however, on account partly of
anatomic and partly of experimented and clinical work a high degree of
localization has been found to exist in the cerebellum. From the anatomic
point of view it has been found that in certain groups of animals, such as
the uugulata, the postero-medial lobule of the cerebellum is very large;
whereas the lobuli ansiformes are small. In another group, the carnivora,
the opposite obtains, the lobuli ansiformes being greatly developed and the
postero-medial lobule quite small.
868
THE CENTRAL NERVOUS SYSTEM
By studying these developmental differences in relationship to the activi-
ties of the muscular system, Bolk suggested that movement of those regions
of the body which are affected by muscle groups on both sides — for example,
the head, neck or trunk — would be represented on the cerebellar cortex by
an unpaired center — that is, a center occupying a middle position — and that
this would be capable of exercising an influence equally upon the muscles
of both sides. Movements of the limbs would require an entirely different
type of coordination, since they are not accustomed to act together, unless
for certain movements, as walking. Based on these theoretic considerations
Bolk found a definite correspondence to exist between the variations in the
development of certain cerebellar lobules and the functional importance of
certain muscle groups, and the general conclusions deducible from his and
Fig. 223. — Diagrams to represent respectively a ventral view of the left half and a dorsal
view of the right half of the human cerebellum illustrating the scheme of subdivision according
to Bolk. (From photographs of specimens from the Anatomical Museum, Western Reserve Medical
School.) (From Davidson Black.)
correlated work may be summed up as follows (cf. Davidson Black16) : The
lobus anterior cerebelli (see Fig. 223) contains the centers for the coordi-
nation of the muscle groups of the head (eyes, tongue, muscles of mastica-
tion, muscles of expression), and of the larynx and pharynx. The lobus sim-
plex contains centers for the coordination of the muscles of the neck. The
lobulus medianus posterior contains the unpaired centers for the synergic
movements required by the right and left extremities for the purposes
of progression. On the other hand paired centers for the extremities —
those centers that have to do with the independent movements of each limb
of the same side of the body — are located in the lobuli ansiformes et para-
mediani (crus primum and crus secundum). The centers for the rest of
FUNCTIONS OP THE CEREBELLUM
869
the trunk and tail region are located in the remainder of the cerebellum.
These conclusions are the basis of the accompanying map of the cerebellum.
Basing his work on these anatomic conclusions, Van Eijnberk has studied
the effect of circumscribed extirpation of certain lobules of the cerebellum
on the muscular control of the different parts of the body, with the following
results. Total or partial extirpation of the lobulus simplex produces side to
side oscillations of the head, indicating the removal of the influences of the
cerebellum that control the movements of the muscles of the neck. Complete
extirpation of the crus primum of the lobuli ansiformes causes as an imme-
diate— irritative — effect dynamic disturbances of the fore limb of the same
side, replaced later by a condition of atonia, which makes the limb hang
limp, and of asthenia, which makes it feeble in its movement when it
is excited to contract. Extirpation of the crus secundum has a similar
Fig. 224. — Schema of the parts of the mammalian cerebellum spread out in one plane. (After Bolk
by Van Rijnberk from Luiciani. Op. cit.) On the right side of the figure the relation of the
different lobules to the functional development of the musculature is indicated according to the
theory of Bolk noted in the text. (From Davidson Black.)
influence on the muscles of the hind limb of the corresponding side. Extir-
pation of both crura of the lobulus ansiformis causes marked asthenia and
atonia in both fore and hind limb on the same side as the lesion. A char-
acteristic disturbance in walking develops as a late effect of this extirpation.
It has been termed the ''hen's gait." Extirpation of the lobulus para-
medianus causes rotation on the longitudinal axis of the body, with pleuro-
thotonus to the operated side. (Fig. 224.)
Just as in the case of cerebral localization, so in cerebellar we find that
within each of the largest centers a more particular localization can be made
out; thus, in each of the centers for the upper and lower extremities,
there is a definite arrangement of subsidiary centers for the direction of
the activities of antagonistic muscle groups concerned in the movements of
particular joints. It must be remembered, however, that in all these cases
no real paralysis is produced by extirpation, but only a want of coordina-
870 THE CENTRAL NERVOUS SYSTEM
tion on account of the fact that the sthenic, tonic and static impulses re-
quired for muscular harmony are not properly elaborated.
After some time, as in the case of complete cerebellar extirpation, the
symptoms gradually disappear, but they can be obtained more or less char-
acteristically in practically all animals, at least in all those that have been
investigated, including dogs and monkeys.
It will be of interest to consider for a moment the possible causes for the
ultimate disappearance of the symptoms of cerebellar extirpation. These
are either : (1) an organic compensation by the uninjured parts of the cere-
bellum, or (2) a functional compensation by the voluntary centers of the
cerebrum. Although the former of these methods of compensation may
sometimes develop after partial destruction of the cerebellar cortex, it can
not of course explain the recovery which we have seen to occur after the
entire cerebellum has been removed. The most important compensation no
doubt is effected by the cerebrum, as the following observation clearly in-
dicates. If half of the cerebellum of a dog is destroyed, and the animal
kept alive until the symptoms of cerebellar extirpation have entirely dis-
appeared, it will then be found, if the cerebral center on the opposite side
is removed, that the symptoms return in their original severity. After this
second operation the powers of standing in the erect position and of
walking are permanently lost.
CLINICAL OBSERVATIONS
Application of these laboratory results has been recently made in the
clinic, the most important contribution having come from the clinic of
Barany, who for his work was awarded the Nobel prize. In cases of abscess,
cysts, or regional agenesia, it is now possible to determine the exact site of the
lesion in the cerebellum. To effect this localization, it has been necessary to
work out certain clinical tests. The most important of these is called the
index test. This is described by Davidson Black as follows : ' ' The patient 's
eyes being closed, he is asked to execute a simple movement in a given
direction with one of his extremities. For example, the forearm being
firmly supported, the patient's index finger is extended and brought into
contact with that of the observer; the patient is then required to move his
finger vertically downward and then to return it to its previous position.
The test is repeated a number of times, both in the vertical and in the
horizonal direction, and if any tendency toward deviation from the plane of
movement be present, its direction is noted. By slight modification of the
foregoing procedure it is possible to test each of the limb segments in all
positions of rotation, pronation or supination."
The index test is applied (1) without previously producing nystagmus
FUNCTIONS OP THE CEREBELLUM
871
and (2) after producing artificial nystagmus. The artificial nystagmus is
produced by spinning the patient two or three times, and consists of con-
stant lateral movements of the eyeballs, quick in the direction in which the
artificial movement took place, and slow in the opposite direction.
In a normal subject, previous to spinning the index test shows no devia-
tion, but after the production of artificial nystagmus a deviation is noted
in the direction corresponding to the slow jerk of the nystagmus (reaction
Fig. 225.
Fig. 226.
• Figs. 225 and 226 represent respectively the inferolateral and the posterior aspect of the human
cerebellum indicating certain cerebellar localizations according to Barany. (After Barany, from
Andre'-Thomas et Durupt. Op. cit.) N. VII, Nervus facialis; N. IX, Nervus Glossopharyngeus;
N. XII, Nervus hypoglossus.
The signs in the above diagram indicate the exact localization of the centers for the tonus of
the musculature concerned in some of the movements of the right arm and leg, (g) marks the
center for downward movements of the arm; X, for abduction of the arm; O, adduction of the
hand; + adduction of the arm; ±, adduction of the hip. N. V. indicates Nervus trigeminus;
N. VI, Nervus abducens; N. VII, Nervus facialis; N. IX, Nervus glossopharyngeus; N. XII,
Nervus hypoglossus. (From Davidson Black.)
deviation). When a cerebellar lesion exists, the index test performed on
a patient without nystagmus sometimes causes a spontaneous deviation in
the segment of the body corresponding to the position of the lesion on the
cerebellar cortex, but more frequently, if it is applied after the production
872 THE CENTRAL NERVOUS SYSTEM
of artificial nystagmus, the reaction deviation in that segment will fail to
be obtained. The exact site of the cerebellar lesion is diagnosed partly
from the nature and direction of the deviation which is produced and
partly from the segment of the limb in which it occurs, the explanation
for the disturbances being that interference with the cerebellar control of
one muscle group causes the antagonistic muscular groups to perform their
movements in an exaggerated manner, so that the segment moves too much
in their direction.
Barany's conclusions so far may be summarized as follows:
(1) The centers for the extremities are located on the cortex of the
hemispheres in the semilunar (superior and inferior) and digastric lobules
(see Fig. 225). The representation is uncrossed or homolateral, thus con-
trasting with cerebral localization, in which it is crossed or heterolateral.
(2) Within each of these chief centers there is a further localization,
which however does not refer to anatomic groups of muscles but rather to
the functional performances of the different segments of the limb. Thus,
within the arm centers there are subsidiary centers concerned in the
movements of the limb in the various planes in rotation, in pronation
and in supination. It is a functional rather than an anatomic localization.
(3) When a center concerned in the movements of the limb in a certain
direction, e. g., to the right, is suddenly destroyed, a spontaneous devia-
tion is produced in the opposite direction (to the left).
CHAPTER C
THE CEREBELLUM AND THE SEMICIRCULAR CANALS;
FUNCTIONAL TESTS
The cerebellum serves as the great nerve center to which are transmit-
ted, through the various proprioceptors, the impulses which, as it were,
inform it as to the exact degree of muscular effort required to maintain
the animal in its various postures. It is, as Sherrington puts it, the head
ganglion of the proprioceptive system. Such impulses from the muscles,
tendons, etc., could not, however, supply information regarding the exact
position of the body in space. For this purpose special receptors con-
nected with the eighth nerve are provided in the semicircular canals.
These, it will be remembered, are three in number on either side, each canal
consisting of a semicircular bone tube attached to the vestibule of the
internal ear ; and they are arranged so that they lie at right angles to one
another in the three planes of space. The three canals on either side are
thus disposed so as to form an arrangement like a V-shaped armchair with
the back inwards. This arrangement causes the posterior vertical canal
of one side to be in the same plane as the superior vertical canal of the
opposite side, the external canals being in the horizontal plane on both
sides. The arangement will be plain from the' diagram (Fig. 227).
Within the osseous canals are suspended membranous tubes, which do not
fill the canals. The canals, etc., contain fluid, but are not completely
filled. The osseous as well as the membranous canals are dilated at one
end to form the ampulla, and it is here that the vestibular division of the
eighth nerve ends in a structure called the ' ' crista acustica, ' ' consisting of
hair cells supported by sustentacular cells. The nerve terminates in fine
arborizations between the hair cells. In the saccule and utricle are struc-
tures similar to the crista, called the maculae acusticae. These structures are
receptors specialized for the purpose of responding to changes in the
position of the head, and therefore of the body in general. When the
head moves in a certain plane of space, the fluid in the membranous canals
and in the utricle and saccule on account of inertia undergoes a certain
movement, which acts on the hairs of the hair cells and thus sets up a
stimulus. According to the degree of the stimulation in the various
ampullae, which again will be dependent upon the plane or planes in
which the movement of the head has occurred, impulses are transmitted
873
874
THE CENTRAL NERVOUS SYSTEM
through the vestibular nerve, and these impulses ultimately reach the
cerebellum.
The experimental evidence for these conclusions regarding the functions
of the semicircular canals is very strong. Thus, after destruction of all
the canals — an operation which is particularly easy in the pigeon — the
animal behaves very much the same as after cerebellar destruction. After
some months these disorders disappear, because the cerebellum learns to
control the movements of the body from other proprioceptive impulses,
particularly those of sight. If such a recovered animal is bandaged, the
symptoms return in all severity. This compensation is furthermore an
educative process, for it does not occur when the cerebral centers as well
Fig. 227. — The semicircular canals of the ear, showing their arrangement in the three planes of
space. (From Howell's Physiology.)
as the semicircular canals are removed, and it can be abolished in a re-
covered animal by removal of the cerebral cortex.
Many observations of great interest have been made concerning these
labyrinthine sensations by Pike and others, but we can not discuss them
further here. One point of interest, however, is that forced movements
in definite planes are induced by removal of a canal. Removal of the
horizontal canals, for example, causes continued nodding movements of
the head in the plane of the injured canals. An experiment of great sig-
nificance was performed by Ewald to show the effect of causing a move-
ment of the fluid in one of the canals. For this purpose a bony canal was
opened at two places by a dental drill. Through the hole farther from the
ampulla, amalgam was introduced so as to block the backward movement
of fluid, and into that nearer the ampulla a fine tube was inserted con-
nected with a rubber bulb. By manipulating the bulb, the membranous
CEREBELLUM AND THE SEMICIRCULAR CANALS 875
canal could be compressed and currents set up in the endolymph. It was
found that these currents always caused the animal to move its head and
eyes in the plane of the canal that was being stimulated and in the direction
of the current of endolymph.
Finally, visual impressions supply much of the information the cerebel-
lum requires, the close association of the movements of the eyeballs with
cerebellar and labyrinthine disturbances being well recognized. The nys-
tagmus already described in connection with Barnay's tests is dependent
upon this association. The symptoms and sensations of giddiness or nausea
produced by rotation of the body, or by unusual movements such as those
of a boat, are no doubt due to the irregular and unusual variety of laby-
rinthine sensations which they excite.
In a word, then, the function of the cerebellum is to receive proprio-
ceptive impulses from the body along with labyrinthine and visual im-
pressions and to integrate and develop from them impulses which, by
being transmitted to the cerebral and other nerve centers that dominate
muscular movements, so coordinate the nerve discharges from them that,
when muscular movement occurs, it does so in relationship to the previous
position of the animal and in the most efficient way to attain the object
for which the movement was made. . The cerebellum is the head nucleus
of the proprioceptive system.
THE ASSOCIATION BETWEEN THE EYE MOVEMENTS AND THE
SEMICIRCULAR CANALS
The close association between the eye movements and the semicircular
canals is indicated by the occurrence of nystagmus when the ear is stimu-
lated either electrically or by means of moderately cold water impinging
on the membrana tympani. The latter method of inducing nystagmus is
styled the caloric, and is employed in the examination of candidates for
the aviation service. Its value over the tests of nystagmus after rotating
the body and the index test already described depends on the fact
that it enables us to test each vestibular apparatus separately.
Water at 68° F. is allowed to run through a stop nozzle into the ex-
ternal auditory canal, free of wax, from an irrigation bottle placed
about 3 feet above the head, which is meanwhile tilted at an angle of 30°
forward. In about 40 seconds a rotary nystagmus with the direction of
the jerk to the side opposite to the douched ear should be evident, or
dizziness complained of. The reaction test is then applied and im-
mediately afterward the head is inclined at an angle of 60° backward
from the perpendicular, when a horizontal nystagmus to the side op-
posite to the douched ear should develop. Deviation is again tested.
876 THE CENTRAL NERVOUS SYSTEM
The procedure is repeated on the other ear. If it takes longer than 90
seconds for the nystagmus to appear, the vestibular apparatus of that
side is abnormal. Absence of the reaction deviation after the douching is
a certain sign of internal ear disease.
It is undoubtedly essential that these tests should be most carefully
applied to all would-be aviators. They frequently reveal lesions of the
vcslibular apparatus or the cerebellum in subjects who had thought
themselves perfectly normal, and who indeed may have boasted of their
powers of equilibrium because they imagined that freedom from seasick-
ness or failure to become dizzy in dancing indicated a high development
of this function. There can be no doubt that many aviators have gone to
their death because of impairment in the ear mechanism. When on "terra
firma" the muscular sense and cutaneous sensations often make the
vestibular weakness of no consequence, but when deprived of these con-
tributory sensations and dependent on the ear-balance mechanism alone,
as in flying, any weakness becomes a serious menace.
CHAPTER CI
THE AUTONOMIC NEKVOUS SYSTEM
In discussing the physiology of the central nervous system, we have
broken away from the traditional textbook treatment of the subject in
that we have left practically untouched any description of the course of the
various nerve tracts or of the position of the nerve centers. "We have
pursued this policy in the belief that the study of these details of structure
belongs just as surely to the anatomist as does the structure of other parts
of the body, notwithstanding that to trace the course of the nervous path-
ways he may have to call to his aid the physiologist and clinical neurologist.
There is one part of the nervous system, however — namely, the involuntary
or autonomic — the physiology of which it is impossible to discuss apart
from its anatomy, because this has depended very largely on physiologic
methods for its elucidation. Until such methods were emphasized and
while anatomy alone was depended upon, little could be learned of the
functions and connections of the sympathetic chain and of the various
nerve plexus that compose the involuntary nervous system. We shall here
review briefly the general anatomic plan of this system as described by
Gaskell.17
GENERAL PLAN OF CONSTRUCTION
The plan of the involuntary nervous system is much the same as that
of the voluntary, the main points of difference being dependent upon the
location of the neurons composing the reflex arcs. It will be remembered
that there are three of these: the receptor, the internuncial, and the ef-
fector neurons (page 782). The receptor neurons have the same position
for both systems ; namely, the posterior root ganglia (Fig. 228) . The inter-
nuncial neurons of the involuntary system, like those of the voluntary,
have their cells in the spinal cord, where they are represented, in the
thoracic region, by the cells of the lateral horn of gray matter ; in .the
sacral region, by a similarly placed collection . of cells ; and in the bulbar
region, mainly by the dorsal nucleus of the vagus. The main cause for
the difference between the two systems is dependent on the course of the
fibers of the internuncial neurons ; in the involuntary system they leave the
spinal cord before connecting with the effector neuron nerve cells, which
are contained in the various ganglia found throughout the body, whereas
877
878
THE CENTRAL NERVOUS SYSTEM
in the voluntary, they remain within the spinal cord, and terminate on
the effector neurons, which are the anterior horn cells.
The outflow from the spinal cord of involuntary internuncial fibers,
which we shall hereafter style connector fibers, occurs along the an-
terior spinal roots, but is somewhat irregular in distribution, because
it is interrupted in the cervical and lumbar regions, where the nerve
plexus for the extremities come in. There are, therefore, three main
regions of outflow for the connector fibers — a thoracicolumbar, a bulbar,
and a sacral; and the fibers (Fig. 229) do not behave in the same manner
in all of them. The fibers of the thoracicolumbar region form the so-
called sympathetic system, and run by the corresponding white rami
communicantes either immediately to the ganglia of the sympathetic
Post root
gang.
us
Fig. 228. — Diagram to illustrate the different arrangements of the internuncial neurons of the
voluntary and involuntary nervous systems. In both systems the afferent fiber terminates (by col-
laterals) around a ceU of the gray matter of the cord. In the voluntary system this cell is sit-
uated in the posterior horn, and its axon travels to an anterior horn cell. In the involuntary
system, on the other hand, it is located in the lateral horn, and its axon leaves the cord by the
anterior root and travels by the white ramus into a sympathetic ganglion, where it connects with
a nerve cell, whose axon forms the postganglionic fiber. . (From Gaskell.)
chain, or by the splanchnic nerves to the abdominal ganglia. In the gan-
glia are situated the cells of the effector neurons. The fibers of the sacral
region connect with effector neurons, forming the pelvic ganglionic group
(pelvic nerves, nervi erigentes) ; and those of the bulbar outflow with
effector neurons located either peripherally or in the ganglia of the
vagus and the seventh, ninth, and eleventh cranial nerves. In the mid-
brain there is a fourth group of involuntary connector fibers running to
effector neurons found in the ciliary ganglia.
The anterior roots of many of the spinal nerves are therefore not
Fig. 229. — Diagram of the sympathetic nervous system to be used along with Fig. 233. The
preganglionic fibers are in red, and the postganglionic in black. S.c., superior cervical ganglion;
I.e., inferior cervical ganglion; T, stellate ganglion; S.p., great splanchnic nerve; C, ganglia of
solar plexus; m., inferior mesenteric ganglia; h., hypogastric nerves; N.E., nervus erigens. The
arrows indicate the direction of nerve conduction. The numerals indicate the spinal nerves.
(From Howell.)
---Spinal cord
} Sympathetic chain
,n";--->Solar ganglion
A/)
Fig. 230. — Diagram (after I.angley) showing the manner of connection of the fibers compos-
ing the great splanchnic nerve. The left-hand diagram represents the usual arrangement, the
preganglionic fibers (black) passing through the ganglia of the sympathetic chain and having
their cell stations in the solar ganglion, from which the postganglionic fibers (red) then emerge
to run to their destination along the blood vessels. The right-hand diagram shows a possible
exceptional arrangement.
THE AUTONOMIC NERVOUS SYSTEM 879
composed entirely of fibers belonging to voluntary effector neurons, but
also of connector fibers of the involuntary system. These ' are distin-
guished from the voluntary fibers by being much smaller in diameter;
indeed it was by this characteristic that Gaskell succeeded in tracing the
course of the involuntary fibers.
In brief, therefore, the fibers of the internuncial neurons of the volun-
tary nervous system are confined within the central nervous system,
where they are contained mainly in the white columns of the spinal cord, the
pyramidal tracts, for example, being composed of internuncial fibers
from the cerebral neurons; the corresponding fibers of the involuntary
nervous system (connector), on the other hand, run out of the- cord with
the anterior roots to effector neurons situated either in the ganglia of
the sympathetic chain or in peripheral localities. Just as the voluntary
internuncial fibers give off many collaterals, so do those of the involun-
tary system, so that an impulse transmitted by one internuncial neuron
may excite a broad field of effectors. We shall see later that it is through
these collaterals that reflex responses can apparently often be excited
by the stimulation of the central ends of nerves — such as the hypogastric
to the bladder — after all connections with the central nervous system
have been severed. (Fig. 230.)
To elucidate the further course of the involuntary fibers, and deter-
mine the location of the effector neuron nerve cells, it becomes necessary
to supplement anatomic with physiologic methods of investigation. The
various functions of the innervated parts — vascular changes, muscular
movements, glandular activity — are observed by the usual methods of
the physiologist, and the nerve roots or nerves believed to contain the
involuntary fibers either cut or stimulated. If a change is observed in
the functions, it indicates that part at least of the involuntary nerve
supply is contained in the nerve structure that has been cut or stimu-
lated. Such a result does not, however, inform us as to whether the
fiber is that of the connector or effector neuron — whether it is pre-
ganglionic or postganglionic. This may be determined in many cases
by observing whether nerve degeneration occurs as a result of cutting
the fibers, but the most useful method for answering the question is
that discovered by Langley by the use of nicotine, which in certain con-
centrations specifically paralyzes the synaptic connections between the
connector and the effector neurons. If a weak (1 per cent) solution of
this alkaloid is painted on a ganglion or peripheral nerve plexus in
which the connector neuron finds its effector nerve-cell, it will break
the nerve path, so that physiologic responses produced by stimulating
the preganglionic fibers become no longer elicitable. When the involun-
tary connector fibers run through several ganglia, as in the sympathetic
880 THE CENTRAL NERVOUS SYSTEM
chain, it becomes possible, by systematically painting the ganglia with
nicotine, to tell exactly in which of them the fiber finds its effector
nerve cell.
The course and functions of the effector neurons of the three outflows —
bulbar, thoracicolumbar, and sacral — vary in many details and must be
considered separately.
THE THORACICOLUMBAR OUTFLOW, OR SYMPATHETIC
SYSTEM PROPER
The connector fibers are sharply confined in their outflow from the
cord between the first thoracic and the fourth lumbar segments, and
they run by the white rami communicantes to the sympathetic chain,
where some of them connect with effector nerve cells in its ganglia, while
others run beyond the chain to find their effector cells in collateral gan-
glia represented by the semilunar, superior and inferior mesenteric and
the renal in the abdomen. The fibers of the effector cells, often called
postganglionic, are distinguished from the connector or preganglionic
fibers by being nonmedullated. Those derived from cells in -the lateral
sympathetic ganglia proceed to their destination either by way of the
gray rami communicantes to the segmental nerves after the fusion of
the anterior and posterior spinal roots, or by the outer walls of the blood
vessels. (Fig. 231.)
The effector neurons supply the following structures:
1. The blood vessels and heart.
2. The musculature of the sweat glands.
3. The musculature of the hair follicles and other muscles lying under
the skin.
4. The musculature of the so-called segmental duct, which is repre-
sented in the adult by the uterus, Fallopian tubes, ureters, etc.
5. The sphincters of the intestine.
Regarding the innervation of the Hood vessels, the exact situation of
the ganglia in which the effector neurons are situated and of the nerve
roots which contain the connector fibers, is shown in the accompanying
table (page 881).
It is clear that the innervation of the blood vessels is practically con-
tinuous, the effector neurons being situated both in the lateral and in the
collateral chain of ganglia. Those of the former run to the vessels of
structures innervated by the cranial and spinal segmental nerves, while
those of the latter supply the vessels of the abdominal and pelvic viscera.
The connector fibers to the sweat glands are also strictly confined to
the thoracicolumbar system, the cell station being found in the ganglion
Post, root
Ant. root —
Preganglfonic fiber
'---Sympathetic ganglion
rami
Postganglionic fiber
Fig. 231. — Diagram (after Langley) to show the manner in which a preganglionic fiber,
emanating from the spinal nerve by the white ramus communicans, connects in a ganglion of the
sympathetic chain with a nerve cell (red), the axon of which then proceeds as the postganglionic
fiber (red) by way of the gray ramus communicans back to the spinal nerve, along which it
travels to the periphery. It will be observed that the preganglionic fiber does not form its
synapsis in the first ganglion it encounters.
THE AUTONOMIC NERVOUS SYSTEM 881
stellatum for the fore limb, and the lower lumbar and upper sacral
ganglia for the hind limb. When they are stimulated, the muscular
fibers surrounding the sweat glands contract and squeeze out the sweat.
SITUATION OF BLOOD SITUATION OP MOTOR ROOTS CONTAINING CONNECTOR
VESSELS GANGLION CELLS NERVES
Head and neck. Superior cervical ganglion. 1, 2, 3, 4, 5, thoracic; 2, 3, 4, give
maximum effect.
Heart. Ganglion, stellatum and in- .1, 2, 3, 4, 5, thoracic; 2, 3, give
ferior cervical ganglion. maximum.
Anterior extremity. Ganglion stellatum. 4, 5, 6, 7, 8, 9, thoracic, and 10
slightly.
Posterior extremity. 6th lumbar, 7th lumbar, and 11, 12, 13. thoracic; 1, 2, lumbar
1st sacral ganglion. and 3 slightly.
Kidney. Eenal ganglion. 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
thoracic ; 1, 2, 3, 4, lumbar.
Spleen. Semilunar ganglion. 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
thoracic; 1, 2, 3, lumbar.
Abdominal viscera. Superior mesenteric ganglion 6, 7, 8, 9, 10, 11, 13, thoracic ; 1, 2,
and semilunar ganglion. 3, lumbar.
Pelvic viscera. Inferior mesenterie ganglion. 1, 2, 3, 4, lumbar.
(Gaskell)
The ganglia for the pilomotor fibers are more widespread (extending
from the fourth thoracic to the coccygeal ganglia).; but the connector
fibers are again strictly confined to the thoracicolumbar region. Stimu-
lation of these fibers causes movement of the hairs, or on hairless animals,
the condition called "goose skin."
The Motor Nerves of the Muscles Surrounding the Segmental Duct. —
It will be observed that the connector fibers to the abdominal and pel-
vic viscera are collected into two special nerve trunks, the greater and
the lesser (or lumbar) splanchnics. The collateral ganglia (semilunar
and superior and inferior mesenteries) with which these connect, have
nothing to do with the segmental nerves, but their nerve cells send fibers
(postganglionic) which supply the various viscera not only with vaso-
motor fibers but also with the "sympathetic" fibers, which we have seen
exercise such an important control over their glandular and muscular
functions.
All of the fibers contained in the lumbar splanchnics do not, however,
have their cell stations in the inferior mesenteric ganglia, but run
through it and proceed in the hypogastric nerves to find their effector
cells on the musculature of the various structures that are developed
from the Miillerian and Wolffian ducts — i. e., of the ureters, uterus, Fal-
882 THE CENTRAL NERVOUS SYSTEM
lopian tubes and vas deferens. Many of the fibers of the hypogastric
nerves are therefore those of involuntary internuncial neurons.
The ileocolic and internal anal sphincter muscles of the intestines and
internal vesical sphincter receive their nerve supply from effector neurons
situated in the superior and inferior mesenteric ganglia, the internuncial
fibers arising from the thoracicolumbar region. It is possible that the
other sphincters of the intestinal canal — viz., the cardiac and pyloric
sphincters of the stomach — are similarly innervated. (Fig. 232.)
Great aid in working out these nerve connections is received by study-
ing the effect of epinephrine, which acts specifically on those tissues that
are supplied by the sympathetic nervous system.* Epinephrine has no
effect on tissues innervated by the bulbar or sacral outflows, and it
develops its action peripherally, being indeed more potent on a dener-
vated organ even after all its nerves have been allowed to degenerate.
Advantage of this action of epinephrine has been taken in the investi-
gation of doubtful cases of sympathetic innervation, such as in the cere-
bral, coronary, and pulmonary blood vessels. The outcome of these in-
vestigations has been discussed elsewhere.
THE BULBOSACRAL OUTFLOW OR THE PARASYMPATHETIC
SYSTEM
From the medulla oblongata arise involuntary connector neurons,
which are carried mainly by the vagus nerves but partly by the seventh,
ninth and eleventh cranial nerves to effector nerve cells situated periph-
erally on the structures to which the nerves run (Fig. 233). These include
in a general way the muscles and glands of the alimentary canal and
its derivatives as far as the end of the small intestine. In the small
intestine itself the cells of these motor neurons are those of Auerbach's
plexus found between the two muscular coats. In the diverticula, which
include the lungs and the gall bladder, the nerve cells to which the
vagus fibers run are also situated peripherally.
The sacral outflow occurs through the second and third sacral nerves,
the fibers joining to form a single nerve (the pelvic nerve or nervus
erigens) on each side. This passes directly to the bladder, where it
connects with a plexus, often called the hypogastric, which extends over
the bladder and neighboring portion of the rectum. The branches run
to connect either with the nerve cells of the ganglia of the plexus itself,
or with nerve cells situated on the walls of the large intestine and blad-
der. The pelvic nerve makes its connections with the periphery in the
•Its action is always the same as that which is produced by stimulation of the sympathetic nerve
supply, whether this effect is one ot stimulation or inhibition.
Splanchnic
(Poafgan
Head e NecK
(Postqangij
Vagus—
-Medulla ===Key —
PreGangiionic Sympathetic
PostGanglionic Sympathetic
Pre Ganglionic Bulbo-Sacral
(Para Sympathetic)
PostGanglionicBulbo-Sacral
Arm
1 (Post
Heads-
Neck
(Pregan'gj
Sympathetic
Ganglia
Jhoracic
Splanchnic
-v nerve
-Coellac Plexus &
^-\ 5gp.Mes.Gang.
.Leg
(PreyangJ
Arm .
(Pregang)
Lumbar
.Splanchnic nerve
Abdominal
Viscera
(Precjang.)
-Inf. Mes.Gang.
.Leg
(Post
gang.)
illeck.
Pelvic visceral nerve
Fig. 232. — Diagram showing the main parts of the autonomic nervous system to be used along
with Fig. 233. For the sake of clarity several of the preganglionic fibers of the sympathetic
autonomies are omitted, but the position of their egress from the cord is indicated in the side
notes. The diagram shows clearly the distribution of the bulbosacral autonomic system by way
of the vagus and the first, second and third sacral nerves.
THE AUTONOMIC NERVOUS SYSTEM 883
same manner as the vagus. Taken together these two nerves supply the
musculature of the gastrointestinal tract, including the cloaca, the
vagus as far as the end of the small intestine, and the pelvic nerve
from this point on. It must of course be remembered that certain
muscles— namely, the sphincters of the small and large intestine —
receive their nerve supply from the sympathetic (page 882). Just as
structures innervated by the sympathetic are peculiarly susceptible to
the action of epinephrine, it has been discovered that those innervated
by the bulbosacral system are very susceptible to the action of acetyl-
choline, which is present in ergot. They are not acted on by epinephrine,
nor are the structures upon which this acts affected by acetyl-choline.
AXON REFLEXES
At this place it is convenient to consider for a moment the phenome-
non which has already been referred to as an axon reflex. It was dis-
covered that when one of the hypogastric nerves was cut and the central
end stimulated there was a reflex contraction of the bladder and the in-
ternal anal sphincter, along with vasoconstriction in the region of the rec-
tum and that this occurred, even after disconnecting the inferior mesen-
teric ganglion from the spinal cord by cutting the lumbar splanchnic nerves.
Injection of nicotine immediately abolished the response. It looked as
if reflex action was possible through the ganglion; which would justify
the name "sympathetic" originally given to the involuntary nervous sys-
tem in the belief that the ganglia were centers for local reflex actions.
Further investigation showed, however, that this reflex is not similar to
those occurring in the voluntary system, but is dependent upon the
presence of a collateral on internuncial fibers that run through the in-
ferior mesenteric ganglia to nerve cells situated peripherally on the
walls of the bladder and rectum. The collaterals terminate by synapsis
around nerve cells in the ganglion, the axons of which, as we have seen,
run to the bladder, the rectal blood vessels, and the internal sphincter
ani. The evidence for this explanation depends on the observation that
the axon reflex is no longer possible after the lumbar splanchnics have
been cut and time allowed for their fibers to become completely degen-
erated.
Similar reflexes depending on collaterals have been found in the lateral
chain, and there can be little doubt that they are of frequent occurrence
throughout the whole involuntary nervous system, just as they are,
within the spinal cord, in the voluntary. It is because of these collaterals
and the fact that nerve fibers transmit impulses in both directions
that a stimulus transmitted through one or a limited number of connector
884 THE CENTRAL NERVOUS SYSTEM
neurons may excite a broad field of effectors and cause a widespread
effect.
FUNCTIONS OF AUTONOMIC NERVES
The functions of the autonomic nerve fibers have been discussed in
connection with the structures which they supply, and we shall require in
this place only to review them in a general way.
Two opposed effects may be obtained: stimulatory (augmentory) and
inhibitory; and these may be produced through one nerve by its being
stimulatory for one set of muscle fibers and inhibitory for another set
in the same viscus. The branches running from the inferior mesenteric
ganglion to the colon,, for example, are augmentory (constrictor) for the
blood vessels and inhibitory for the muscular walls of the colon.
The greatest interest centers on the inhibitory impulses. They are
best known in connection with the vagus nerve to the heart, the sympa-
thetic to the small intestine, and the hypogastric to the musculature of
the bladder. It is interesting to compare the nature of inhibition in the
involuntary and voluntary systems. In the latter, it will be remembered,
inhibition can occur only through the internuncial neurons and the ef-
fector nerve cell, stimulation of the effector nerve fiber never having
any other than an augmentor effect. It is quite otherwise in the involun-
tary nervous system, for stimulation of the effector nerve fiber, after
complete destruction of the effector nerve cell, -is still followed by a typical
inhibition. This, it will be remembered, may be demonstrated on the frog
heart by applying electric stimulation to the white crescentic line after
paralyzing the effector nerve cells by nicotine. The same may also be
shown in the case of the chorda tympani, where stimulation of the post-
ganglionic fibers in the hilus of the gland causes dilatation of the blood
vessels after paralysis of the ganglion by nicotine, vasodilatation being
of course a phenomenon of inhibition.
It is a difficult matter to designate precisely' Which fibers in any part
of the involuntary nervous system are inhibitory and which augmentory.
Indeed, as mentioned above, one fiber may perform both functions. In
cases where the existence of inhibitory fibers is doubtful, great aid is
afforded by the use of ergotoxine, an alkaloid of ergot, which possesses
the remarkable property of specifically paralyzing the augmentor nerves
of the sympathetic system (but not of .the parasympathetic) ; that is,
the same fibers as are stimulated by epinephrine. When, a particular
structure is supplied with augmentory and inhibitory fibers by a com-
bined sympathetic nerve, electric stimulation or the application of epi-
nephrine usually gives only augmentory effects; after the injection of
Lac/iryma/q/and
Nasal mucosa — .;y-:— -
Sublingual /f
gland I '/,
W
Submaxillary gland
SubmaxillarytSublingbal)
ganglion
Iliocecal
sphincter*
B/a
Ve5/ca/ sphincter-Jr
Urethral sphincter//;
<-r".
^=
Cran/a/ and 5acra/ nervej
mofor = red
inhibitory = Wue
Thoraclco-lumbar nerves
motor =.ye//ow
inhibitory*qreeh
'Postganglion'tc fibers
are dotted, thus —
N.XI
Jup. cervical ganglion
Thyroid gland
Inf.cervical ganglion
Ansa subclavia
•Stellate (/L* Thoracic)
ganglion
fibers
Pilo motor muscle
Celiac (Semilunar)
ganglion(Solarplex)
Splanchnic nerves
Sympathetic chain
.1
[ \~Lumbarsplanchnics
Sup. Mesenteric
ganglion
Inf. ttesenteric
ganglion
hypogastric nerves
T> P HailecK
PelvidHypogastricjnterilidc) < sphincter \ ^
plexus. (YesicalSt rectal portions) Pelvic nerves (Nervus erigens)
Fig. 233. — Schematic representation of the involuntary nervous system. (From Jackson.)
THE AUTONOMTO NERVOUS SYSTEM 885
ergotoxine, however, a reversed effect is observed; namely, inhibition
instead of augmentation. By taking advantage of this fact, Dale has
been able to demonstrate in the hypogastric nerves inhibitory fibers to
the uterus, and Elliott has demonstrated the inhibitory action of epi-
nephrine on the muscles of the ureter in the dog. Inhibitory 'fibers have
also been discovered by these methods in the great splanchnic nerves, in
the nerve roots supplying the kidney, and in the cervical sympathetic
supplying the blood vessels of the mucous membrane of the mouth, etc.;
that is, in nerve trunks which previously were believed to contain only
augmentory fibers. The accompanying diagram from Jackson will give
an idea of the currently accepted views concerning the distribution of
augmentory and inhibitory fibers. (Fig. 233.)
THE AFFERENT FIBERS OF THE AUTONOMIC SYSTEM
It has long been known that the exposed viscera are remarkably insen-
sitive. This experience is in accord with the observation that the supply
of afferent fibers to the viscera is relatively very small. In the hypo-
gastric and probably in the great splanchnic nerve, Langley computes
that only about one-tenth of the medulla ted fibers are afferent. At the
two ends of the alimentary canal, where cooperative reflexes between
the somatic musculature and the viscera are of importance, a greater
number of afferent fibers are found in the autonomic nerves; for ex-
ample, in the pelvic nerve about one-third of the fibers are afferent, and,
as we have frequently seen, the vagi contain large numbers of them
coming from the lungs, stomach, and no doubt from other viscera.
The afferent visceral fibers, as above stated, arise like those of the
voluntary system, from the ganglion cells of the posterior roots. They
travel in company with the connector fibers through the white ramus
communicans, so that the stimulation of the central end of one of these
may cause reflex rise in blood pressure and other movements.
It is found that, after opening the abdominal cavity under local
anesthesia, cutting and suturing of the viscera may be continued without
causing any pain. When the viscera are inflamed, however, and under
certain conditions of stimulation, such as the distention of the bile ducts
with biliary calculi, or the violent contraction of the intestines, excruci-
ating pain may be evoked. This pain is frequently not localized hi the
viscera, but is referred to certain parts of the surface of the body, and
it has been shown by Mackenzie and by Head that it is referred to the
area of skin which is supplied with sensory nerves by the same segment
as that to which the afferent autonomic fibers of the particular viscus
run. It has further been shown that vascular disease may cause sensi-
886 THE CENTRAL NERVOUS SYSTEM
tivity of the corresponding cutaneous areas, so that clinical methods are
available for localizing the site of the disease by studying the exact
position and extent of the referred pain or skin tenderness.
NERVOUS SYSTEM REFERENCES
(Monographs and Original Papers)
iParker, G. H.: Proc. Am. Philos. Soc., 1911, i, 217-225.
^Head, H., and Rivers, W. H. R.: Brain, 1908, xxxi, 323-450.
sMeek, W. J. : Am. Jour. Physiol., 1911, xxviii, 356-360.
*Bruce, A. Ninian: Arch. f. exper. Path. u. Phannakol., 1910, Ixiii, 426-433.
4aSherrington, C. S. : Numerous papers on reciprocal innervation of antagonistic
muscles, Proc. Roy. Soc., Vol. B, 66; also in Jour. Physiol., xxii, xxxiv, xxxviii,
xliii, and Quart. Jour. Exper. Physiol., ii.
sHolmes, Gordon: Brit. Med. Jour., 1915, ii, Nov. 27, Dec. 4 and 11.
ePike, F. H. : Am. Jour. Physiol., 1909, xxiv, 124-152.
7Jolly, W. A.: Quart. Jour. Exper. Physiol., 1910, iv, 67-87.
sLombard, W. P.: Jour. Physiol., 1889, x, 122-148.
sCollier, J.: Lancet, April 1, 1916, 711.
icRanson, S. W., and von Hess, C. L. : Am. Jour. Physiol., 1915, xxxviii, 128.
"Head, H., and Thompson: Brain, 1906, xxix, 537.
i2Sherrington, C. S., and Brown, T. Graham: Jour. Physiol., 1913, xlvi, Proc. Physiol.
Soc., p. xxii.
isBrown, T. Graham, and Sherrington, C. S.: Proc. Roy. Soc., 1912, 85, B, 250-277.
"Gushing, Harvey: Proc. Am. Physiol. Soc., Am. Jour. Physiol., 1909.
isLuciani, L.: Kleinhirn, Ergebnisse der Physiol., 1904, I.
isBlack, Davidson: Cerebellar Localization in the Light of Recent Research, Jour. Lab.
and Clin. Med., 1916, i, 467.
irGaskell, W. H. : The Involuntary Nervous System, Monographs on Physiology, ed. by
E. H. Starling, Longmans, Green & Co., 1916.
Other Monographs not Specifically Referred to in the Context
•
18Sherrington, C. S. : (1) The Integrative Action of the Nervous System, Silliman Lec-
tures, Yale University. Scribner's Sons, New York. (2) Shafer's Textbook of
Physiology, II. Young J. Pentland, London, 1899.
J. S.: Recent Researches on Cortical Localization and on The Function of
the Cerebrum in Further Advances in Physiology, ed. by Leonard Hill, London,
E. Arnold, 1909.
INDEX
Abdominal respiration, 307
Abnormal pulses, 276
Absorption, in general, 13
from stomach, 456
of fats, 691
Acapnia, 292
Accessory food factors, 584
Acetaldehyde, 708
Acetoacetic acid, 683, 709
Acetone, 683, 709
Acid:
buffer action, 36
excretion of, by kidneys, 46
number of fats, 687
total concentration of, 32
Acidity, actual degree of, 23
Acidosis :
ammonia-urea ratio during, 616
compensated, 39
in diabetes, 683
in nephritis, 683
in starvation, 569
relationship to alveolar CO2, 354
relationship to breathing, 354
theory of, 38
uncompensated, 39
Acids, of urine, 524
Actual degree of acidity and alkalinity, 23
Adenine, 635
Adenosine, 638
Adjusters, 783
Adrenal glands and diabetes, 673
Adrenaline (see Epinephrine)
Adsorption, 65
compounds, 70
conditions influenced by, 67
effect of chemical forces on, 68
effect of electric changes on, 67
everyday reactions depending on, 66
of gases, 66
Afferent fibers of autonomic system, 885
Afferent spinal pathways, 830
Age, 584
effect on creatinine excretion, 624
Alanine, 600, 603, 606, 649, 666
Albolene absorption, 692
Albuminuria, 519
Alkali retention, determination of, 48
Alkaline buffer, 36
Alkaline reserve, 38
measurement of, 41
Allantoin, 636, 639, 645
Allied reflexes, simultaneous integration
of, 823
successive integration of, 823
Alloxan, 635
Alveolar air:
clinical investigation of, 347
estimation of gases in, 344
Fridericia method, 340
Haldane method, 340
Pearce method, 345
tension of CO,, 46, 339, 356
during breathing in confined space,
357
tension of oxygen, 339
Ambard's equation, 527
in acid excretion, 48
Amboceptor, 96
Amino acids, 597
and energy output, 541
in blood, 606
chemistry of, 598
determination of, 599
fate of, 610
groups, 598
in growth, 576
in tissues, 607
in urine, 530, 620
structure of, 602 > 603
Aminoacetic acid (see Glycocoll)
Aminopropionic acid (see Alanine)
Ammonia :
ammonia-urea ratio:
influence of acidosis on, 616
in disease, 620
influence of liver on, 617
as reserve alkali, 616
excretion of, 615
excretion of acid in combination with, 46
of urine, 530
Ammonium carbamate, 616
Ammonium carbonate, 616
Amoeba, 782
Amylases, 81, 90, 491
Amylolysis, 491
in stomach, 454
Amylopsin, 491, 656
Anacrotie wave, pulse, 203
Analysis (psychic), 858
Anaphylactie reaction, 595, 601
Anaphylaxis, 89
Anarthria, 862
Anastomosis, intestinal, 470
887
SS8
INDEX
Anemia, 93
bloodflow in, 283
Anesthesia, 831
Aneurism, bloodflow in, 284
pulse in, 143, 200
Angina pectoris, fibrillation in, 196
Animal calorimeter, 536
Anions, 16, 59
Antagonistic muscles, 818
Antagonistic reflexes, 824
Anterior roots, 787
Anticoagulants, 99
Antidromie impulses, 234
Antiferments in blood, 89
Antithrombin, 104, 112
Antitoxins, 69
Antitrypsin, 90
Aortic regurgitation, pulse in, 131
Apesthesia, 838, 851
Apex beat, tracing of, 275
Aphasia, motor, 860, 862
sensory, 862
subcortical, 862
Apnea, nervous element in, 332, 362, 365
Apparatus for measuring respiratory ex-
change, 554
Appetite juice, nature of, 440
Arc, reflex, 784
Arginase, 81, 616
Arginine, 605, 616, 627
Aromatic sulphates, 632
Arrhythmia of sinus, 266, 277
Arterial pressure, 122
Arteries, bloodflow in, 198
Arteriosclerosis, diastolic pressure in, 143
Aspartic acid, 605, 666
Asphyxia, 311
Assimilation limit, 652
Association areas, cerebral, 852, 861
neurons, 783, 785
Astasia, cerebellar, 8C7
Asthenia, 867
Asthma, dead space in, 311
Ataxy, cerebellar, 866
Atonia, cerebellar, 867
Atophan, 651
Atropine, effect on glands, 422
Auditory center, 851
Auricle, pressure in, 148
propagation of beat in, 191
Auricular curve, contour of, 153
Auricular fibrillation, 196, 269, 280
Auricular flutter, 196, 269, 279
Auriculoventricular orifice, 148
bundle, 183
node, 183
Ausculatory method (of blood pressure),
130
Autocatalysis, 77
Autonomic nerves, cerebral, 423
sympathetic, 423
Autonomic nervous system, 877
afferent fibers of, 885
Autonomic nervous system — font M
axon reflexes in, 883
bulbosacral outflow, 882
connector fibers of, 878
functions of, 884
general plan of construction, 878
parasympathetic, 882
thoracicolumbar outflow, 880
internal vesical sphincter, 882
Axon, 784
reflexes, 797, 883
Azelaic acid, 712
Bacillus coli communis, 500
Bacteria, in intestine, 499, 657
in stomach, 482
Bacterial digestion, 499
Balance, energy, 535
. material, 543
sheet of body, 543
Banting cure, 571
Basal heat production, 538
Basal ration, 5.76
Basophile cells, 96
"Bends" in caisson workers, 402
Benzoic acid, 630, 710
Benzoyl chloride, 631
Beriberi, 584
Beta-hydroxy butyric acid, 709
Bile, 442
and fat digestion, 690
chemistry of, 494
constituents of, 492
from gall bladder, 492
functions of, 493
pigments of, 495
salts, 494
Bilirubin, 495
Biliverdin, 495
B-imidazolylethylamine, effect on blood
vessels, 397
Birds, removal of liver from, 618
Blood :
absorption into, 13
amino acids in, 606
amount in body, 135
antiferments of, 89
circulation of, 122
dissociation curve of, 383
fat of,
estimation, 696, 697
variations in, 697
ferments of, 89
gases of, transportation, 379
general properties of, 85
mass movement of, 281
means by which gases are carried, 390
oxidation in, 396
proteases of, 89
proteins of, 87
origin, 88
INDEX
SS9
Blood— Cont'd
quantity of, in body, 85
refractive index of, 88
specific gravity of, 86
sugar level of, 657
regulation, 671
transfusion of, 93, 135, 139
viscosity of, 140
volume of, 136
water content of, 86
Blood cell, red, fate of, 93
origin of, 92
regeneration of, 93
stroma of, 91
white, 96
Blood clotting, 98
in diseases, 111
in physiologic conditions, 110
influence of calcium on, 103
influence of tissues on, 104
intravascular, 107
methods of retarding, in drawn blood,
• 99
negative phase of, 108
theories of, 106
time of, 100, 108
visible changes during, 98
Blood corpuscles in mountain sickness, 401
Bloodflow :
clinical conditions affecting anemia, 283
cardiovascular diseases, 284
fever, 284
diseases of nervous system, 285
mass movement of, 208
movement in veins, 214
variations in, 282
velocity of, 206
visceral, 212
Blood gas manometer, 381
Blood platelets, 97
Blood pressure, 122
diastolic, 127
effect of hemorrhage on, 135
effect of pleural pressure on, 306
factors maintaining, 134
H-ion of blood on, 237
mean arterial, 123
in shock, 290
systolic, 127
tracing, 125
Blood vessels, 880
elasticity of, 142
tone of, 236
Body fluids, reactions of, 35
Body weight and energy production, 539
Botulism, 503
Bowman, capsule of, 507
Bradycardia, 193
Brain :
circulation in, 247
vasomotor nerves, 252
volume of,' 250
Breathing, in compressed air, 399
in rarefied air, 360
periodic, 363, 371, 376
Brownian movement, colloids, 57
Bruits, 158
Buffer action of blood, 374
Buffer substances, 36
Building stones of protein, 597
Bulbosacral outflow, 882
Butyric acid, 709
C
Cadaverine, 629
Caffeine, 635
Caisson disease, 402
cause of, 403
decompression of workers, 406
prevention, 404
symptoms, 402
working conditions in, 408
Calcium ion, influence on clotting, 103
influence on heart, 166
Calcium rigor, 166
Calomel electrode, 30
Calorie, 535
Calorimeter, 535
animal, 536
Benedict, 537
bomb, 537
hand, 281
respiration, 536
Eussel-Sage, 537
Calorimetry, direct, 546
indirect, 546, 554
Canals, semicircular, 873
removal of, 874
Cannabin, 577 '
Capillary analysis of colloids, 56
Carbamino reaction, 599
Carbohydrates, absorption of, 657
assimilation limits, 652
digestion of, 656
and growth, 583
metabolism of, 652
production from protein, 665
saturation limit, 652
Carbon balance, 547
Carbon dioxide, combining power, 42
effect on respiratory center, 352
estimation in blood, 390
output, 550
volume percentage in blood, 391
Carbon dioxide tension, 337
in alveolar air, after exercise, 367
estimation of, 339, 344
in mountain sickness, 361
in periodic breathing, 375
in arterial blood, 337
in venous blood, 342
Carbonic acid (see Carbon dioxide)
Carboxyl group, 598
Cardiac decompensation, 311
890
INDEX
Cardiac depressor nerve, 239
Cardiac muscle, physiologic characteristics
of, 176
Cardiac pouch (stomach), 453
Cardiac sphincter, 448
Cardiorenal disease, bloodflow in, 284
energy output in, 542
Cardiograms, 275
Cardiovascular disease, bloodflow in, 284
Casein, 488, 576
Caseinogen, 488
Catalase, 90
Cations, 16
Catalysts, 72
Catalytic power, 23
Celenterates, nervous system of, 782
Cellulose, digestion of, 500
Centers :
association, 852, 855
diabetic, 672
motor, 843
sense,
auditory, 851
visual, 851
sensory, 850
word centers, 862
Cephalin, 689
Cereals and growth, 581
Cerebellar ataxy, 866
Cerebellum :
ablation of, 869
clinical observations, 870
extirpation of, 869
functions of, 865
lobes of, 868
localization of function of, 867
Cerebral circulation, 247
Cerebral compression, 253
Cerebral cortex, stimulation of, 844
structure of, 852
Cerebral localization, 843
clinical observations, 849
hemispheres, removal of, 840
Cerebral vessels, ligation of, 247
Cerebrospinal fluid, 248
Cerebrum, higher functions of, 860
CH, method of expressing, 27
Cheyne-Stokes breathing, 371, 377
Chlorides, urine, 513
Cholesterol, 494, 688
estimation of, 697
Choline, 689
Chorda tympani, 231, 396, 423
Chromatolysis, 801
Chromatine, 638
Chromosones, 638
Chyme, 456, 482
Circle of Willis, 247
Circulation of blood:
control of, 216
influence of gravity on, 244
mass movement of blood, 208
through the heart, 257
Circulation of blood — Cont'd
through the liver, 255
through the lungs, 253
time of, 213
Circulation time, 206
Clinical application, circulation, 259
nervous system, 828, 849, 862
respiration, 310, 399
Clotting of blood (see Blood clotting)
Coagulative ferments, 82
Cod-liver oil, nutritive value, 706
Coefficient of oxidation, 393
Coefficient of solubility of gases, 337
Cold spots, 792
Collaterals, 784
Colloids :
Brownian movement, 57
capillary analysis, 56
characteristic properties of, 50
diffusibility of, 51
dispersion means, 54
dispersoid, 54
electric properties of, 55
osmotic pressure, 57
electrophoresis, 56
external phase, 54
gelatinization, 61
heterogeneous, 51
homogeneous, 51
imbibition, 62
internal phase, 54
isoelectrie point, 64
lyophobe, 60
mutual precipitation of colloids, 56
osmotic pressure of, 141
size of colloid particles, 53
suspensions, 53
suspensoids and. emulsoids, action of
electrolytes on, 63
Tyndall phenomenon, 51
Compensated acidoses, 39
Complemental air, 300
Compressed air sickness, 399
cause of symptoms, 403
prevention of, 404
treatment of, 406
Concentration cell, 30
Concentration point, auricles, 185
Conception, 861
Concept, 861
Conditioned reflexes, 481, 856
Conductivity, determination of, 17
equivalent, 19
molecular, 19
specific, 17
Conductivity cell, 18
Conglutin, 538
Construction of autonomie nervous sys-
tem, 877
Contracture, extension, 806
Cooking, 593
Coronary circulation, 257
Coronary vessels, vasomotor nerves, 268
INDEX
891
Corpora quadrigemina, 840
section behind, 840
section in front of, 840
Corpuscles of blood, red, 91
white, 96
Cortex, removal of, 843
Coughing, 300, 412
Cranial cavity, pressure in, 251
Creatine, 606, 613
chemistry of, 622
estimation of, 623
in disease, 626
metabolism of, 624
origin of, 626
Creatinine, 613
chemistry of, 622
coefficient, 624
estimation, 623
in urine, 529
metabolism, 624
of blood in disease, 651
origin of, 626
Crista acustica, 873
Critical concentration, 8
Crossed extension reflex, 804
Cuorin, 689
Current of action of heart, 187
Cyanosis, 360, 400
Cysteine, 603
Cystine, 577, 592, 604
Cystosine, 637
Cytases, 463
D
Dalmatian dog, purine metabolism of, 646
Dalton's law, 336
Dead space, 302, 310
Deafness, 864
Deamidization, deaminization, 501
Deaminizing enzyne, 639
Decerebrate rigidity, 808
Decerebration, 843
Decolorization of liquids by charcoal, 66
Decompression, 406
Defecation, 470
blood pressure during, 412
Defibrinated blood, 101
Degeneration, successive, 813
Deglutition, 445
Delayed conduction, 270, 276
Delirium cordis, 195
Dendrites, 784
Depression of freezing point, 10
of urine, 523
Depressor nerve, 238, 239, 240
Depressor substances, 397
Dessert, physiologic value of, 437
Detoxication compounds, 629
Detoxication process, 501
Dextrins, 491, 656
Dextrose (see Glucose)
Diabetes :
acidosis in, 684
and the ductless glands, 678
assimilation limits in, 652
blood examination in, 659
blood fat in, 699
center, diabetic, 672
early diagnoses of, 652
energy output in, 542
experimental, 672
fat metabolism in, 683
ketosis, 683
pancreatic, 678
nervous, in man, 674
permanent, 676
phlorhizin, 665
postprandial hyperglycemia, 659
renal, 661
starvation treatment in, 684
treatment of, 653
Diabetic acidosis, 684
Diabetic center, 672
Diabetic gangrene, 258
Dialuric acid, 645
Dialysate, 52
Dialysis, 12
method, colloids, 51
Diaphragm, action of, 320, 321
physiology of, 324
Diastolic filling of heart, 153
Diastolic pressure, 127, 132
measurement of, in man, 128
Dicrotic notch, 202
wave, 203
Diet at different ages, 590
of different communities, 589
Dietetics, 588
Differential manometer, 381
Diffusion, 12
Digestibility of foods, 593
Digestion, by pancreatic juice, 489
in intestine, 489
in stomach, 481
mechanism of, 444
Digestive glands:
control of,
hormone, 425
nervous, 423
general physiology of, 418
microscopic changes during activity,
418
Dispersion medium, colloids, 54
Dispersoid, colloids, 54
Dissociation, 16, 17
JMssociation constant, 19, 388
Dissociation curve:
of blood, 383
of hemoglobin, 383
influence of salts on, 385
influence of H-ion concentration on,
386
influence of temperature on, 386
Dissociation hypothesis, applications of, 21
Dissociation, rate of, 380
892
INDEX
Diuresis, 578
Diuretics, 578
Diver 's palsy, 402
Douglas method, 544, 558
Dropped beat, 271
Dtt Bois formula, 541
Ductless glands, 729
in diabetes, 678
Dyspnea, 314, 349
Dystrophy, isolation, 808
E
Earth worm, nervous system of, 783
Eck fistula, 617
Eclampsia, 620
Edema, 62, 120
Edestin and growth, 577
Effectors, 782
Elastin, digestion of, 486
Electric conductivity, 16
Electric currents, development of, 29
Electric properties of colloids, 55
Electrocardiograms, 158, 259
normal, 261
standardization of, 260
ventricular complex, 262
waves of, 261
P-wave, 189, 261
T-wave, 220, 263
Electrocardiograph, 260
Electrocution, cause of death in, 195
Electrolytes, 16
action of, on colloids, 63
Electrolytic solution pressure, 29
Electrophoresis of colloids, 56
Electrostatic attraction, 29
Emboli, 107
Emetics, 450
Emotional glycosuria, 675
Emphysema, 311, 314, 324
Empyema, 324
Emulsions, 688
Emulsoids, colloids, 60
Endocrine organs, 729
• Endoenzyme, 71
Endogenous metabolism, 615
of purines, 647
Energy balance, 535
Energy output, and age, 541
and body weight, 549
and disease, 542
and muscular work, 549
and sex, 541
and surface area, 540
in starvation, 568
Enterokinase, 443, 489
Enzymes, 71
action of temperature on, 74
amylases, 81
and catalysis, 72
antienzymes, 81
arginase, 81
E nzymcs — Cont 'd
coagulative ferments, 82
conditions of activity, 82
endoenzymes, 71
glyoxylase, 82
invertases, 81
lipases, 81
nature of, 72
oxidases, 82
peculiarities of, 80
peroxidases, 82
properties of, 73
proteases, 80
reversibility of action of, 25, 77
specific action of, 73
types of, 79
urease,' 82
velocity constant, 74
Epieritie receptors, 790
Epilepsy, Jacksonian, 849
Epinephrine, 236, 502
and diabetes, 673
Equilibrium, nitrogen, 571
Equivalent, conductivity, 19
Erepsin, 490, 601
Ergastoplasm, 420
Ergot, 502
Ergotoxine, 209
Erythroeytes, 91
fate of, 94
regeneration of, 93
Escapement, 218
Esophagus, during swallowing, 446
inhibition of, 447
peristaltic wave in, 447
Esters, 686
Ester value, 687
Ethereal sulphates, 501, 632
Excelsin, 577
Exogenous metabolism, 615
Exophthalmic goiter, 756
energy output in, 542
Excretion of acid combined with ammonia,
46
Excretion of urine, 507
Extension contracture, 45, 806
Extensor thrust, 57
reflex, 805
Exteroceptors, 788, 822
Extrasystole, 266
Eyes, movements of, 847
F
Factor safety, in diet, 592
Fatigue of reflexes, 825
Fats:
absorption of, 691
chemical theory, 693
mechanistic theory, 692
and growth, 583
blood, 696, 697
destination of, 699
INDEX
893
Fats, blood— Cont 'd
determination, 696
during absorption, 698
during fasting, 698
variations in, 697
chemistry of, 686
depot fat, 699, 700
destination of, 701
desaturation of, 705, 712
digestion of, 690
fat dust, 696
liver fat, 699, 701
metabolism of, 696
tissue fat, 699, 706
transportation to liver, 702
Fatty acids, 686
acid number, 687
breakdown of, 709
ester value, 687
formation from carbohydrates, 701,
707 •
in liver in disease, 703
iodine value, 688
melting point, 687
Eeichert-Meissl value, 688
saponification value, 687
Feces, 499, 521
Ferments (see Enzymes)
Ferments in blood, 89
Fever, bloodflow in, 284
cold-bath treatment, 284
purine excretion during, 648
Fibers, anterior root, 100
connector, 193
internuncial, 802
Fibrillation, auricular, 196, 269
ventricular, 195
Fibrin, 99
fibrin needles, 99
source of, 101
Fibrin ferment (see Thrombin), 102
Fibrinogen, 87, 101, 103, 111
Filtration, 13
Final common path, 787, 821, 824
Fistula, biliary, 492
gastric, 434
salivary, 430
Flexion-reflex, 804, 821
Flutter, auricular, 269
Food:
accessory factors of, 593
cooking, importance of, 593
effect of, on circulation, 243
effect on creatinine excretion, 624
laxative qualities, 594
palatability, 593
Food factors, accessory, 584
Foodstuffs, rate of leaving stomach, 458
Forced breathing, 324
Formaldehyde titration, amino acids, 487
Formation of solid surface films, 66
Freezing point constant, 10
Freezing point, depression of, 10
Fridericia's method for alveolar air, 340
Frontal visual center, 851
Fructose, 666
Functions of autonomic nerves, 884
Fundus of stomach, 452
G
Gallstones, 494
Galvanometer, string, 187, 259
Ganglia, 784
Gas in stomach, 462
Gas laws, 3, 336
Gases, adsorption of, 66
coefficient of solubility, 337
estimation of, 344
partial pressure of, 336
solution of, 336
tension of, 336
transportation in blood, 390
Gaskell's clamp, 175
Gastric contents, regurgitation of, 449
Gastric digestion, 481
rate of, 487
Gastric fistula, 434
Gastric juice, quantity secreted, 440
strength of, 441
Gastric secretion, 432
hormone control of, 437
local stimulation of, 438
nervous control of, 434
Gastric tube, 453
Gastrin, 439, 456
Gastroenterostomy, 460
Gastrointestinal contents, reaction of, 505
Gelatin, 578
Gelatinization, 61
Glands, changes during activity, 422
electric changes, 422
normal conditions of activity, 430
oxygen consumption of, 396, 421
respiration of, 396
Globulin, 577
Gliadin, 576
Glomerulus, 507
Gluconeogenesis, 662, 677, 680
direct method, 663
indirect method, 664
in normal animals, 667
Glucose, 708
fate of absorbed, 662
glucose to nitrogen ratio, 664
injections, intravenous, 653
subcutaneous, 656
parenteral assimilation, 656
tolerance for, 653
utilization of, in tissues, 677
Glutarnic acid (see Glutaminic acid)
Glutaminie acid, 605, 667
Glutein, 577
Glutelin, 577
Glyceric aldehyde, 665
Glycerol, 665
894
INDEX
Glycocholic acid, 494, 631
Glyeine, 494, 603
Glycinin, 577
Glycocoll, 601, 630, 667, 710
Glyeogen, 662
fate of, 669
sources of, 662
Glycogenase, 662
Glycogenolysis, 669
hormone, 676
nervous, 672
postmortem, 670
Glycolaldehyde, 665
Glycolysis, 677
Glyconeogenesis (see Gluconeogenesis)
Glycosuria, alimentary, 659
emotional, 675
postprandial, 659
relation to sugar of blood, 660
renal, 661
Glycuronates, 630
Glycuronic acid, 630, 631, 632
Glyoxal, 631
Glyoxylase, 82, 666
Glyoxylic acid, 631
G-N-ratio, 664
Goiter, exophthalmic, 542
Gout, 648, 650
etiology of, 650
guanine, 640
uric acid excretion in, 648
Grading of intensity of reflex action, 809
Gram molecule, 3, 5
Gram molecular solution, 22
Gravity, on circulation, 244
compensation for, 245
Growth, 574
accessory factors, 585
basal ration, 576
carbohydrates and, 583
curves of, 576
curves of inhibition, 579
fats and, 583
inorganic salts and, 586
lysine and, 578
proteins and, 574
trypanophane and, 578
vitamines, 584
Guanidine, 605, 622
Guanine, 635
gout, 640
Guanosine, 638, 639
Giinsberg reagent, 487
H
Haldane-Barcroft apparatus, 45
Haldane gas apparatus, 559
Haldane's method for alveolar air, 340
Heart :
action of, 144
auricular curve, 153
diastole of, 145
Heart— Cont'd
isometric period in, 149
muscle, properties, 176
nutrition of, 161
opening and closing of valves, 154
oxygen requirements- of, 396
oxygen supply of, 164
perfusion of outside body, 161
postsphygmic period, 150
presphygmic period, 149
pressure in, 146
pumping action of, 134, 144
resuscitation in situ, 164
rhythmic power in, 170, 174
sounds of, 157
systole of, 145
utilization of glucose in, 681
vagus control of, cold blooded, 217
vagus control of, mammalian, 220
vagus terminations in, 225
ventricular curve, 146
work of, 212
Heart beat :
• arrhythmia of, 266
myogenie hypothesis of, 171
neurogenie hypothesis of, 170, 172
origin of, in cold-blooded animals, 170
origin of, in mammalian, 182, 189
pace maker of, 174.
propagation of, 224
sympathetic control of, 223, 227
ultimum moriens, 185
vagus control of, 217, 220
Heart block, 174, 270, 276
effect of vagus on, 219
Heart disease, vital capacity of lungs in,
314
Heart-lung preparation, 158
Heat production and age and sex, 541
and body weight, 539
surface, 540
disease, 542
Heat spots, 792
Heat value of foods, 535
Hematocrit, 7
Hematoporphyrin, 496
Hemiplegia, 258
Hemodromograph, 200
Hemoglobin, 91
dissociation constant, 388
dissociation, curve of, 380, 382, 383
estimation of, 92
rate of dissociation, 386
relationship to bile pigments, 496
specific oxygen capacity of, 379
transportation of O2 by, 390
Hemolysis, 7, 95
Hemolytie jaundice, 93
Hemophilia, 112
Hemopoietic activities of bone marrow, 93
Hemorrhage, 59
immediate effects of, 137
recovery from, 138
INDEX
895
Hemorrhagic diseases, 112
Henle, loop of, 507
Hepatic artery, flow in, 255
Heterocyclic compounds, 604
Hexoses, 652
Hibernating animal, metabolism of, 549
Hibernation, breathing during, 374
Higher functions of cerebriim, 860
H ion or hydrogen ion, 168
H-ion concentration, 22
after hemorrhage, 142
catalytic power of, 23
determination of, 31
of intestinal contents, 505
law of mass action and, 26
method of expressing, 27
method of measurement:
electric method, 29
indicator method, 32
standard solutions for, 34
H-ion concentration in blood:
effect on dissociation curve, 386, 389
effect on respiratory center, 335
Hippuric acid, 530, 630, 710
Hirudin, 100
Histamine, 397, 502
Histidine, 606, 623
Homogentisic acid, 502, 531
Hordein, 578
Hormones, 3, 729
in control of circulation, 216
respiratory, 349
Howell theory (blood clotting), 106
Hunger, 471
Hunger contractions :
alcoholic beverages and, 478
control of, 476
during starvation, 475
in esophagus, 474
inhibition of, 477
in stomach, 471
nerve centers and, 479
remote effects of, 474
rhythmic, 471
splanchnic nerve and, 477
vagus nerve and, 477
Hiirthle manometer, 126, 146 •
Hydrocephalus, 249, 253
Hydrochloric acid, amount of, 482
and emptying of stomach, 460
functions of, 482
source of, 483
Hydrogen ion (see H ion)
Hyperacidity, 461
Hyperesthesia, 831
Hyperglycemia, in pancreatic diabetes,
680
postprandial, 659
splanchnic, 673
Hyperpnea, 349, 359
Hyperthyroidism, 756
Hypertonie solution, 6
Hypertonicity, 63
Hypogastrie nerves, 797
Hypothyroidism, 755
Hypotonic solution, 6
Hypoxanthine, 635
Ignition juice, 438
Ileocolic muscles, 882
Ileocolie sphincter, 467, 469
Imbibition, 62
Imidazole and growth, 586, 604, 623
Imidazole ring, 623
Imidazolylethylamine, 397, 426, 502
Immediate induction, 823
Impulses, nature of, 830
Index test, 870
Indican, 632
Indicator method, list of indicators, 33
Indole, 501, 604, 632
Indoxyl sulphate of potassium, 632
Induction, immediate, 823
successive, 824
Inhibition, reciprocal, 814
Inhibitory effects of autonomic nerves, 884
Inner vation, reciprocal, 814
Inorganic constituents of urine, 531
Inorganic salts and growth, 586
Inosine, 639
Inosinic acid, 637
Inspiration, negative pressure during, 305
Integration of allied reflexes, 822
Integration of nervous system, 809
Intercostal muscles, 319
Internal anal sphincter muscles, 882
Internal vesical sphincter, 882
Internal respiration, 378
Intestinal bacteria, 657
Intestinal juice, control of, 442
Intestinal obstruction, 470, 504
Intestinal secretions, 441
Intestine :
absorption from, 13
anastomosis of, 470
bacterial digestion in, 499
digestion in, 489
law of, 466
movements of:
large, 468
clinical conditions effecting, 470
small, 463
nature of, 466
nervous control of, 467
Intracardiae pressure curves, 146, 151
Intracranial pressure, 253
Intragastric pressure, 454
Intrapleural pressure, 304
Intrapulmonie pressure, 299
Intra vitam anticoagulants, 100
Intravascular clotting, 107
Inulin, 664
Invertase, 81, 492, 657
Invertebrates, segmented, 783
Inverting enzymes, 657
896
INDEX
Involuntary fibers, course of, 879
Iodine value of fats, 688
lonization, 16
Irradiation in nervous system, 826
Irreversibility in reflexes, 810
Isoeleetric point, 64
Isoleucine, 604
Isomaltose, 79
Isometric period, 149
Isotonic solution, 6
Jacksonian epilepsy, 849
Jugular pulse tracing, 274
Juice, gastric, 434
intestinal, 442
pancreatic, 441
K
Keith and Flack, conducting tissue in
heart, 185
Kent, bundle of, 185
Ketonic acid, 708
Ketosis, 683
Kidney, oxygen requirements of, 396
removal of, 621
structure of, 507
Knee-jerk, 804, 815, 828
reinforcement of, 829
Lactalbumin, 577
Lactam, 649
Lactase, 491, 657
Lactic acid, 397, 603, 676, 665, 708
effect on respiratory center, 376
in mountain sickness, 362
produced by exercise, 367, 413
Lactim, 649
Language, 860
Latent period, 809
Laws of gases, 336
of mass action, 23
applied measurement of H-ion concen-
tration, 26
Lead poisoning, 650
Lecithin, 689
estimation of, 697
in bile, 498
in blood, 696, 699
Leech extract, 100 .
Legumelin, 578
Legumin, 578
Lesions of nervous system, 835
Leucine, 604, 666
Leucocytes, 96
sensitizing of, 70
transitorial, 97
Leucocythemia, 648
Levulose, 656
Levy and Rowntree method, 41
Limulus, heartbeat of, 172
Lipase, 25, 90, 491, 687
Lipemia, 699
Lipoids of blood, 699
Lissauer-tract, 831
List of indicators, 33
Litten's diaphragm phenomenon, 321
Liver :
circulation through, 255
disease of, 620
glyeogen in, 662
metabolism of fats in, 701
perfusion of, 618
removal of, 617
urea formation in, 617
Local irritants, 243
Localization, cerebral, 843
Locke solution, 168
Loven reflex, 244
Lungs, circulation through, 253
mode of expansion of, 325
Lymph:
absorption into, 13
electric conductivity, 16
filtration in, 118
formation and circulation, 115
formation of, 15
Lymph spaces, 115
Lymphagogues, 119
Lymphatics, 115
Lymphocytes, 96
Lyophobe colloids, 60
Lysine, 592, 605
Lysine and growth, 576
M
Maculae acusticse, 873
Maintenance, diets for, 579
Malingerers, 42
Maltase, 491, 657
Maltose, 491
Manometer :
blood-gas differential, 382
Hiirthle, 124, 146
mercury, 123
. optical, 146
spring, 126
valved mercury, 152
Mark-time reflex, 806
Mass action, 23
Mass action and H-ion concentration, 26
Mass movements of blood, 281
Mastication, 444
Mechanics of respiration, 299
Medulla, section above, 839
Megacaryocytes, 103
Melting point, fats, 687
Membrane synaptic, 798
Memory, 786
Mercury manometer, 123
INDKX
897
Metabolism:
calculations, 544
endogenous, 615
exogenous, 615
general, 534
in starvation, 566
normal, 570
of carbohydrates, 652
of fats, 686
of proteins, 595
of purines, 637
special, 534
Methyl glyoxal, 665
Methyl group, 598
Methyl purines, 635
Methylation, 627
Methylglyoxal, 665, 666
Mett's method, 487
Microcytes, 94
Microtonometer, 339
Mid-capacity of lungs, 311
Milk, clotting of, 488
Miniature stomach, 433
Minimal air, 300
Mononuclear leucocytes, 96
Morawitz theory, blood clotting, 107
Motor areas, ablation of, 843
stimulation of, 844, 846, 848
Motor nerves of segmental duct muscles,
881
Mountain sickness, 360, 399
adaptation to, 400
alveolar CO2 in, 360
blood corpuscles in, 401
Movements, of intestine, 463
of stomach, 452
Municipal food statistics, 591
Muscarine, action on heart, 226
Muscle, cardiac, properties of, 176
refractory period, 178
respiration in, 395
staircase phenomenon (treppe), 177
skeletal, 177
respiration in, 394
Muscles, antagonistic, 818
Muscular exercise, 243, 539
circulatory changes during, 410
effect on metabolism, 551
effect on respiration, 366
H-ion during, 413
purines during, 647
redistribution of blood during, 415
respiratory changes during, 410
temperature of blood during, 415
Mutual precipitation of colloids, 56
My enteric reflex, 796
Myogeuie hypothesis of heartbeat, 171
Myxedema, 755
energy output in, 542
N '.
Narcotics and blood fat, 698
Necrosis of liver, 620
Negative pressure in ventricle, 152
Nephelometer, 697
Nephrectomy, 621
Nephritis, 650
acidosis in, 683
urea retention in, 528
Nerves :
of skin, 796
network, 4, 29
regeneration of, 36
segmental distribution of, 837
specific properties of, 789
vasodilator, 797
Nerve cells, 33, 799
Nervi erigentes, 231
Nervous control:
of gastric secretion, 434
of ileocolic sphincter, 468
of intestinal glands, 442
of intestinal movements, 467
of pancreas, 427
of salivary glands, 423
of stomach movements, 458
Nervous diabetes, 672
in man, 674
Nervous system:
autonomic, 877
bulbar fibers, 882
functions of, 884
sacral fibers, 882
thoracicolumbar fibers, 880
effect of section at various levels of:
anterior root, 99, 835
just behind medulla, 839
just behind post. corp. quad., 840
just in front of ant. corp. quad., 840
posterior roots, 836
spinal cord, 839
evolution of, 718
influence on excretion of urine, 519
integration of, 786, 809
Network, nerve, 796 .
neurofibrils, 800
neuropile, 784, 797
Neurogenie hypothesis, of heart, 172
Neurons, 784
association, 783, 785
intermediate, 802
internuncial, 802
Neutrality, regulation of, 36 -
Nicotine, 233
action on vagus, 226
Nissl bodies, 800
Nitrogen :
excretion of,' premortal rise, 566-
in starvation, 566
undetermined, urine, 613
Nitrogen balance, 570
Nitrogenous constituents of urine, 523
Nitrogenous equilibrium, 571
Nitrogenous metabolites, in starvation,
568
898
INDEX
Nociceptive, 795, 804
impulses, 832
reflex, 825
Noeud vital, 327
Nonelectrolytes, 16
Nonthreshold substances, 512
Normal acid, 22
Normoblasts, 93
Nuclease, 638
Nucleic acid, 637, 689
Nuclein ferments, 90
Nucleins, 637
Nucleoside, 638
Nucleotide, 638
Nystagmus, 871, 875
O
Obesity, Banting cure for, 571
Oleic acid, 868
Olein, 868
Opsonins, 70
Organs, loss of weight during starvation,
568
perfusion of, 618
Ornithine, 616, 631
Ornithuric acid, 631
Orthopnea, 313, 318
Oscillatory method of blood pressure, 130
Osmometer, 5, 230
Osmosis, 4
Osmotic pressure, 4, 10
and formation of lymph, 13
and hemolysis, 7
and plasmolysis, 8
measurement by depression of freez-
ing point, 11
in physiologic mechanisms, 13
in production of urine by kidneys, 14
of transfusates, 141
Ovalbumin, as food, 577
Ovovitellin, as food, 577
Oxidases, 82
Oxidation of blood, 387
Oxybutyric acid, 616, 683, 709
Oxygen :
coefficient of oxidation, 393
determination of, 562
estimation in blood, 390
requirements of tissues, 393
tension in alveolar air, 340, 344
tension in arterial blood, 337
transportation by blood, 379
volume percentage in blood, 390
Oxygen insufficiency, and periodic breath-
ing, 373
effect of, on respiration, 350, 359
Oxygen supply of heart, 164
Oxyproteic acid, 629
Pacchionian body, 249
Pain:
sensation of, 832
Pain, sensation of — Coiit'd
transmission in eorji, 830
sense, 795
Palatability, 593
Palmitic acid, 686, 707
Pancreas :
hormone control of, 420
histologic changes of, 429
oxygen requirements, 396
nervous control of, 427
sugar metabolism and, 678
Pancreatic diabetes, 678
Pancreatic digestion, 489
Pancreatic juice, 441
and fat digestion, 690
secretion of, 420, 426
Pancreatin, 490
Parasympathetic system, 882
Paroxysmal tachycardia, 269, 278
Partial dissociation, 271
Partial pressure of gases, 336
Pathways, sensory, in spinal cord, 830
Pelvic ganglionic group, 878
Pentose, 664
Pepsin, action of, 485
products of, 486
Pepsinogen, 485
Peptides, 601
Peptone, 105, 486
Perception, 861
Perfusion, of kidney, 631
of liver, 618
Perfusion fluid, of heart, 165
Perfusion of heart, 161
Periodic breathing, causes of, 372
types of, 371
Peripheral resistance, 134, 229
Peristalsis :
in esophagus, 446
in large intestine, 468
in small intestine, 465
in stomach, 453, 456
Peristaltic rush, 466, 470
Peristaltic wave, 465
Pernicious anemia, energy output in, 542
Peroxidases, 82
PH, 27
Phagocytes, 97
Phenaceturic acid, 710
Phenol, 501
Phenolacetie acid, 502
Phenolphthalein, 482, 525
Phenylacetic acid, 631, 710
Phenylalanine, 604
Phenyl group, 604
Phlorhizin, 664, 665
Phosphates, excretion of, 47
Phosphate solutions for H-ion, 34
Phosphates of urine, 532
Phospholipins, 689
in bile, 498
Phrenic center, 328
Physicoehemical basis, 1
INDEX
899
Physiologic processes depending on ad-
sorption, 69
Pigments, absorption of, 117
Piloearpine, action on heart, 226
Pilomotor fibers, 880
Pitot's tubes, 201
Plasma, 99
Plasmolysis, 8
Platelets, of blood, 97, 106
Plethora, 86
Plethysmograph, 209, 230, 273, 303
Pleurisy, 324
Plexus of Auerbach and Meissner, 466, 796
Pneumothorax, 305
Poikilocytes, 94
Polygraph, 273
Polyneuritis, 584
Polynuclear cells, 96
Polypeptides, 487, 601
Polyphosphoric acid, 637
Polysaccharides, 489
Polysphygmograms, 273
Portal vein, bloodflow in, 255
Postdicrotic wave, pulse, 203
Postprandial hyperglycemia, 659
Postcentral convolutions, 850, 854
Posterior roots, 787, 836
Postsphygmic period, 150
Postural reflexes, 826
Potassium, microchemical test for, 421
Potassium ions, on heart, 167
Potential acidity of urine, 524
Precentral convolutions, 843, 854
Precipitins, 595-
Predicrotic wave, pulse, 203
Prefrontal region, 854
Premature beats, 277
Premortal rise, 566
Presphygmie period, 149
'Pressor impulses, 238, 239, 240
Pressure :
intrapleural, 304
effect of, in blood pressure, 306
intrapulmonic, 299
negative, 305
osmotic, 10
Pressure pulse, 127
Principle of Willard Gibbs, 66
Proline, 604
Proprioceptive impulses, 865
Proprioceptors, 822
Prosecretin, 426
Proteases, 89
Protein sparers, 571
Proteinases, 80
Proteins :
as colloids, 63
bacterial digestion of, 501
chemistry of, 597
metabolism of, 595, 613
end products, 613
minimum requirement, 572, 592
of blood, 88
Proteins — -Gont 'd.
relative value of, for growth, 611
salting out of, 60
Proteose, 486
Protopathic impulses, 831
Protopathic receptors, 790
Protothrombin, 103, 106, 111
Psychopathology, 862
Ptomaines, 502, 629
Ptyalin, 491, 656
Pulmonary circulation, 253
Pulmonary ventilation, 350
Pulses, 198
abnormal, 276
alternans, 181
bigeminus, 181
contour of wave, 200
length of wave, 199
palpable, 201
pressure, 127
pulse curves, 202
pulse waves, 189, 200, 203
rate of transmission, 198
velocity, 200
venous, central, 205, 274
venous, peripheral, 205
Purkinje fibers, 184
Purine bodies (see Purines)
Purines :
chemistry of, 529, 613, 634
endogenous, 641, 643
exogenous, 641
metabolism of, 637
in starvation, 569
synthesis of, 646
Putrefaction, intestinal, 501, 530
Putrescine, 629
Pyloric . canal, 452
Pyloric sphincter, control of, 456
Pyloric vestibule, 453
Pyramidal cell lamina, 854
Pyrimidine bases, 636, 637
Pyruvic acid, 600, 708
B
Runii communicantes, 233
Eaynaud's disease, bloodflow in, 258
Eeaction deviation, 871
Eeaction of urine, 524
Eeactions depending on adsorption, 66
Eeactions of body fluids, 35
Eeceptors, 782, 788
distance, 785
epicritic, 790
external, 788, 822
internal, 788
of skin, 790
projicient, 785, 788
proprio, 822
protopathic, 790
temperature, 791
touch, 793
900
INDEX
Reciprocal inhibition, 814
action of strychnine on, 819
Eeciprocal innervation of blood vessels,
241, 814
Bed blood corpuscles, origin of, 92
Eeduction of blood, 387
Referred pain, 885
Reflex, conditioned, 431
unconditioned, 431
Reflex arc, 784
after effect, 810
grading of intensity, 809
irreversibility of conduction, 810
latent period, 809
oxygen deprivation, 813
properties of, 13, 29, 49
refractory period, 811
summation, 810
Reflex conduction, resistance of, 813
Reflexes :
allied, simultaneous integration of, 823
antagonistic, 824
axon, 797
Babinski, 807
conditioned, 856
cremasterie, 856
crossed extension, 804
extensor thrust, 805
fatigue of, 825
flexion, 804, 821
integration of allied, 821
interaction between, 821
irradiation of, 826
mark-time, 806
myenteric, 796
nature of, 825
nociceptive, 825
postural, 826
unconditioned, 431, 856
Refractive index, blood, 88
Refractory period, 811
Refractrometric methods, 88
Regeneration of erythrocytes, 93
Regulation of neutrality, 36
Regurgitation of gastric contents, 449
Reichart-Meissl value of fats, 688
Reinforcement of knee-jerk, 829
Renal diabetes, 661
Renal function, theories of, 511
Rennin, 488
Reserve alkalinity, measurements of, indi-
rect methods, 42, 46
measurement of, titration methods, 41
Residual air, 300, 311
Respiration :
abdominal, 307
beyond the lungs, 378
during muscular exercise, 410
in compressed air, 402
in rarefied air, 399
mechanics of, 299
movements of diaphragm in, 321
movements of ribs in, 319
Respiration calorimeter, 536
Respiratory center, 327
afferent impulses to, 331, 332
automaticity of, 329
hormone control of, 335, 349
reflex control of, 331
sensitivity to alveolar CO,, 357
stimulation by OCX, 352
subsidiary, 328
Respiratory changes in muscular exercise,
410
Respiratory exchange :
according to body weight, 550
and body temperature, 551
clinical method for determining, 554
in diabetes, 678
and muscular exercise, 551
and temperature of environment, 551
in tissues, 393, 397
Respiratory hormone, nature of, 349
Respiratory movements, 315
Respiratory passages, pressure of air in,
299
Respiratory quotient, 545
in diabetes, 678
influence of diet on, 547
influence of metabolism on, 549
influence of muscular activity on, 370
Respiratory tracings, 303
Respiratory valves, Pearce's, 554
Reticulated erythroblasts, 93
Reversible action of enzymes, 25
Ribs, movements of, 315
musculature of, 319
undulatory movements of, 317
Right lateral connection, heart, 185
Rigidity, decerebrate, 808
Rolandic fissure, 855
Roots, 787
anterior, 787, 835
posterior, 787, 836
Rhythmic segmentation, 464
Sacral outflow, 882
Salicylates, 648, 657
Saline injection, effect on blood pressure,
139
Saliva, control of secretion, nervous, 423
psychic, 431, 856
normal secretion, 431
Salt, dietetic value, 586 -~
Salted blood, 100
Salting of proteins, 60
Saponification, 687
Sarcosine, 623
Saturation limits, 652, 654
Scratch reflex, 805, 821
Scurvy, 585
Sea anemone, nervous system of, 783 •
Second wind, 415
Secretory fibers, varieties of, 424
INDEX
901
Secretin, 425
chemical nature of, 426
mechanism of action of, 420
Secretion (see under various glands)
general considerations, 418
Segmental distribution of nerves, 837
Segmentation movements, 463
Segmented invertebrates, nervous system,
783
Semicircular canals, 873
eye movements and, 875
removal of, 874
Semilunar valves, 150, 155
Semipermeable membrane, 4
Sense, temperature, 791
touch, 793
pain, 795
Sensory centers, 850, 851
Serine, 603
Serum albumin, 87
Serum globulin, 87
Sex, effect on creatinine excretion, 624
effect on energy output, 541
Sham feeding, 435
Shell shock, 287
Shock, 287
anesthetic, 288
blood pressure in, 290
experimental investigations, 289
gravity, 287
hemorrhagic, 288
nervous, 289
recovery from, 805
secondary symptoms of, 295
spinal, 288, 803
surgical, 289
treatment of, 295
vasomotor control in, 290
Sinoauricular node, 185, 266
Sinus arrhythmia, 266, 277
Sinus bradycardia, 266, 277
Skatole, 501, 632
in urine, 531
Skeletal muscle, respiration in, 394
Skin, receptors of, 790
Soap, 686
Sodium ions, 166
Solution of gases, 336
Solutions :
gas laws and, 3
gram molecular, 5, 22
hypertonic, hypotonic, and isotonic, 6
nature of, 3
Sorensen method for estimating amino
groups, 599
.Sounds, cardiac, 157
recording of, 158
Specific conductivity, 17
Specific dynamic action,, 538
Specific gravity of urine, 522
Sphingomyelin, 689
Sphygmic period, 273
Sphygmograph, Dudgeon's, 201
Spinal animal, 804
Spinal column, 786
Spinal cord :
ablation of, 839
in laboratory animals, 803
in man, 806
hcmisection of, 831
sensory pathways in, 830
successive degeneration in, 813
Spinal pathways, afferent, 830
Spinal shock, 803, 807
Spirometer, 556
Splanchnic circulation in shock, 292, 294
Splanchnic nerve, 233, 672
Sponges, nervous system of, 782
Stalagmometer, 65
Standard of neutrality, 26
Standard solutions, preparation of, 34
Stannius' ligature, 176
Starvation, 566
acidosis during, 569
cause of death, 570
effect of creatinine excretion, 625 „
energy output during, 568
excretion of nitrogen, 566
loss of weight, 568
nitrogenous metabolism, 568
purines during, 569
secretion of gastric juice during, 476
sensations during, 475
sulphur during, 569
treatment of diabetes, 684
Statistical method, in diet control, 589
Stearic acid, 687
Stokes-Adams syndrome, 193
Stomach :
arrangement of food in, 455
digestion in, 481
emptying of, 456
effect of pathologic conditions on,
460
rate of, 458
gas in, 462
miniature, 433
movements of, 451
effect on food, 454
Stroma of red cell, 91
Stromuhr, 207
Strychnine, action on reciprocal inhibition,
819
Subarachnoid space, 116, 248
Subcostal angle, 321
Subcostal borders, 321
Subdural space, 116
Submicrons, 54
Substantia-gelatinosa, 831
Successive induction, 824
Successive regeneration, 813
Sugar, storage of, 662
Sugar level in blood, 657
Sugar metabolism (see Carbohydrates),
652
relation of pancreas to, 678
902
INDEX
Sulphates, ethereal, 632
Sulphates, of urine, 532
Sulphur, excretion of, 614
in starvation, 569
Summation in reflexes, 810
Superior laryngeal nerve, influence on res-
piration, 334
Supplemental air, 300
Surface area, and energy output, 540
Surface tension, measurement of, 64
Surgical shock, 289
Survival period, 580
Suspensions, 51
Suspensoids, colloids, 60
Swallowing, 445
center, 447
of liquid food, 448
nervous control of, 447
sounds produced by, 449
x-ray during, 449
Sweat glands, 880
Sympathetic control of heart, 227
afferent, 223
Sympathetic nerve, 424
Sympathetic system, 878, 880
Synapsis, 784, 797, 819
Synaptic fatigue, 296
Synaptic membrane, 798
Syntonin, 486
Systolic index, 128
Systolic pressure, 127
measurement of, in man, 128
Tabes dorsalis, 286
Tachycardia, paroxysmal, 269
Tactile impulses, 833
transmission in cord, 833
Taurine, 494
Taurocholic acid, 494
Temperature :
after-effect, 792
effect on metabolism, 551
sensation of, 792, 832
transmission in cord, 832
Temporary association, 857
Tendon jerks, 828
Tension of CO, in venous blood, 342
of gases in alveolar air, 46, 339
Tetanus, in stomach, 471
Tetanus toxin, action on reciprocal inhi-
bition, 819
Theine, 635
Theobromine, 635
Thermoesthesiometer, 791
Thoracic operculum, 316
Thoracicolumbar outflow, 880
Thrombin, 102
Thrombogen, 106
Thrombokinases, 106
Thromboplastin, 106, 111
Thrombosis, 107
Thrombus formation, 113
Thymic acid, 649
Thymine, 637
Tidal air, 300
Tissot method, 544, 556
Tissue fluid, 116
Tissue juice, 117
Tissues :
amino acids in, 607
influence of, on clotting, 104
oxygen requirements of, 393, 397
utilization of glucose by, 681
Titrable acidity and alkalinity, 22
Tonometer, 338, 381
Tonus rhythm, of stomach, 471
Torcular'herophili, 250
Touch, discrimination, 794
localization, 37, 795
sense, 793
Toxins, 69
Transfusion of blood, 135, 139
Trephining, 253
Treppe, 178
Trichlorlactamide, 635
Trimethylamine, 629
True colloidal solutions, 51
Trypsin, 426, 428, 601
action of, 489
Trypsinogen, 426, 428
Tryptophane, 592, 596, 604, 632
and growth, 576, 578
Tubules, uriniferous, function of, 517
Tumors and diet, 582
Turbidity of colloids, 51
Turck's method, 115, 842
Tyndall phenomenon, colloids, 51
Tyrodes solution, 168
Tyrosine, 502, 604, 632, 666
Tryptic digestion, products of, 490
U
Ultramicroscope, 800
Uncompensated acidosis, 39
Unconditioned reflex, 431, 856
Undetermined nitrogen, 613, 629
Undulatory movement of ribs, 317
Urea, 527, 608
in blood, 610
during disease, 651
excretion of, 615
Urease, 82, 610
Uric acid, 529, 531, 614, 618
amount of, 522
bases of, 531
chemical nature of, 634
endogenous excretion, 647
in disease, 651
metabolism of, 643
of blood, 648
synthesis of, 644
under drugs, 648
Uric acid diathesis, 634
INDEX
903
Uricase, 640
Uricemia, 650
Uricolytic index, 641
Urine :
acids of, 531
amino acid, 530
aromatic oxyacids of, 530
chlorides of, 531
composition, 521
creatinine of, 529
depression of freezing point of, 523
hippuric acid, 530
homogentisic acid, 531
inorganic constituents of, 531
nitrogenous constituents of, 523
normal organic salts of, 523
phosphates, 532
physical processes involved in produc-
tion of, 14
purine bodies of, 529
rate of excretion, 643
reaction of, 524
skatole, 531
solid constituents of, 525
specific gravity of, 522
sulphates of, 532
total potential acidity of, 524
urea of, 527
Uriniferous tubule, 507
Urobilinogen, 496
Utilization limit, 653
Vagus, 878
control of heart, 217
impulses, afferent, 222
Vagus center, effect of nicotine on, 226
location of, 222
tonieity of, 221
Vagus nerve, influence on respiration, 332
Valine, 604, 606
Valves, cardiac, mechanism of, 154
auriculoventricular, 154
semilunar, 155
Van Slyke method for acidosis, 42, 44
Van Slyke method for amino groups, 600
Vascular reflex, 283
Varicose veins, 214
Vasoconstriction, 229
Vasoconstrictor fibers, 229
methods of detecting, 229
of extremities, 233
of head, 233
of viscera, 233
origin of, 232
Vasodilator fibers, 234
methods for detecting, 229
origin of, 234
Vasodilator nerves, 797
Vasomotor center:
afferent impulses, 238, 239
chief center, 235
effect of H-ion of blood on, 238
Vasomotor center — Cont 'd
hormone control of, 237
subsidiary centers, 235
Vasomotor fibers, 231
origin of, 232
Vasotonic impulses, 240
Veins, disappearance of pulse in, 205
Velocity constant, enzymes, 75
Velocity, mean lineal, 206
pulse, 200
Venous blood, tension of CO, in, 342
Venous outflow, 230
Venous pulse tracing, 273
Venous return to heart, 292
Venous sinus, 248
Ventilation of lungs, 350
Ventricle, curves of pressure in, 146, 148,
151
Ventricles :
conductivity tissue of, 184
fibrillation, 195
spread of beat in, 192, 194
Vignin, 578
Viscera, blood supply of, 247
Visceral bloodflow, 212
Visceral sensitiveness, 885
Viscosity of blood, 140
Visual center, 851
Visual psychic areas, 854
Visual sensory area, 854
Vital activity, 14
Vital capacity, 300, 313
in disease, 314
Vital theory of urine excretion, 572
Vitamines, 584
Vividiffusion, 606
Volition, 786
Vomiting, 449
W
Water content of blood, 86
Water hammer,
in blood pressure measurement, 133
Wheatstone bridge, 18
White crescentic line, 226
Wiggers manometer, 146
Willard Gibbs, principle of, 66
Word blindness, 863
Word centers, 862
Word deafness, 863
Xanthine, 635
Xanthine oxidase, 639
Xanthosine, 639
X-rays, in study of stomach, 433
movements of stomach seen by aid of,
451
Zein, inadequacy for growth, 578
Zymogen granules, 420, 421, 429
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